Arundineae. An erect, compactly tufted perennial grass, 15–130 (−250) cm high when flowering, forming either tussocks or extensive swards. Rootstock more or less creeping, with both stout and fine roots. Culms erect, slender to somewhat stout, stiff, smooth, with one node towards the base, disarticulating at this node; the basal internode up to 5 cm usually becomes swollen and club-shaped in late summer or autumn and filled with food reserves. Leaf sheaths rounded on the back, smooth, hairy at the top; ligule a dense fringe of short hairs; leaf blades flat, 3–12 mm wide, long-tapering from near the base to a fine point, 10–45 cm long, sparsely pilose, and completely deciduous in the winter. Panicles erect, variable, ranging from very dense and spike-like to open and very loose, dark to light purple, brownish, yellowish, or green, 3–60 cm long, 1–10 cm wide, and have slender, smooth or minutely rough, branches with short, finely ciliate pedicels. Spikelets 4–9 mm long, tapering to a long point, loosely 1–4-flowered, breaking up at maturity beneath each lemma, with a rough axis. Glumes persistent, lanceolate, acute, shorter than the lemmas, membranous; the lower glume 0–1-veined, 1.5–3 mm long, the upper 1–3-veined, 2.5–4 mm long. Lemmas spaced, tapering to an obtuse apex, bluntly 3–5-keeled, firm, smooth, 4–6 mm long. Paleas with minutely rough keels; in fruit, the caryopsis is enclosed by the hardened lemma and palea. Stamens 3, exserted, anthers large (1.5–3 mm long), violet-brown; stigmas purple, styles terminal, very short.
Molinia caerulea is a very variable species owing to a combination of phenotypic plasticity and genecological variation. Morphological plasticity is expressed as variation in overall size, in the length and width of leaves, and especially in the length, width and colour of the panicles (Hubbard 1968).
Polyploid races of M. caerulea occur in Britain. Within M. caerulea sensu stricto two subspecies may be recognized, but many intermediates occur (Fl. Eur. 5; Sell & Murrell 1996); tetraploid plants forming clumps of a number of single-culmed plants of smaller stature (culms usually < 65 cm) and panicle size (usually < 30 cm) are referable to ssp. caerulea, whereas taller diploid and decaploid tussock-building plants with culms usually 65–125 (160) cm, spreading panicles mostly 30–60 cm long and leaves 8–12 mm wide, belong to ssp. arundinacea (Schrank) K. Richt. (ssp. altissima (Link) Domin, for which known localities in the British Isles are listed by Trist & Sell 1988).
Two diverse populations of M. caerulea, one growing on a calcium-rich, alkaline, LeBlanc waste tip (pH 7.54 ± 0.13), the other on an acid moorland (pH 3.9 ± 0.1), were investigated by Salim et al. (1995). A detailed morphometric comparison of the two populations, in terms of the key morphological features which characterize the two subspecies described above, suggested that both are M. caerulea ssp. caerulea. The two populations could be distinguished, however, by differences in height, leaf width, leaf length and gross flower size of plants growing in the field. But an electrophoretic analysis of isoenzymes could not separate the two populations, indicating that they are genetically similar and that differences in size and flowering are presumably plastic responses to environmental factors. When plants from both populations were, however, grown experimentally under identical conditions in both acid and alkaline artificial growing medium, the acid population plants consistently showed significantly greater growth than the alkaline population plants regardless of the nature of the growth medium. This implies that the differences in growth in field conditions may not be merely phenotypic plasticity, but that the two populations may possibly be edaphic ecotypes.
Molinia caerulea is a common native species usually abundant and frequently dominating large areas, often to the exclusion of other flowering plants. Ssp. caerulea is widespread in open situations on moors, heaths, bogs, fens, mountain grassland, cliffs and lake shores, always on at least seasonally wet, acid sandy, or peaty ground throughout the British Isles. The much larger ssp. arundinacea (ssp. altissima) occurs in tall vegetation with dense ground cover in fens, fen-scrub, and in fen-like vegetation by rivers and canals, sometimes shaded by Betula and Salix spp., on somewhat base-rich mineral soils with fluctuating water-table; scattered but frequent in suitable places in central and southern Britain, but very scattered in Scotland and southern Ireland (Stace 1997).
I. Geographical and altitudinal distribution
Molinia caerulea is locally abundant especially in the north and west of the British Isles (Fig. 1) where it occurs in all vice-counties. The area of land in Britain dominated by this species has been estimated at around 600 000 ha or 10% of the uplands (Bunce & Barr 1988). It is widespread in Europe except for some islands, but mainly on mountains in the south (Fig. 2). Outside Europe the species occurs in North Africa, Caucasus and Siberia; introduced in North America.
The altitudinal range of M. caerulea in the British Isles extends from sea level to an altitude of 613 m in the English Lake District, in Wales to 701 m on Snowdon, in Ireland to 747 m in Down and to an upper limit of 914 m in the Scottish Highlands (Alt. range Br. Pl.). It ascends to 500 m in northern Norway, to 1230 m at Hardangervidda in southern Norway (Atl. N.W. Eur.), and in the Alps up to 1900 m (Vergl. Chor.).
(a) climatic and topographical limitations
Molinia caerulea is most commonly found on flat ground and on gentle to moderate (< 40°) slopes suitable for the formation of peaty gleys or deep peats, and is generally not favoured by any particular aspect.
It tolerates a wide range of irradiance; characteristic habitats include both open, submontane grasslands and mires and the lighter phases of Betula pubescens woodland. Molinia caerulea persists in moderate shade, but tends to produce few flowers under these conditions.
Molinia caerulea grows on a diverse range of soil types. These include calcareous surface water gleys over Carboniferous limestone, often drift-covered, as in Upper Teesdale (Pigott 1956), and in calcareous, strongly irrigated mud of silt, sand, gravel and humus over Dalradian limestone in the Scottish Highlands (Pl. Comm. Scot.). In the Norfolk Broads it occurs on ground-water gleys over alluvium in the lower reaches of river valleys, and on basin peats in the upper reaches (Perrin 1961). It is found on blanket peat and basin and flushed peat of low base status in south-west Galloway, and non-calcareous gley soil of low base status in Aberdeenshire (Birse & Robertson 1976). A number of soil types that support the growth of M. caerulea in the British Isles are described by Avery (1990): stagnogley-podzol over lithoskeletal siltstone and sandstone in the Ashdown Forest, East Sussex; humus-ironpan stagnopodzol on loamy drift at Bloxworth, Dorset; ferric stagnopodzol over lithoskeletal mudstone and sandstone or shale at Brendon, Devon; orthic gley soils over loamy drift at Llangadog, Dyfed, and Epping Forest, Essex; and bog peat at Risley Moss, Cheshire, at St Clether, Cornwall and Hartford, Devon. It occurs on brown podzolic soils of the Manod series over base-poor Lower Palaeozoic shales and mudstones close to the Llyn Brianne reservoir, Mid Wales (Soulsby & Reynolds 1994).
Molinia caerulea has a bimodal pH distribution with peaks of abundance on both highly acidic soils (pH < 4.0) and calcareous soils of pH > 7.0 (Grime et al. 1988). In British mires it is equally abundant in ‘bog’, with pH generally < 5.0, low Ca2+, and Cl− and SO42− as the main inorganic anions, and in ‘fen’ with pH generally > 6.0, high Ca2+ and HCO3− (Cooper & Proctor 1998; Wheeler & Proctor 2000). At both high and low pH, M. caerulea tends to be associated either with moist grassland soils or with soligenous peat having a well-oxygenated soil profile (Armstrong & Boatman 1967). It is most abundant and grows vigorously, however, on sites where there is ground-water movement, good soil aeration and an enriched nutrient supply (Jefferies 1915; Rutter 1955; Webster 1962a; Loach 1966, 1968a; Sheikh 1969a). See also VI (E).
James (1962) carried out pot experiments in a cold glasshouse with seedlings of M. caerulea grown in John Innes no. 1 potting compost made using an acidic loam (pH 5.4) unmodified or modified by the addition of CaCO3 or CaSO4, to determine the factors which might bring about the natural exclusion of the species from well-drained calcareous soils. He found that a high concentration of calcium as such is not inimical to the growth of the species under both free-draining and waterlogged soil conditions. When the phosphate level of the well-drained calcareous experimental soil was increased very greatly, the plant grew successfully, suggesting that the inability to obtain phosphorus is a factor that naturally excludes M. caerulea from dry calcareous soils.
The nutrient content of soils from three adjacent and closely related wet-heath communities containing M. caerulea as a major component in Bramshill Forest, north-east Hampshire, was determined by Loach (1966). The wet heath site in which the plant was most abundant (Molinietum) had the largest total soil nutrient content and greatest water-table depth and fluctuation, although the topsoil was phosphorus-deficient in comparison with many other soils. The other two wetter sites were also deficient in phosphorus. Loach (1968a) assessed experimentally the relative abilities of the sites to supply nutrients to M. caerulea from the dry matter yields and nutrient uptake of tillers replanted into six small enclosures cleared of standing vegetation in each of the three sites. Tissue concentrations of nutrients in shoots of the plant were greatest in the wet heath and least in the valley bog, and these differences were associated with large and small dry matter yields, respectively. The relative nutrient status of these wet-heath soils can be gauged from the phosphorus concentrations ranging from 30 to 60 mg per 100 g dry tissue, which are extremely low compared with the 100–200 mg per 100 g more commonly found. The availability of the soil nutrients for the growth of M. caerulea was assessed experimentally in a glasshouse, where near-optimal conditions of water supply could be maintained thus eliminating the effects of differences in the degree of waterlogging which occur in the field (Loach 1968a); the growth response of seedlings planted in soil cores from each site to additions of the major nutrients N, P and K, alone and in combination, was monitored. Phosphorus additions, especially in combination with the other elements, caused very large growth increases in all soils. The increase was greatest in the valley bog soil, where there was a fivefold rise from an initially low yield to a level not significantly different from that of the wet-heath soil. The results of a further experiment, in which soil cores were freely drained or waterlogged, showed that drainage without added nutrients did not improve the growth of M. caerulea in valley bog soil, confirming the extreme nutrient deficiency of this soil type. This also suggests that in the three wet-heath sites the differences in the abundance of M. caerulea caused by waterlogging are not to be explained solely in terms of its effect on nutrient uptake.
A further experiment was carried out by Sheikh (1969b) in the enclosures cleared of standing vegetation by Loach (1968a); the effects of additional nutrients (N, P, and K) and competition between transplanted material of M. caerulea and Erica tetralix were determined after two seasons’ growth. Without additional nutrients there were large differences between sites in the growth of M. caerulea: poor growth on the valley bog (453 mg dry weight per plant) and best on the wet-heath plots (768 mg). The response to nutrients was marked (mean plant dry weight 1315 mg) and the interaction of nutrition with sites was such that site differences disappeared when nutrients were applied. There were no significant effects of competition with E. tetralix on the growth of M. caerulea.
Green shoots of dominant M. caerulea collected between mid-July and mid-August from 36 widely ranging, undisturbed, unfertilized habitats throughout Britain were analysed for inorganic element concentrations, together with matching analyses of the soil nutrients (Rowland et al. 1999). Soil pH ranged between 3.1 and 5.8, and the humus content varied widely. There were significant correlations (P < 0.001) between loss-on-ignition and N, P, K, Na, Mg and Ca (P < 0.05) in the soil. Concentrations of Ca, Mg, Zn and Mn in the plant material were significantly correlated (P < 0.001) with extractable levels in the soil. However, the lack of correlation between the measures of soil availability for N, P and K and plant tissue concentrations may indicate that rapid shoot growth in the spring is supported more from stored nutrients, translocated back from the basal internodes, than from concurrent absorption from the soil. The average nutrient demands of M. caerulea for P, K, Ca, Fe and Na were relatively low compared with the mean concentrations in 21 other grass species sampled in the Vegetation Nutrient Survey, whereas they were relatively high for N, Mn and Zn.
Widespread acidic grassland dominated by peat-forming M. caerulea (Molinietum caeruleae) is closely related to the main peat vegetation of the northern and western hills, to which it is often marginal (Tansley, Br. Isl.). In the Southern Uplands it is characteristic of considerable areas of grassland on peaty gley podzols (Veg. Scot.). Molinia caerulea freely colonizes the drier parts of the ‘blanket bog’ of western Ireland and Scotland, and its associates are predominantly wet peat plants (Tansley, Br. Isl.). In the Western Highlands and in Galloway, extensive Molinieto-Callunetum, which occurs on shallow ombrogenous peat, can be regarded either as blanket bog or wet grass-shrub heath. This association often grades into floristically poor Molinia-Myrica mire, where the soligenous influence becomes more definite and in which the co-dominant M. caerulea often shows a dense tussocky habit (Pl. Comm. Scot.). In southern England, M. caerulea is perhaps the most characteristic plant of damp or wet heath and it is nearly as widespread as it is in the above mire communities and on certain types of fen in England and in Northern Ireland (Tansley, Br. Isl.). On the peat of the East Anglian Fens, when Cladium mariscus sedge fens are cut at frequent intervals (every 2 years), apparently stable communities with abundant or dominant Molinia caerulea are produced. Such communities were once widespread but nowadays are quite small in extent (Perring et al. 1964; Wheeler 1980). In these fens, Molinietum is intermediate between luxuriant base-rich fen and acidic bog (Tansley, Br. Isl.). The results of a survey carried out between 1967 and 1969 (Pigott & Wilson 1978) show that an association in which M. caerulea was dominant, and formed a tussocky grassland rooted in a black fibrous peat, occupied a wide zone in the northern and eastern parts of Esthwaite North fen. Although neither mown nor grazed in 1967–9 the species occupied the same area as it did in 1914–16 when part of the area was still cut for hay (Pearsall 1918). In the Scottish Highlands, M. caerulea also occurs locally in wetter places in pinewoods, birchwoods and oakwoods (Steven & Carlisle 1959; Veg. Scot.). In two western oakwoods (Blechno-Quercetum petraeae) in North Wales, M. caerulea occurs in wet areas differentiated by Sphagnum palustre and often S. recurvum (Edwards & Birks 1986); it is present in acidic mires (pH 3.8) at Coed-y-Rhygen; at Coed Ganllwyd it dominates acid boggy areas (pH 4.0) and also occurs in areas of seepage that are base-enriched (pH 4.8).
The National Vegetation Classification records M. caerulea in a range of communities (Rodwell 1991a,b, 1992, 1995), and provides community descriptions, distribution, affinities, habitat details, and floristic tables. Molinia caerulea is ubiquitous in the following communities which are synonymous with vegetation referred to above and often termed a Molinietum or ‘Molinia grassland’: in Molinia caerulea–Potentilla erecta mire (M25), which occurs throughout western Britain, and is especially frequent in south-west England, Wales and southern Scotland, on moist but well-aerated, acid to neutral, peats and peaty mineral soils where there is at least a slight degree of nutrient enrichment; and in the warmer lowlands of southern Britain in Molinia caerulea-Cirsium dissectum fen-meadow (M24), a community of moist to fairly dry peats and peaty mineral soils, circumneutral but somewhat mesotrophic. There are other mire communities in which M. caerulea is constant throughout. In particular, the Scirpus cespitosus(Trichophorum cespitosum)–Erica tetralix wet heath (M15), a compendious vegetation type with few constants and wide variation in the pattern of dominance and in associated flora. Molinia caerulea, Trichophorum cespitosum, Erica tetralix and Calluna vulgaris are all of high frequency, but of the four species, M. caerulea is the most consistent overall and it is often abundant. The community occurs widely at lower altitudes in western and northern Britain, particularly in the western Highlands of Scotland, in south-west Scotland and Wales, and, less extensively, in the Lake District, Dartmoor and Exmoor, characteristically on moist and generally acid and oligotrophic peats and peaty mineral soils. In the eastern and southern lowlands of Britain the Scirpus (Trichophorum)–Erica wet heath is replaced by the Erica tetralix–Sphagnum compactum wet heath (M16), which is characteristically dominated by varying proportions of Erica tetralix, Calluna vulgaris and Molinia caerulea depending on environmental factors, and occurs on acid and oligotrophic mineral soils or shallow peats that are at least seasonally waterlogged.
Molinia caerulea is abundant in Scirpus cespitosus (Trichophorum cespitosum)–Eriophorum vaginatum blanket mire (M17), a community on waterlogged ombrogenous peat, dominated by mixtures of monocotyledons, ericoid sub-shrubs and Sphagna, among which Calluna vulgaris, Erica tetralix, Eriophorum angustifolium, Narthecium ossifragum, Potentilla erecta, Sphagnum capillifolium and S. papillosum are the other constant species; it is largely confined to western Britain, being especially widespread in the western Highlands of Scotland and the western Isles and running down through south-west Scotland, the Lake District, west Wales and south-west England.
A number of mire communities containing M. caerulea are widespread but local in England and Wales. It is consistently present and often greatly abundant in the field layer of Betula pubescens–Molinia caerulea woodland (W4), in which it shares constancy together with Sphagnum recurvum/palustre, a mire woodland community that is widespread but local throughout the lowlands and upland fringes of Britain, characteristically on thin or drying ombrogenous peats around the margins of blanket mires; also on soligenous peats in valley mires and on peaty gleys flushed with base- and nutrient-poor waters. In Narthecium ossifragum–Sphagnum papillosum valley mire (M21), M. caerulea is abundant in better aerated situations and also constant together with Calluna vulgaris, Drosera rotundifolia, Erica tetralix, and Eriophorum angustifolium, a community of the southern lowlands of Britain on permanently waterlogged, acidic and oligotrophic peats. In Schoenus nigricans–Juncus subnodulosus mire (M13), M. caerulea is locally prominent and also a constant together with Carex panicea, Potentilla erecta,Succisa pratensis, Calliergon cuspidatum and Campylium stellatum, a community that is widespread but decidedly local throughout lowland England and Wales, and confined to peat or mineral soils in and around mires irrigated by base-rich, highly calcareous waters. Molinia caerulea is generally abundant in Schoenus nigricans–Narthecium ossifragum mire (M14), together with the other constants Anagallis tenella, Erica tetralix, Aneura pinguis, Campylium stellatum, Scorpidium scorpioides, Sphagnum auriculatum and S. subnitens, which occurs in the oceanic south-west of Britain, very locally in Cornwall, east Devon, south-east Dorset and the New Forest, characteristically on peats and mineral soils irrigated by moderately base-rich and calcareous ground waters.
Molinia caerulea is the most common dominant in Molinia caerulea–Crepis paludosa mire (M26) together with a substantial block of constants and companions, Carex nigra, C. panicea, Equisetum palustre, Filipendula ulmaria, Potentilla erecta, Ranunculus acris, Succisa pratensis, Valeriana dioica and Calliergon cuspidatum, a very local community of moist, moderately base-rich and calcareous peats and peaty mineral soils in the submontane northern and Craven Pennines; and abundant in the Carex echinata and Juncus acutiflorus subcommunities of the Carex echinata–Sphagnum recurvum/auriculatum mire (M6), which are virtually ubiquitous in the upland fringes on soligenous peats and peaty gleys irrigated by base-poor waters in the submontane zone in Britain.
In a number of local heathland communities M. caerulea is constant: it is obviously preferential in the Molinia caerulea subcommunity of the Calluna vulgaris–Ulex minor heath (H2), which is particularly extensive in the New Forest; locally extensive in Ulex minor–Agrostis curtisii heath (H3) together with the other constants Calluna vulgaris, Erica cinerea and E. tetralix, on impoverished acid soils with impeded drainage, confined to south Dorset and Hampshire; in Ulex gallii–Agrostis curtisii heath (H4), very similar to the previous example but with the replacement of the one gorse by the other together with Potentilla erecta as an additional constant, on a variety of acid soils in the warm oceanic parts of south-west Britain; on the Lizard peninsula in Cornwall, M. caerulea is frequently of high cover in Erica vagans–Schoenus nigricans heath (H5) together with the other constants Anagallis tenella, Carex pulicaris, Erica tetralix, Festuca ovina, Potentilla erecta, Serratula tinctoria, Succisa pratensis, Ulex gallii and Campylium stellatum, confined to wet base-rich but calcium-poor mineral soils and shallow peats; often abundant in the Agrostis curtisii and Molinia caerulea subcommunities of the Erica vagans–Ulex europaeus heath (H6), with the other constants Agrostis canina ssp. montana, Carex flacca, Erica cinerea, Filipendula vulgaris, Potentilla erecta, Ulex gallii, and Viola riviniana, characteristic on oligotrophic, circumneutral or fairly base-rich, but not lime-saturated, free-draining brown earths, found only on the Lizard peninsula in Cornwall; and also in the Molinia caerulea subcommunity of the widely distributed Calluna vulgaris–Deschampsia flexuosa heath (H9), which primarily occurs on lowland sites in the southern Pennines and North York Moors with more scattered local occurrences through the Midland plain.
Molinia caerulea occurs frequently in Carex rostrata–Calliergon cuspidatum/giganteum mire (M9), a community with a widespread but rather local distribution; Carex dioica–Pinguicula vulgaris mire (M10), of widespread but local occurrence throughout northern England and Scotland on soligenous mineral soils and shallow surface peats kept very wet by base-rich, calcareous and oligotrophic waters; Hypericum elodes–Potamogeton polygonifolius soakway (M29), which extends in a well-defined zone from west Surrey, through the New Forest to the south-west Peninsula, up through Wales and into Galloway, on shallow soakways and pools in peats and peaty mineral soils; Juncus subnodulosus–Cirsium palustre fen meadow, Briza media–Trifolium spp. subcommunity (M22b), widely distributed on suitably wet and base-rich soils through the southern British lowlands, particularly in East Anglia, north Buckinghamshire and Anglesey; in the Juncus acutiflorus subcommunity of the Juncus effusus/acutiflorus–Galium palustre rush-pasture (M23), widespread in western Scotland and Wales, exceedingly common at low to moderate altitudes; in Calluna vulgaris–Erica cinerea heath (H10), which occurs widely particularly in south-west Scotland on acid to circumneutral, free-draining soils; and in Agrostis curtisii grassland (U3) on moist base-poor soils, in response to burning and grazing, around the upland fringes of the south-west and in southern parts of the New Forest.
In a number of mire communities M. caerulea is occasional: Carex rostrata–Sphagnum recurvum mire (M4) which is widespread but local throughout the north-west of Britain; Carex rostrata–Sphagnum squarrosum mire (M5) with a widespread but fairly local distribution in the north-western parts of Britain; Carex demissa–Saxifraga aizoides mire (M11), a community largely confined to Scotland, where it is common in the southern and central Highlands but also occurs more locally in the Southern Uplands, the Lake District, the northern Pennines and in north Wales; Erica tetralix–Sphagnum papillosum raised and blanket mire (M18), of widespread but local occurrence through the lowlands of Wales and north-west Britain up to the Clyde-Moray line; in the Erica tetralix subcommunity of the Calluna vulgaris–Eriophorum vaginatum blanket mire (M19), found at lower altitudes through Wales and Strathclyde; in the Juncus effusus–Holcus lanatus subcommunity of the Filipendula ulmaria–Angelica sylvestris mire (M27), distributed down the western seaboard of lowland Britain; and also in Calluna vulgaris–Juniperus communis ssp. nana heath (H15) of rather patchy occurrence along the western side of the more northerly mountains, especially Beinn Eighe and Foinaven.
In the following communities M. caerulea is also found occasionally: Salix pentandra–Carex rostrata woodland (W3), which occurs locally throughout the submontane zone of northern Britain; in the Rhytidiadelphus triquetrus subcommunity of the Quercus petraea–Betula pubescens–Dicranum majus woodland (W17) found in the more continental parts of eastern Scotland; Sesleria albicans–Galium sterneri grassland, Helianthemum canum–Asperula cynanchica subcommunity (CG9) confined to the Carboniferous Limestone in the lowlands of South Lakeland and north Lancashire; and in Carex paniculata sedge-swamp (S3) widespread around the open-water transitions of the Shropshire meres.
Molinia caerulea is scarce in the following woodland communities: Salix cinerea–Galium palustre woodland (W1); Salix cinerea–Betula pubescens–Phragmites australis woodland (W2); Fagus sylvatica–Deschampsia flexuosa woodland (W15); Quercus spp.–Betula spp.–Deschampsia flexuosa woodland (W16); and Quercus petraea–Betula pubescens–Dicranum majus woodland (W17).
Molinia caerulea is also scarce in a number of grassland and montane communities: Festuca ovina–Agrostis capillaris–Thymus praecox (polytrichus) grassland (CG10); Festuca ovina–Agrostis capillaris–Alchemilla alpina grass-heath (CG11); Dryas octopetala–Carex flacca heath (CG13); Deschampsia flexuosa grassland (U2); Festuca ovina–Agrostis capillaris–Galium saxatile grassland (U4); Luzula sylvatica–Geum rivale tall-herb community (U17); and Pteridium aquilinum–Galium saxatile community (U20).
Molinia caerulea is scarce in a number of mire and swamp communities: Sphagnum auriculatum bog pool community (M1); Sphagnum cuspidatum/recurvum bog pool community (M2); Eriophorum angustifolium bog pool community (M3); in the Empetrum nigrum ssp. nigrum subcommunity of the Calluna vulgaris–Eriophorum vaginatum blanket mire (M19); Anthelia julacea–Sphagnum auriculatum spring (M31); Ranunculus omiophyllus–Montia fontana rill (M35); Phragmites australis swamp and reed-beds (S4); and in Phragmites australis–Peucedanum palustre tall-herb fen (S24).
Molinia caerulea is scarce in a number of heathland communities: Calluna vulgaris–Scilla verna heath (H7); Calluna vulgaris–Ulex gallii heath (H8); Calluna vulgaris–Vaccinium myrtillus heath (H12); Calluna vulgaris–Cladonia arbuscula heath (H13); Calluna vulgaris–Racomitrium lanuginosum heath (H14); and in Calluna vulgaris–Vaccinium myrtillus–Sphagnum capillifolium heath (H21).
In the submontane belt of Central Europe, purple moor-grass (Molinia caerulea) has become the litter grass par excellence of unmanured litter meadow communities. These communities, including the extreme examples of the floristically rich Cirsio tuberosi-Molinietum on base-rich damp soils and the acid-soil Junco-Molinietum, which are referred to the alliance Molinion, in the order Molinietalia, in the class Molinio-Arrhenatheretea, are now to be found only in the plains north of the Alps and in valleys of the outer Alps themselves (Ellenberg 1988).
In the lowlands and to a lesser extent in the montane belt of Central Europe, three groups of pure deciduous woodland types can be distinguished in which M. caerulea occurs in a subassociation on moist-soil, Betulo-Quercetum molinietosum; in subatlantic Birch-Oak woods (Betulo-Quercetum) particularly characteristic of and frequent on the diluvial sands of the northern plain in north-west Germany and neighbouring areas; in southern Central Europe Oak woods (‘Quercetum medioeuropaeum’); and in Insubrian Birch-Downy Oak woods (‘Betulo-Quercetum insubricum’) found in high rainfall areas on the southern edge of the Alps and which contain submediterranean species. In the north-west German plain when the Molinia subassociations of Betulo-Quercetum and Fago-Quercetum are planted with Pine the communities are described as Molinia-Dryopteris-Pine forest and Molinia-Rubus-Pine forest, respectively (Ellenberg 1988).
Molinia caerulea occurs with high frequency in subcontinental acid-soil mixed Spruce-Oak-Pine woods, Pino-Quercetum, in north-east Germany and in north Poland. In Central Europe, in communities dominated by Pinus sylvestris, M. caerulea ssp. arundinacea has a high frequency in Moorgrass-Pine woods (Molinio-Pinetum) in soft clay marls over limestone on slopes with a fluctuating dryness in the submontane to montane belts in the rainy area surrounding the Alps. In a similar woodland type on lime-rich mineral soils, on gravel which is covered by a layer of sand on terraces no longer subject to flooding along the Isar river south of Munich, M. caerulea ssp. arundinacea occurs in Dorycnio-Pinetum molinietosum (Ellenberg 1988).
In treeless intermediate mires, M. caerulea is frequent in ‘lagg’ bog communities assigned to the raised bog order Sphagnetalia magellanici in the class Oxycocco-Sphagnetea, which for example are found in the Harz, Germany. In north-west Germany it also occurs in hollows which temporarily fill with water and which alternate with hummocks in true raised bog; a few years after raised bog has been drained the old surface turns into heathland, and if this secondary vegetation is burnt from time to time tussocks of M. caerulea become dominant. It also occurs in the fragments of acid-soil dwarf-shrub heath (Genisto-Callunetalia) mostly maintained by sheep grazing, and in moist sand heath (Genisto-Callunetum molinietosum) whose distribution centre lies in Atlantic north-west Europe (Ellenberg 1988).
In the Netherlands, Westhoff & den Held (1969) record M. caerulea in a number of communities: a differential of the alliance Erico-Sphagnion in the order Sphagnetalia magellanici together with Erica tetralix; in the Empetro-Genistum tinctorium community a differential species of subassociation molinietosum; in Frangula-Salicetum auritae (Franguletea) a differential together with Betula pubescens and Erica tetralix; in Cirsio-Molinietum (Molinietalia) subassociation agrostietosum; occurs in the Carici elongatae-Alnetum subassociation betuletosum pubescentis; also in subassociation molinietosum of Querco roboris-Betuletum woodlands. Further floristic lists and references to M. caerulea are provided by Schaminée et al. (1995).
IV. Response to biotic factors
Molinia caerulea is tolerant of burning and grazing, and is particularly important in acidic upland areas where it dominates large areas of poorly drained land. On the Isle of Rhum, north-west Scotland, burnt M. caerulea-dominant range was grazed much more heavily by red deer, the only large herbivore present, than surrounding unburnt Molinia stands (Miles 1971). At many sites, the burning and grazing history affects the cover of the plant and determines whether the vegetation is wet heath or grassland. On mixed grass and heather on poorly drained mineral soils, burning at about 3–6-year intervals shifts dominance to M. caerulea (Miles 1988). Where grass is already dominant, frequent burning favours M. caerulea at the expense of other grasses (Grant et al. 1963). Controlled grazing studies on small plots (Jones 1967) have demonstrated that heavy grazing, particularly if fertilizer is also added, favours Agrostis spp. at the expense of M. caerulea, whereas in ungrazed or very lightly grazed swards the latter achieves dominance (Jones 1967; Job & Taylor 1978).
Molinia caerulea has a grazing value intermediate between that of Agrostis-Festuca grassland and impoverished Nardus grassland (McVean 1952). In free grazing situations or in traditional extensive sheep production systems, M. caerulea is usually little grazed, especially in summer when the sheep concentrate on the Agrostis-Festuca areas (Hunter 1962; King & Nicholson 1964). During autumn the ungrazed leaves are shed; thus, the loose litter which accumulates impedes access by the sheep to the green leaves of other grasses in winter and to the fresh growth of all grasses in spring. The dead material dilutes the quality of the diet and reduces the attractiveness of the Molinia communities for subsequent grazing. When managed to control the accumulation of previously under-utilized dead herbage, these communities can provide high-quality feeding value for grazing animals from June to August (Grant et al. 1985). The rapidity of growth and tallness of M. caerulea in June–July suggest that, in summer, such grassland is more suited to grazing by hill cattle than sheep.
The level of grazing by cattle that can be sustained while maintaining Molinia dominance has been investigated in long-term grazing experiments carried out at two sites in southern Scotland, Cleish Hills, Fife (230 m) and Bell Hill, Roxburghshire (255–280 m), by Grant et al. (1996). Plots continuously grazed by cattle for 6 years, with animal numbers adjusted twice weekly to maintain target lamina lengths, compared treatments where 66% rather than 33% of the annual leaf production was removed by grazing. The rates of leaf extension in M. caerulea were reduced at the higher level of grazing. In comparison with areas protected from grazing during the final year of treatment only, the biomass of Molinia and other grasses in areas open to grazing showed that the taller Molinia was utilized to a much greater extent than the other grasses. After 6 years of grazing, the biomass of M. caerulea at 33% utilization was reduced by 46–65% compared with ungrazed exclosures, while at 66% utilization it was reduced by 86%. Basal internode size was greatly reduced in the grazed plots compared with the ungrazed exclosures, with effects on tiller base size being more important than variation in concentrations in determining amounts of starch, total water soluble carbohydrates, N, P, and K on a per tiller basis. Floristic diversity was increased on grazed compared with ungrazed areas. The cover of M. caerulea was decreased and that of other broad-leaved grasses increased by grazing; at the lower rate of utilization the cover of M. caerulea appeared to be levelling off (at around 60–65% after 3–5 years) while at the higher rate a continued downward trend was evident. A second experiment was carried out in the field over a 4-year period at Cleish, Fife (230 m) and at Sourhope, Roxburghshire (450 m), using Molinia tussocks as the experimental unit (Torvell et al. 1988; Grant et al. 1996). This experiment was designed to investigate the long-term responses of M. caerulea to timing, frequency and severity of defoliation. Tussocks were left uncut or cut annually to remove 33% or 66% of lamina material, either once per year in June (at peak leaf growth), July or August (leaf extension growth ceasing), or repeatedly in June, July and August. The results showed that frequency and severity of defoliation were more important than timing in their effects on Molinia. Three years of repeated light defoliation, compared with uncut controls, reduced leaf production in a fourth uninterrupted growing season by 40%, while repeated heavy defoliation reduced it by 78%. Reductions in both the numbers and size of tillers contributed to this result. Single annual cuts reduced leaf production at 66% lamina removal only when they were made late in the season. These results are consistent with earlier investigations of the short-term responses of M. caerulea to defoliation; though the weight of leaf (mg tiller−1) in the first season of cutting was unaffected by defoliation, the weights of storage organs (the roots and new season’s basal internodes) were reduced by defoliation (Latusek 1983; Torvell et al. 1988; Thornton 1991). This led to the expectation, borne out by the longer-term work, that the effects of defoliation on leaf production would be deferred in time. Nutrient richness, however, also influences tillering and vigour of growth in Molinia. In a pot experiment using rooted basal internodes protected from rain but otherwise exposed to ambient conditions, Thornton (1991) found that secondary tillers were produced at high N but not at low N supply. He also found that leaf extension was increased on defoliated plants at low, but not at high N, and the adverse effects of defoliation on root dry weight and root : shoot ratios were proportionally greater at low N.
The Central European litter meadows dominated by M. caerulea are traditionally managed by being mown every year or at least every second year; this cut must not be made before the end of September, when the haulms and leaves of the plant are fully mature. The straw has to be removed from the meadow, otherwise the particular floristic composition will be lost (Ellenberg 1988). Since there is now much less demand for litter for bedding as a result of the modernization of housing for cows, the conservation of the large unmanured litter meadow complexes has become a pressing problem. The simple mowing and removal of the straw has become too expensive. Up to a point, the mowing can be replaced by burning off and this certainly maintains the dominance of the fire-tolerant M. caerulea. However, after many years of research in Switzerland, it has been discovered that the only way to maintain the species combination is to mow the purple moor-grass meadow in the autumn every second year by machine and remove the cut material to a place where it can be burned (Ellenberg 1988).
In upland areas of England and Wales, and in parts of southern and western Scotland, there has been a considerable increase in M. caerulea at the expense of Calluna vulgaris since the start of the Industrial Revolution (Chambers et al. 1979). This shift in vegetation composition has been variously attributed to burning and/or grazing regimes and increased atmospheric nitrogen and sulphur deposition (Todd et al. 2000). Samples taken from two localities on Exmoor have been subjected by Chambers et al. (1999) to recently developed techniques of plant macrofossil counting and to conventional pollen analysis: data confirmed the recent ousting of C. vulgaris and the rise to dominance of M. caerulea in a wet heath community (a variant of NVC type M15), but also gave evidence of an earlier unsuspected presence of Molinia; the overwhelming dominance of Molinia in a mire community (NVC type M25) was also a recent phenomenon, but only partly at the expense of C. vulgaris. The palaeological evidence indicated a greater antiquity and former abundance of M. caerulea than is often appreciated and suggested that, over the past millenium, vegetation dominance has alternated between heather moorland (Callunetum) and grass moor containing at least some Molinia, while the former Calluna-dominated wet heath itself developed originally from grass moor. These findings have important implications for conservation and management.
In the Netherlands dry heathlands were, until a few decades ago, generally dominated by Calluna vulgaris. Since the 1970s and still ongoing, these heathlands have shown striking shifts in species composition; C. vulgaris has been largely replaced by Molinia caerulea. This change has been attributed to an increase of N availability in these originally nutrient-poor ecosystems, triggered by stress and disturbance factors which cause opening of the Calluna canopy, e.g. senescence, frost, drought or by heather beetle (Lochmaea suturalis) attacks (Aerts 1989; Aerts & Heil 1993). Wet heathlands, originally dominated by Erica tetralix, have become monospecific stands of M. caerulea (Aerts & Berendse 1989). This change took place during a period in which the availability of nitrogen and phosphorus in these originally nutrient-poor ecosystems increased owing to the accumulation of litter and humus. Earlier when these heathlands formed part of the Dutch agricultural system, this accumulation was slowed down, or interrupted at regular intervals by sod cutting, grazing by sheep or burning. In many wet heathlands in the Netherlands the accumulation of litter and humus has led to mineralization rates of nitrogen as high as 100–130 kg ha−1 year−1 (Berendse et al. 1987), but not until 10 years after sod removal. Nitrogen availability also increased in both types of heathland because of the increased atmospheric deposition of ammonium, which is caused by the increased NH3-emission from heavily manured pastures and fields in the surroundings of the heathlands; total atmospheric input rose to 30–50 kg N ha−1 year−1 in the 1980s (Bobbink et al. 1992).
In recent reviews, Bobbink et al. (1998) and Aerts & Bobbink (1999) have drawn attention to the competitive ability of Molinia caerulea in Dutch wet Ericion tetralicis heathlands. Clearly, M. caerulea is a better competitor and can replace Erica tetralix at high nitrogen or phosphorus availability or both, as demonstrated by the responses to applied fertilizer or lowering of the water-table in both a container micro-ecosystem experiment and in the field (Berendse & Aerts 1984; Aerts & Berendse 1988). Lowering the water-table initially leads to higher mineralization rates (Berendse et al. 1994; Aerts & Ludwig 1997), so the change in the competitive balance between E. tetralix and M. caerulea, after lowering the water-table, might also be a result of changes in nutrient availability. Increased atmospheric deposition combined with minimal leaching (Berendse 1990) leads to observed availabilities that are sufficient to explain replacement of E. tetralix by M. caerulea. The high competitive ability of M. caerulea in these situations appears to depend on a combination of a high potential productivity, a high percentage biomass allocation to the roots, an extensive root system exploiting a large volume of soil, and plasticity in the spatial arrangement of leaf layers over its tall canopy (Aerts et al. 1991).
A combination of burning, grazing and herbicide applications has been applied in factorial combinations to control M. caerulea in upland moorland communities in three regions of Britain, where it is replacing Calluna vulgaris as the dominant species: Exmoor, North Peaks and Yorkshire (Todd et al. 2000). Of the treatments applied, glyphosate (Roundup Biactive, Monsanto, High Wycombe, UK) had the most significant effect on Molinia at all study sites. The use of a suite of selective herbicides has been assessed by Milligan et al. (1999). These laboratory trials showed that for Molinia, glyphosate was the most effective and only herbicide to provide a reduction in root growth, suggesting that this could reduce regeneration via the roots. However, even extremely low levels of glyphosate can reduce shoot growth in C. vulgaris significantly. Two selective herbicides tested, quizalofop-ethyl (Pilot, AgroEvo UK) and sethoxydim (Checkmate, Rhône Poulenc), reduced various other measures of Molinia growth but did not damage Calluna, and these two graminicides are recommended for further field trials.
V. Response to environment
Molinia caerulea is frequently dominant over large areas to the exclusion of all other flowering plants. Typically it forms dense circular tussocks from 8 to 20 cm in diameter at the base and raised to a similar height above the soil; above the base the leaves stand up boldly, 20–40 cm high; the inflorescences, spike-like panicles, rise above the foliage on wiry stalks 25–75 cm long (Jefferies 1915). According to Proffitt (1985), however, tussocks may include more than one genotype.
The distribution of the tussocks themselves is very variable. They may be scattered, or provide an almost continuous cover of large tussocks which has the appearance of a rough hummock and hollow system, or in other situations seem to form a shorter lawn-like sward but which, on close inspection, is found in its characteristic tussocky form with systems of litter-lined runnels.
(b) performance in various habitats
Estimates of the maximum above-ground dry matter production by stands of M. caerulea have been made in a number of habitats in Britain: 500 g m−2 in poor fen/bog sampled from eight sites in England and Scotland (Pearsall & Gorham 1956); and 540 g m−2 in a Molinia-dominated flush in Bramshill Forest, Crowthorne, Berkshire (Loach 1968b).
In the Netherlands, the estimates of maximum above-ground dry matter production are generally higher than in Britain and this has been attributed to the high atmospheric NHx-deposition (Aerts 1989): 541 g m−2 in wet heath (Berendse et al. 1987); 670 g m−2 in dry heathland, Genisto-Callunetum (Aerts 1989); and 984 g m−2 in wet heathland (Aerts & Berendse 1989); root biomass production by M. caerulea in wet heathland was 1080 g m−2 year−1, equal to above-ground production (Aerts et al. 1989).
A detailed study of the seasonal pattern of production and nutrient uptake and release by M. caerulea was made in Molinietum in Bramshill Forest (Loach 1968b). The dry weight and nutrient content of the standing crop were measured on four occasions during the year. At its peak in August, the 5.4 t ha−1 of dry matter contained the following amounts of major nutrients (kg ha−1): N 68, P 3.4, K 56, Ca 4.9 and Mg 2.8. Much of the N, P and K content of the August crop was taken up early in the season, whilst Mg and Ca were absorbed more slowly. The basal storage organs (internodes) of the previous year made a major contribution to bud formation, but relatively little to early growth in the spring. New basal internodes were already well formed in August, and their nutrient content did not increase at the time of leaf fall in autumn. The tussock, however, was shown to contain a considerable reserve of nutrients, with the exception of K, which was readily leached from the plant material. Litter losses were high (3.9 t ha−1 dry matter), and represented a loss of about 80% of the nutrient content of the standing crop.
Subsequently, Morton (1977), using experimental covered and open plots in Molinietum in nearby Swinley Forest, Berkshire, determined the mechanism of the nutrient loss from the M. caerulea sward in autumn and early winter. About 75% of the N and P in the leaves was withdrawn before abscission, and about 90% of the K, Ca and Mg was returned to the soil by leaching from the leaves. Molinia caerulea therefore appears to have a pattern of internal cycling of nutrients analogous to those of deciduous trees.
The performance of M. caerulea in the Molinietum, compared with the valley-bog site in Bramshill Forest, was assessed by Sheikh (1969b); mean values derived from 50 single tiller samples in each site were: total leaf area 25.9 and 16.9 cm2, total dry weight of basal internode 6.6 and 5.8 mg, and total dry weight of buds 0.86 and 0.62 mg, respectively, all significantly different at P = 0.05.
(c) effect of frost, drought etc.
Particularly in upland areas (Jefferies 1915), the flat thin leaves of M. caerulea which expand early in the year suffer from exposure in cold spring weather. Plants show a prevailing orange and yellow coloration, but as summer advances the yellows give place to a richer green, although the leaf-tips are withered and dead. This phenomenon occurs also in the much hardier leaves of other moorland grasses such as Nardus stricta, but in M. caerulea the withering involves a length of leaf that is quite exceptional and which is succeeded, passing down the blade, by a zone in which the fading colours indicate the inability of the leaf to withstand the severities of the habitat of this plant, giving the appearance of autumn tints all the year round.
VI. Structure and physiology
Molinia caerulea has a condensed rhizome but it can extend vertically and lift new daughter tillers between 2 and 10 cm above surrounding tiller levels, especially in heavily crowded areas of tussock. It has an enormous root system that forms a dense tangle at the top and penetrates to a great depth (> 80 cm). The roots are of two types, both covered by root-hairs: strongly twisted cord-roots about 1.5 mm in diameter, 15–45 cm long; and fibrous roots 0.3–0.8 mm in diameter, 5–13 cm long, branching freely in all directions (Jefferies 1915, 1916).
Proffitt (1985) has described in detail the growth and development of the root system. It remains active for three seasons, and lateral root growth occurs in the second and third seasons. Root initiation corresponds with an increase in soil temperature to 10 °C and total root length increases until there is a rapid decline in growth at the time of leaf abscission, in response to a loss of assimilate production. This period of active growth can be divided into two parts. Firstly, branching growth which occurs early in the year, pre-anthesis, directly involves the production of primary lateral branches from the distal portion of the axis roots of the previous season’s vegetative tillers, and in the third year the production of secondary lateral roots developed from the distal portion of the primary laterals. Both these primary and secondary lateral roots, which are produced at the beginning of the season, aid the development of the new reproductive and vegetative tillers. Secondly, extension growth, which occurs mainly after anthesis, involves the production of unbranched axis roots by the newly formed reproductive tillers and the vegetative tillers which extend from their bases. Such axis roots will overwinter and branch in the following spring. There is particular ecological significance in the longevity of this root system, because the second and third year old roots assume the role played by early new root growth seen in other grass species; thus carbohydrates stored in the basal internode may be released for tiller growth alone.
Roots of M. caerulea are confined to soil pores exceeding 150 µm, with a maximum in the 300–600 µm pore diameter class, although the diameters of the roots are much smaller than those of the pores in which they are found. These pores may be air-filled or water-filled at particular soil water tensions (Sheikh & Rutter 1969).
The root system of M. caerulea responds to waterlogged soils by a change in orientation in poorly aerated or anaerobic soil horizons (Armstrong & Boatman 1967); the adventitious roots become orientated into a horizontal growth pattern along a tolerable redox plane, and in this way they avoid the deeper, more intensely reducing and hostile horizons. Under these circumstances the lateral roots of this species invariably grow upwards towards more oxidizing conditions. In root systems of M. caerulea exposed to a fluctuating water table, a ‘shaving brush’ effect is developed as a survival mechanism against the extremes of anaerobiosis. The root apices are killed in the rising water table but the root bases remain healthy and new laterals rapidly grow from these and adjust to the new level. If the water table oscillates frequently, the cycles of death and regrowth produce a mass of brush-like roots. One of the possible ways in which M. caerulea achieves a degree of tolerance to soil anaerobiosis is by radial oxygen loss from roots, the rate of which has been determined by Armstrong (1967a) as 14 ng cm−2 min−1 (in the apical centimetre of root of length 6–10 cm), compared with 128 ng cm−2 min−1 in the wetland species Eriophorum angustifolium. Protection by radial oxygen loss implies the formation of an oxygenated zone in the rhizosphere which forms a buffer between the cells of the root and the hostile environment. The contribution of radial oxygen loss to the exclusion of soil-borne toxins by the oxidizing activity of M. caerulea roots is, however, much less than enzyme-mediated oxidation within the root and at its surface which might account for up to 90% of the total (Armstrong 1967b).
The root system of M. caerulea also responds to waterlogged soils by means of aerenchyma in the cord-roots which arises by the tearing and dissolution of cortical cells rather than by their separation. Webster (1962a) has shown that this aerenchyma contains between 15% and 20% oxygen in the gaseous state, and it may therefore provide an important diffusion path to root apices in water-saturated soil.
Tillers bearing basal buds which are able to act as overwintering organs are those that have not flowered in the previous year; the apices of these flowerless, overwintering tillers had not fully entered into a reproductive phase of growth before the previous year’s climatic conditions restricted growth and initiated leaf abscission. When a season is experimentally extended, the flowerless, overwintering tillers will produce buds which flower by the autumn; there is no innate reason for them not to, only the approach of bad weather (Proffitt 1985). The number of new buds on an overwintering basal internode is usually between 2 and 4 per tiller. Normally the rhizome remains no more than a cellular connection between two basal internodes. The following season’s shoots represent the development of terminal buds of short rhizomes and growth is continued sympodially from the axillary buds. Only after leaf lamina expansion is complete, and not before tiller initiation as in many other grasses, when a tiller is no longer dependent on the stored reserves of the parental internode, do the new axis roots emerge. In consequence the roots appear a considerable time after the growth of their complementary buds. Assimilates are stored in the form of starch as a reserve for the following season. In autumn the nitrogen, phosphorus and potassium concentrations in the basal internodes and nitrogen and phosphorus in the roots are increased, but especially phosphorus.
Stomata occur on both sides of the leaves; they are c. 29 µm in length × 17 µm wide with a pore c. 15 µm in length × 1.5 µm wide. Those on the upper side are present in the furrows between the 11–15 ridges on either side of the midrib; on the under surface they are in rows almost opposite to those on the inner surface (Jefferies 1916; Farragher 1974). Stomatal frequency has been recorded as 250 mm−2 on the upper surface and 25 mm−2 on the under surface (W.J. Davies, personal communication).
The cells in the piliferous layer of the cord-roots frequently show the presence of mycorrhiza, the mycelium being visible within these cells and on their surface and occasionally penetrating to the next row of supporting cells (Jefferies 1916). Rooted tillers of M. caerulea examined by Eason et al. (1991) had 30.4 ± 3.4% root length colonized by the vesicular-arbuscular (VA) endophyte. Roots collected in April 1999 from Gisburn Forest, north Lancashire, showed about 15–20% infection by VA mycorrhizal fungi. Vesicles, arbuscules and fairly coarse VA hyphae were present, mostly confined to the finer roots and the outermost layers of the cortex (R. Francis, personal communication). Harley & Harley (1987) record reports of VA mycorrhiza in M. caerulea from continental Europe.
(c) perennation: reproduction
A semirosette hemicryptophyte. Reproduction is both by vegetative spread and sexual reproduction, weighted towards the former strategy. Molinia caerulea regenerates in situ through lateral vegetative spread, particularly in habitats favouring the non-tufted growth form. It produces much small seed, which is wind dispersed, germinates in the spring and readily colonizes bare ground, yet establishment is sparse. A persistent seed bank has been recorded (Chippendale & Milton 1934; Proffitt 1985).
Molinia caerulea ssp. caerulea is tetraploid (2n = 36) and ssp. arundinacea can be diploid (2n = 18) (Sell & Murrell 1996) or decaploid (2n = 90) (Fl. Eur. 5; Stace 1997). The 2C nuclear DNA content of the tetraploid (2n = 36) has been recorded as 4.9 pg per nucleus by Grime et al. (1988).
(e) physiological data
The effect of wind on plants derived from tillers of M. caerulea collected from seven sites in West Wales, from both exposed and sheltered habitats, has been assessed experimentally by Pitcairn & Grace (1984). Plants were raised in standard conditions in a peat and sand mixture adjusted with limestone to pH 4.0. Replicates from each collection were grown in a controlled environment room, and in a matched environment of a wind tunnel. These environments were very similar except for wind speed, which was < 0.5 m s−1 in the controlled environment room whilst that in the wind tunnel was 7.4 m s−1, with a turbulent flow. The length of the fourth leaf of the first tiller to develop on the planted tiller was measured every 2 d until its extension was complete. To facilitate comparisons, a logistic function was fitted to the raw data from each leaf. The pattern of growth of the fourth leaf varied with provenance. In four of the seven cases, wind led to reduced final leaf length ranging from 11% to 19%. Material from two sites at high altitude with no natural shelter were among the most wind resistant. There was, surprisingly, the tendency for an increase in the rate of tillering, caused by the high wind treatment, evident in five out of the seven provenances. On the basis of the results for leaf growth and tillering, two provenances were selected for further study, as ‘wind-resistant’ and ‘wind-susceptible’. The susceptible provenance showed the lowest water potentials in the wind treatment (−2.8 MPa), but this was not because it had a more easily damaged leaf surface than the resistant provenance. The essential difference between the two provenances appears to be in the hydraulic pathway from the soil to the leaf surface.
In short-term (24 h) laboratory experiments, Webster (1962b) determined the effects of CO2 and H2S on root and shoot extension in M. caerulea tillers. Plants were grown in a 3 : 1 loam/Sphagnum peat mixture in gas jars, and the rates of extension of the roots, which grew down the inner surfaces of the thick dark paper-wrapped jars, were measured before and after passing gas mixtures containing various concentrations of CO2 and H2S through the soil. The soil was moistened with distilled water and was not waterlogged. Root and shoot extension were significantly reduced by concentrations of both gases in air in the same range as those found under summer field conditions dissolved in the ground-water of the wet-heaths studied previously by Webster (1962a). In a further experiment Webster (1962b) compared the growth of M. caerulea tillers planted into vertical unglazed open-ended pipes, under acid conditions in stagnant and flowing water and at two water-table levels. Water samples were withdrawn at regular intervals from a probe placed in one planted pipe and in an empty pipe in each treatment, and analysed for CO2, H2S and O2 (Webster 1962a). By the end of the second growing season the growth of M. caerulea was significantly reduced in stagnant (CO2 0.7 g L−1) as compared with the flowing (CO2 0.42 g L−1) ground-water, oxygen being absent or deficient (0.3 g L−1) throughout. The concentrations of these gases were within the range of values measured in the field communities. No H2S was found in either of the tanks. With the water-table 15 cm below the surface, the mean (n = 30) number of tillers per cm2 with moving ground-water was 54.5 and with stagnant ground-water it was 25.6 (significant difference 16.7; P = 0.05), and the mean shoot dry weights were 40.0 g and 23.7 g, respectively (significant difference 4.9; P = 0.05). When the water-table was at the surface, the tiller numbers and dry weights more than halved but were still significantly greater in the moving ground-water treatment.
Studies investigating the spring movement of nutrients from stores to growing tissues in M. caerulea have been based upon changes in nutrient concentration (Torvell et al. 1988) or contents (Loach 1968b; Morton 1977) of various plant parts [see V(B)]. A more recent study by Thornton & Millard (1993) has used 15N as a tracer in an experiment using pots of expanded mica, allowing the remobilization of N from M. caerulea roots and basal internode to the shoot to be discriminated from current N root uptake and transport to the shoot. Pots were protected from rain but were otherwise exposed to ambient conditions. Plants were grown for two seasons in a factorial experimental design which involved two levels of N supply and two defoliation levels arranged in a randomized block structure. The potted plants received a complete nutrient solution containing ammonium nitrate at either 0.1 mol m−3 (low N) or 5 mol m−3 (high N), at a rate of 200 cm3 of the appropriate solution weekly, in three equal aliquots. All N supplied throughout the first season was enriched with 15N to c. 5 atom percentage and at natural abundance in the second season. Plants were either undefoliated or had all leaf material removed by clipping on two occasions each season, in July and August (after at least three leaves of all plants were fully expanded) and a second clipping in the following year in May and June, respectively, after visible regrowth had occurred. The increased N supply in the previous season caused significant increases (P < 0.01) in the dry weight, total N content and labelled N content in the newly formed overwintering basal internodes with attached buds and root material. In contrast, defoliation in the first year caused a highly significant decrease (P < 0.001) in the same structures. In the second season there was no significant change in the total N content of whole plants between the end of January and early July for low N plants and up to the end of May for high N plants. However, over the same time intervals the dry weight of the new shoot material increased significantly (P < 0.01): by 0.21 g per plant for low N undefoliated, 0.12 g for low N defoliated plants, 0.91 g for high N undefoliated plants, and 0.80 g for high N defoliated plants. It appears therefore that the initial shoot growth of the plants relied solely upon internal cycling of N. Defoliation reduced the N content of the total plant (P < 0.05) and of the roots and basal internodes (P < 0.001), while it increased the N content of new shoot material (P < 0.005). There were no significant differences in the labelled N content of whole plants when harvested on different dates. Hence, there was no significant net loss of N from the plant roots into the growing medium during the second year of the experiment. A reduction in the labelled content of the overwintered tissue during the following spring was the result of remobilization from both the root and basal internodes. Roots had larger overwintering stores, and showed a greater reduction in their labelled N content throughout the second season, than did basal internodes although reductions were proportionally larger in basal internodes; for example the reduction in labelled N content of roots for high N undefoliated plants was from 95.9 mg (± 13.0) in January to 69.7 mg (± 10.3) per plant in September, and for basal internodes in the same treatment 27.5 mg (± 3.8) reduced to 5.4 mg (± 1.7) per plant. In all treatments, the labelled N content of the new shoot material (leaves + flowers + new basal internodes) increased between the end of January and early June, with concomitant decreases in the labelled N content of the roots plus basal internodes (P < 0.05). Labelled N was therefore being remobilized from the overwintering tissue to support new shoot growth at this time. Since the remobilization mainly occurred before the onset of root N uptake, internal cycling was important for the earliest period of shoot growth, especially the leaves. An increased N supply increased the amount of N remobilized to new shoot growth; however, the proportion of N remobilized from overwintering stores was independent of N supply. Defoliation increased the amount of N remobilized from the roots, and had no effect on the 15N content of basal internodes of plants receiving a low supply of N. Remobilization from leaves supplied flowers in plants receiving a low N supply and both flowers and new basal internodes in plants receiving a higher N supply.
Molinia caerulea, together with six other heathland species, was tested by Troelstra et al. (1995a) for its capacity to utilize NH4+or NO3−as its N source in nutrient culture solution. Seeds were germinated on glass beads and grown for 3–7 weeks, then subsequently raised in a complete nutrient solution (containing 2 mol m−3 NH4+at pH 5.0–5.5) in a glasshouse for several weeks. After precultivation, 60 plants of M. caerulea were selected for each N-treatment. In the continuously aerated and circulated culture solution (automatically adjusted to pH 4), nitrogen was supplied as 2 mol m−3 as NaNO3 or (NH4)2SO4 and a nitrification inhibitor (dicyandiamide, 99 mmol m−3) was added to both treatments. Molinia caerulea and all the other species used in the experiment showed similar growth responses with respect to N source. All seven heathland species, although being putatively adapted to a NH4+-based nutrition, assimilated nitrate-N almost equally well as ammonium-N; for M. caerulea the mean relative growth rates were 105 and 99, respectively, a difference just significant at P = 0.05. However, N source was significantly and consistently correlated with biomass partitioning, as NH4+-fed plants allocated more dry matter to shoots and less to roots when compared to NO3−-fed plants; in M. caerulea, the shoot weight ratio was 0.845 and 0.788, respectively, and the root weight ratio was 0.155 and 0.212, respectively (both differences highly significant at P = 0.001). However, in the Dutch acid heathland soils which support the species, NH4+ is predominant and only a minor part (< 30%) of the mineralized N is nitrified (Troelstra et al. 1990). In a further experiment (Troelstra et al. 1995b), the growth of M. caerulea and three other heathland species was tested in solution cultures at pH 4 with 2 mol m−3 N, supplied as varying proportions of NO3−plus NH4+up to a ratio of 40% nitrate. Exceptionally, compared with the other species, the partial replacement of NH4+by NO3−did not enhance the growth rate of M. caerulea when compared to the NH4+-only treatment in which it absorbed ammonium highly preferentially. Shoot nitrate reductase activity (NRA) was highest in M. caerulea and Deschampsia flexuosa, becoming induced to almost maximum level (4 µmol g−1 DW h−1) at the lowest proportion of NO3-N in the N supply (20%), whereas root NRA was highest in Calluna vulgaris (3 µmol g−1 DW h−1) at this level of supply.
At Malham Tarn, North Yorkshire, Cooper & Proctor (1998) found a high median NRA of 0.62 nkat g [FW]−1 in a range of fen plants, whereas plants on the ombrotrophic surface of the Tarn Moss generally showed little or no activity (median 0.03 nkat g [FW]−1). The highest activity on raised-bog peat was shown by M. caerulea on the rand at the north edge of Tarn Moss (0.11 nkat g [FW]−1).
(f) biochemical data
Using a 15N enrichment technique, Thornton & Bausenwein (2000) have demonstrated experimentally that relationships between N mobilized from roots and swollen basal internodes (to support shoot growth in M. caerulea in spring) and protease activity (ability to degrade azocasein) exist at several levels: between different tissues, temporally throughout the season and between individual clones.
The main features of the seasonal growth cycle of M. caerulea have been described by Jefferies (1915). Growth starts in April or May and is supported by recycled reserves from the roots and the previous season’s swollen, green, stem internodes. Energy (total water soluble carbohydrates) and mineral nutrient reserves stored in the basal internodes are thought to be important in supporting the growth of the new season’s tillers (Torvell et al. 1988). Experimental evidence provided by Thornton & Millard (1993) shows that nitrogen is remobilized from roots and basal internodes to support new shoots, especially leaf growth of M. caerulea in spring, roots supplying more N than basal internodes. The remobilization mainly occurs before the onset of root N uptake and therefore internal cycling is important for the earliest period of shoot growth [see VI (E)]. The basal internodes physically degenerate after a single season (Jefferies 1916; Loach 1968b). Most temperate grass species remain active throughout the winter, even slowly producing new leaves at temperatures just above freezing point. Exceptionally, in M. caerulea the onset of growth is delayed until the spring when solar radiation increases fourfold and mean air temperature increases up to 10 °C (Proffitt 1985). Tillers bearing two series of leaves develop from the basal buds of the previous season. At Crowthorne, Berkshire, the first or lower series of leaves, consisting altogether of five scale and four functional leaves, developed immediately above the rhizome and began to expand in April; the stem elongated during May and June and, in late June or early July, the second or upper series of three functional leaves opened at the tip of the single internode (Loach 1968b). In the Cleish Hills, Fife, and at Sourhope, in the Scottish Borders, the main period of leaf extension growth also occurs during June and early July (Torvell et al. 1988); however, a total of only four to five functional leaves per tiller are usually produced in a season, probably because of the lower temperatures and a shorter growing season at the higher latitude and altitude of the Scottish sites compared with the south of England (Grant et al. 1996). The internodes between the leaves of any one series never elongate, but a single internode of about 5 cm length (representing the stem elongation of May and June), separates the two series. The inflorescence can be dissected out from the ensheathing second series leaves in early July, and expands to open in late July or August. After anthesis, leaf initiation decreases and leaf senescence increases as the mean level of irradiance falls; by September the first series leaves are dead, and the rest gradually die back so that by November all the leaves are dead, and most drop off owing to the formation of abscission layers at the junction of sheath and lamina. Using scanning electron microscopy, Salim et al. (1988) have determined that the leaf abscission zone contains cells which have more than doubled their wall thickness to > 0.4 µm. The line of fracture associated with the zone principally follows the middle lamella, leaving intact cells on the fracture face.
Growth in the following year is continued from overwintering buds at the base of the internode, which develop in the axils of the first series leaves. Seed is set readily from August to October and germination takes place in the field in the following year. However, seedling establishment is sparse and occurs mostly in recently disturbed areas trampled by cattle, or along the edges of drainage channels.
VIII. Floral and seed characters
(a) floral biology
Reproduction is amphimictic and vivipary is unknown. All florets except the most apical are hermaphrodite and protandrous, wind-pollinated; > 100 in an open or dense panicle (1–4 per spikelet).
No hybrids are known.
(c) seed production and dispersal
The compact inflorescences are borne on stalks which generally remain standing during the winter, when winds are high and likely to disperse the seeds and carry them considerable distances; aiding wind dispersal, the palea remains attached to the seed and spreads out from it, while the seed itself is small (Jefferies 1915). A mature seed (germinule) measures only 1.9 × 0.8 mm with a mean air-dry mass of 0.53 mg (Grime et al. 1988).
(d) viability of seeds: germination
Molinia caerulea produces much seed which germinates directly and colonizes bare ground on moorland (Jefferies 1915). However, Grime et al. (1981) found that only an extremely low percentage (3%) of freshly collected seeds of M. caerulea from a heathland community germinated immediately, and similar low rates of germination (11%) in the laboratory at 20/15 °C with a visible radiation flux of 40 W m−2 (‘Warm-white’ fluorescent tubes + tungsten bulbs) over a 15-h day were observed after dry storage at room temperatures for up to a year subsequently. A large subsample of seed was chilled in moist sand at 5 °C for > 45 days to break dormancy and facilitate germination in laboratory experiments to determine the responses to temperature and light; seed germinated over a range from 19 °C to 39 °C (t50 2 days) on a temperature-gradient bar and germination was similar in the light regime described above or in the dark (50% and 40%) whereas at a reduced light level (97 µW cm−2) germination increased to 71%.
(e) seedling morphology
Germination is epigeal. The primary root and coleoptile emerge from the caryopsis at about the same time (Fig. 3). The coleoptile is 3–6 mm tall, somewhat loose and colourless. The first leaf emerges from the tip of the coleoptile, and shortly afterwards a secondary root emerges from near the base of the primary root. Muller (1978) also illustrates a young seedling of M. caerulea at the two leaf stage.
IX. Herbivory and disease
(a) animal feeders or parasites
Free-ranging ponies in the New Forest, Hampshire, show a marked seasonality in their use of vegetation in wet heaths, bogs and natural acid grasslands, matching the time of growth of M. caerulea in each community (Putman et al. 1987). The summer diet of the ponies is primarily of grasses (80–90%) of which M. caerulea contributes to 20% of the diet at this time.
At least 25 phytophagous insect and mite species have been identified on M. caerulea (Table 1). These herbivores include sap-suckers, leaf browsers, gall and mine producers; in particular, it is the principal food plant of the caterpillars of the Scotch argus (Erebia aethiops) and chequered skipper butterfly (Carterocephalus palaemon).
Table 1. Phytophagous insect and mite species recorded from Molinia caerulea. Where information is available, the part of the plant on which the species feeds is shown. Reference numbers follow each description
Sphaeriales. Clavicipitaceae. A form of Claviceps purpurea (Fr.) Tul. with small sclerotia, often regarded as a separate species, C. microcephala (Wallr.) Tul., infects Molinia caerulea, Phragmites communis and Nardus stricta with the development of purple curved sclerotia (ergots) in the place of healthy seed (Webster 1980).
Helotiales. Dermateaceae. Belonium hystrix (de Not.) Höhnel with black apothecia and greyish discs, and Hysteropezizella melatephroides (Rehm) Dennis with erumpent apothecia, rusty or blackish brown with pale yellow discs, are both quite common on dead stems of M. caerulea (Ellis & Ellis 1985).
Hysteriales. Hysteriaceae. Gloniella moliniae (de Not.) Sacc. with dark brown to black hysterothecia is found on thin dead stems of M. caerulea (Ellis & Ellis 1985).
Uredinales. Puccinia brunellarum-moliniae Cruchet and P. nemoralis Juel, both with small brown uredinia and black pulvinate telia, are similar rusts which occur on M. caerulea, with different aecial hosts, the former on Prunella vulgaris and the latter on Melampyrum pratense (Ellis & Ellis 1985).
Sphaeropsidales. Leptostromataceae. Actinothyrium graminis Kunze, with numerous black pycnothyria, is the most common and conspicuous fungus on dead leaves and stems of M. caerulea (Ellis & Ellis 1985).
Although at the present day M. caerulea is abundant or co-dominant on vast stretches of blanket bog in Britain, the humification of plant material in peat of this mire type is so severe that Molinia remains can relatively seldom be found. Occasionally, however, its clusters of tuberized stem-bases and sharply twisted adventitious cord-roots are recognized in bog stratigraphic studies (Godw. Hist.). A recently developed technique of plant macrofossil counting has enabled Chambers et al. (1999) to identify the distinctive epidermal tissues of M. caerulea in upland shallow, relatively highly humified peats developed in historical times.
Prehistorical records are concentrated in the late Flandrian when ombrogenous bogs were actively growing and conditions favoured quick incorporation and preservation of roots and tubers; it seems likely that Molinia occupied the surfaces of the high Pennines at the opening of zone VII; also on various raised bogs, especially on the rand and margins of drainage channels, in zones IV, VIIa, VIIb and VIII (Godw. Hist.).
In historical times, first recorded in 1666 on Mitcham Common, Surrey (First Rec.).
We are indebted to Dr D. Roy for supplying information from the Phytophagous Insects Data Bank, to Mrs J.M. Croft for providing the map from the Biological Records Centre, Dr R. Francis for examining root samples for VA mycorrhizal colonization, and Professor W.J. Davies for determining stomatal number.