M. J. Hayes, Institute of Grassland and Environmental Research, Plas Gogerddan, Aberystwyth, Ceredigion SY23 3EB, UK (fax 01970 823243; e-mail firstname.lastname@example.org).
1Grants for wildlife enhancement in the British Isles have supported the widespread planting of new hedges along field margins. However, much of the planted material, particularly of hawthorn Crataegus monogyna, has been obtained from the continental mainland of Europe. There is a need to assess the implications of this practice for hedgerow performance and for the conservation of indigenous genetic variation.
2One local ecotype and eight commercial provenances (four British and four continental European) of hawthorn were planted in an experimental hedge at both an exposed upland site and a sheltered lowland site. Sections of hedge were planted with or without fencing and with or without mulching in all combinations. Growth and thorniness were assessed over 3 years, and phenology and disease over 2 years.
3At both sites, the most locally obtained provenance had the latest bud-burst, exhibited the least severe symptoms of mildew and was the most thorny. It also showed the greatest height increment at the upland site, but was relatively slow-growing at the lowland site.
4An imported Hungarian provenance had early bud-burst, showed a high growth rate and suffered the most severe mildew. A commercially obtained British native provenance was aberrant in its extremely early bud-burst and other attributes comparable with the Hungarian provenance, indicating the possibility of misidentification at some stage of production or supply.
5In the absence of fencing, at the upland site hawthorn mortality was 100% compared with only 3% at the lowland site. In fenced plots there was c. 320% greater growth when mulching was used.
6The results suggest that for greater establishment success and hence cost benefits in hedge planting, as well as for greater environmental benefits, there should be closer matching of hawthorn provenance to the planting site. The use of commercial material has demonstrated that locally provenanced material can be superior to any commercially available material, and that the current state of the commercial sector is insufficient to enforce the necessary controls over provenance of material used for hedge renovation.
The British landscape consists of a unique tapestry of fields and bordering hedges, a legacy of centuries of mixed farming practices (Pollard, Hooper & Moore 1974; Rackham 1986). Hedges (linear features of woody species) were planted historically to delineate field boundaries and provide a barrier to grazing livestock. However, as a result of agricultural intensification during recent decades with high animal stocking rates and concomitant increases in hedge management costs, many hedges have been neglected or gradually replaced by more convenient post and wire fencing.
Deliberately planted and managed hedges of woody species are rare on a world-wide scale. Outside the UK, they are frequent only in Celtic areas of Europe, e.g. Brittany (Burel & Baudry 1989) and Ireland (Hegarty & Mcadam 1994), and in parts of the Caribbean, e.g. Linderos of the Dominican Republic. In other areas, hedgerows may result from spontaneous colonization of shrubs and trees along fence boundaries, e.g. New Jersey, USA (Forman & Godron 1986).
In the British Isles, the value of hedges and associated field margins as havens for wildlife, and for their contribution to the British scenery, has been increasingly recognized (Wratten 1988; Cummins et al. 1992; Bunce et al. 1994; Macdonald & Johnston 1995). National agricultural and wildlife conservation bodies (e.g. Ministry of Agriculture, Fisheries and Food; English Nature; Countryside Council for Wales) award grants, via a number of agri-environment schemes, to encourage the establishment of new hedges together with the replanting and maintenance of older neglected hedges. Such schemes require a readily available supply of suitable material, and many plant nurseries have specialized in the production of hedging species. However, there has been little scientific work on the relative attributes of different species or of different cultural treatments for hedge planting in terms of establishment success, growth and their potential benefits to wildlife.
Hawthorn Crataegus monogyna Jacq. (authority after Stace 1997), above all species, with its long history of use (Loudon 1822), has commendable properties for hedging. It is readily managed to form a dense hedge by layering and coppicing, is thorny and hence stock-proof, and is hardy, although intolerant to shade and poor drainage (Brooks 1980). Hawthorn foliage acts as a host for a wide range of insect (Kennedy & Southwood 1984) and bird (Osborne 1984) species, and its berries also provide a valuable winter food-source for birds (Lee, Grubb & Bastow Wilson 1991). Although hawthorn is planted widely to encourage wildlife, it has, along with many other widespread species, been somewhat overlooked in terms of its own conservation biology, i.e. how to retain genetic variation and safeguard the geographical pattern of regional forms and ecotypes. Hawthorn exhibits a large degree of morphological variation across mainland Europe (Do Amaral Franco 1968), with several subspecies and growth forms (Good, Bryant & Carlill 1990). It is important to conserve this pattern, especially as the taxonomy of the several closely related and interfertile species has yet to be fully established (D. Gostynska, personal communication; Holub 1992).
Approximately 80% of the hawthorn material obtained from the UK horticultural trade in 1997 was of continental European origin, a practice motivated by supply and cost as seed can be cheaper and more readily available there (A. G. Gordon, personal communication; Forestart Ltd, Hadnall, Shrewsbury, UK, tree seed suppliers). Dunball (1970) also reported the widespread importation of continental hawthorn material to the UK. In a preliminary upland trial in Wales, Jones & Evans (1994) demonstrated apparent differences in growth and morphology between such imported European material and locally collected material. This is an increasingly problematic aspect to the expansion of international trade, but is long standing in that components of biota that also represent commodities are traded across biotic boundaries (DoE 1995). The same process has also been reported in the use of seed of wildflower species in the restoration of diverse grasslands (Akeroyd 1994; Jones & Hayes 1999). In terms of infiltration of alien genetic material into native British wildlife, such movement of exotic provenances for hedging and amenity tree planting (commercial forestry is mainly reliant on exotic species) has probably only been paralleled by freshwater fish-stocking (Maitland 1987; National Rivers Authority 1993). In addition, there is always the danger of importing alien species, and hedges of imported hawthorn material planted along road-verges have often been found to exhibit the characteristics of the hybrid Crataegus × media Bechst. (i.e. C. monogyna×C. laevigata (Poir.) DC.) in areas where one parent (C. laevigata) is completely absent (A. T. Jones, personal observation).
Jones & Evans (1994) concluded that there was a need to compare a wider range of provenances of both continental and British hawthorn, in order to explore variation across western Europe and measure their responses under a range of planting situations. This study compared hawthorn establishment and growth performance over 3 years in newly planted experimental hedges at contrasting altitudes. Benefits of two potential cultural measures to aid hawthorn establishment (mulching and stock-proof protective fencing) were also evaluated. The implications of continued importation of alien material were considered in terms of conservation of indigenous genetic variation in hawthorn and of the potential effects on associated wildlife.
Materials and methods
Experimental site and design
Hawthorn plants of nine provenances were obtained (Table 1). Provenance 1 was collected in 1992 as seed from an upland population (OS grid reference SH 746713) at 350 m a.s.l. Plants were propagated in a nursery at the Institute of Grassland and Environmental Research (IGER, Ceredigion, UK) to produce plants 40–60 cm tall. The remaining eight provenances were obtained commercially as bare-rooted plants in size category 40–60 cm.
Table 1. Source and supplier of Crataegus monogyna provenances used. For the commercially sourced material, latitude and longitude are only approximate and are not guaranteed by the supplier
Institute of Grassland and Environmental Research, Aberystwyth, UK
Higher Heath Forest Nurseries, Whitchurch, UK
Forest of Dean, England
Woodland Improvement and Conservation Ltd, Huntley, UK
Elmcroft Growers, Newent, UK
East Lothian, Scotland
Alba Trees Plc, East Lothian, UK
Dingle Nurseries Welshpool, UK
Maelor Nurseries Ltd, Whitchurch, UK
Alba Trees Plc. East Lothian, UK
Elmcroft Growers, Newent, UK
In February 1995 an experiment was set up at ADAS Pwllpeiran Research Centre in mid-Wales (52°21′15″ N, 3°49′15″ W), at both a lowland and an upland site (a sheltered valley bottom at 150 m a.s.l. and an exposed site at 375 m, respectively). Soils at both sites consisted of typical brown podzolic soils of the Manod series. At each site an experimental hedge was planted, consisting along its length of three replicate blocks, with each block divided into four 10-m strips. Each strip received a different establishment treatment. The establishment treatments consisted of a 2 × 2 factorial design of + or − fencing and + or − mulching. Fenced strips were fenced on both sides with stock-proof wire mesh to prevent any grazing. Mulching was applied by laying down a woven polythene mulch mat (Mypex geomembrane fabric; Radnor Garden Supplies Ltd, Llandrindod Wells, UK) before planting, with slits cut for planting. Within each strip, nine bare-rooted plants of each of the nine provenances were planted in January 1995 in random order as a staggered (zig-zag) double row, at a spacing of 25 cm between plants within rows and between rows, giving a total of 1944 plants for the whole experiment.
At the upper site, pastures adjacent to the hedge were grazed throughout the growing season at a stocking rate of 8·8 ewes ha−1. At the lower site, adjacent swards were grazed with 13·2 ewes ha−1 during the spring, by 66 lambs ha−1 in the autumn and winter, and were left ungrazed for silage production from May to July.
Measurements of hawthorn morphology and size (Table 2) were taken just after planting in February 1995 and again in February 1996 and 1997. Incidence of powdery mildew Podosphaera clandestina Lev. (Khairi & Preece 1978) was assessed by scoring individual plants for degree of infection on two occasions during 1995 (July and August) and on three occasions during 1996 (July and early and late August). The date of initial bud-burst was recorded for individual plants during late-winter/spring of 1995 and 1996, defined as the date on which green foliage was first visible showing from the buds and recorded as the number of days from planting. All plants were checked for evidence of leaf bud-burst at a minimum of weekly intervals.
Table 2. List of morphological characteristics of Crataegus monogyna together with growth and disease attributes assessed
Height of plant (cm), measured from ground to plant apex
Number of branches growing from the main stem
Stem diameter (cm), measured 1 cm from base of main stem
Score of thorniness where:
0 = no thorns
1 = 1–10 thorns on whole plant
2 = thorns covering up to half the plant
3 = 5–10 thorns on each branch
4 = thorns every 2–3 cm along every branch
5 = thorns at least every 1 cm along branches
Score of mildew incidence where:
1 = no disease
2 = < three mature leaves with infection
3 = mature leaves with or without infection but with at least two immature leaves and stem apex infected
4 = as for 3 but with > 1 cm of stem infected
Data were analysed as a randomized-block split-plot design in which the establishment treatments were the main plots. Analyses of variance were constructed using a general linear model, assuming a Poisson error distribution for data on numbers of branches, and normal error distributions for the other characters. With Poisson errors the model is log-linear, so that statistical interactions test deviations from multiplicative effects on the number of branches. To allow for over- or under-dispersion, statistical tests with Poisson errors were based on residual deviance as estimated from the data rather than using its theoretical value of 1.
Type I sums of squares were used for the error structure (plants within plots within blocks within sites), and type II for the treatment effects (provenance, site, fencing, mulching and their interactions) estimated within each error level. All analyses were done using Genstat version 5.4.1 (Genstat 1997). Repeated measures were analysed using the method of Rowell & Walters (1976) as follows. For each plant, means were calculated over all recording times and used for analysis of variation in the mean expression of the character. For each plant, differences between repeated measurements were calculated and used for an analysis of interactions with time; for measures of plant size this provides an analysis of variation in growth rate.
High plant mortality in unfenced plots at the upland site rendered the design non-orthogonal by the end of the second year. Therefore two selections of data were analysed separately. All characters in all treatments were analysed over the first two sampling dates, with annual growth being calculated by subtraction of the first from the second measurement. Height, stem diameter and thorniness were also analysed using data from all 3 years only for the fenced plots. Mean annual growth rates over all 3 years were calculated as (third measurement – first measurement)/2.
Effect of experimental site
Plants at the lowland site grew faster, showing earlier bud-burst and greater mean annual increments in height and stem diameter, and by 1996 they had more branches (Table 3) than plants at the upland site. They also experienced more severe mildew symptoms than in the upland site during the first year. Bud-burst was later in the second year than the first, especially at the lowland site where the difference between years was 15 days compared with 6 at the upland site.
Table 3. Overall effects of planting site on the development of hawthorn. Values are means over all plants from nine provenances under all establishment treatments, and are presented only for those variates that differ significantly between sites. * , ** , *** F-test significant at P = 0·05, 0·01, 0·001, respectively
Both establishment treatments improved growth (plant height increment, stem diameter increment and branch number), thorniness and establishment at both sites (Table 4). Mulching had a larger effect on growth at the lowland than at the upland site, whereas fencing was more beneficial at the upland site. Unfenced plants at the upland site decreased in height during the first 2 years, and by the end of 1996 had all died. In contrast, mortality was only 4% in the fenced plots at the upland site, and 3% and 1%, respectively, in the unfenced and fenced plots at the lowland site. Grazing in unfenced plots at the upland site was sufficiently severe to prevent estimation of bud-burst date for many plants in 1996, especially for the mulched treatment; this is reflected in the high standard error for that treatment in Table 4.
Table 4. Effects of establishment treatment and planting site on hawthorn grown at an upland and a lowland site. Values are means over nine provenances of hawthorn, and are presented only for those variates that differ significantly between sites and establishment treatments (†fenced treatment only). * , ** , *** F-test significant at P = 0·05, 0·01, 0·001, respectively
Provenance effects averaged over sites and treatments
There were large differences between provenances for all characters measured (Fig. 1). Bud-burst was latest in the local provenance, which on average burst buds 5 weeks later than the earliest provenance (Fig. 1a). The local provenance was also the only provenance to burst buds earlier in 1996 than 1995 (Fig. 1b) and was the only provenance with heavily thorned plants. Apart from the Durham and German provenances, the alien provenances had very few thorns (Fig. 1c). The local provenance also showed the least severe symptoms of mildew (Fig. 1d) and the slowest rate of increase in size (Fig. 1e,f) but the third highest rate of branching (Fig. 1g,h).
The Hungarian and Forest of Dean provenances were similar to each other and generally most unlike the local provenance, showing early bud-burst, few thorns, severe mildew infection, and rapid growth rates. The East Lothian and Danish provenances were similar to each other and were intermediate between the local and Hungarian types.
Differences between provenances in response to site and establishment treatments
The provenances differed significantly in their response to planting site and to the establishment treatments, as illustrated in Figs 2–4 for the variates where the differences are most significant. In terms of planting site and fencing, the local provenance was consistently best under harsh conditions and least responsive to favourable conditions for growth. Its bud-burst was 8 days earlier at the lowland than at the upland site, compared with an average of 14 days earlier for the non-local provenances (Fig. 2a). The local provenance showed the highest mean annual increase in height at the upland site but the lowest for the lowland site (Fig. 2b). It showed the lowest loss of height in unfenced treatments during the first 2 years (Fig. 3a). In response to fencing, its annual height increment increased by 12 cm and its number of branches by 60%, compared with averages of 20 cm and 230%, respectively, for the non-local provenances (Fig. 3a,b).
Mulching evoked different patterns of response, in which the local provenance was more, rather than less, responsive to favourable conditions. Its mean annual increase in height was 12 cm greater with than without mulching (Fig. 4a). No other provenance was significantly more responsive and five provenances were significantly less responsive. On average, mulching increased the mean annual height increment by 8 cm. Similarly, mulching increased the rate of branching in the local provenance by 60%, compared with an average of 30% for the non-local provenances, and none of the non-local provenances was significantly more responsive than the local provenance.
Only a few higher order statistical interactions were significant, the most notable being the 3-way interaction between provenance, fencing and planting site. As already shown in Table 4, fencing was more beneficial at the upland than at the lower site. This greater benefit at the upland site was most pronounced in the non-local provenances. For example, for non-local provenances the increase in branching caused by fencing at the upland site was, on average, 3·5 times greater than that caused by fencing at the lowland site. For the local provenance, the corresponding ratio was 2.
The results highlight the dramatic superiority of the hawthorn of most local provenance, an upland ecotype, in the upland site, in comparison with eight non-local commercially available provenances. It was clearly superior agriculturally. It showed better growth under harsh conditions, had a more stock-proof growth form with more tightly packed branches and dense thorns, was less dependent on fencing for good growth, and was relatively resistant to mildew. Its late bud-burst is likely to be associated with better tolerance of cold in late springs.
The local provenance is probably also superior environmentally. For example, its tightly packed branches and dense thorns will provide greater protection for small nesting birds. In addition, the emergence from hibernation of many Lepidoptera species may be timed to coincide with bud-burst of their food plants, for foraging by larvae on new leaves and the avoidance of secondary chemicals built up after the hardening of leaves (Edwards & Wratten 1985). The non-local provenances burst buds up to 5 weeks before the local provenance, and their use for hedge planting is likely to disrupt the plant–Lepidoptera–bird food chain in a way that parallels that resulting from global warming (Buse et al. 1999). The timing and incidence of flowering and fruiting, and the chemical composition of leaves, also have implications for wildlife. For example, Leather et al. (1997) found differences in the relative growth rates of larvae of the pine beauty moth Panolis flammea between provenances of lodgepole pine Pinus contorta. These factors could not be investigated within the time-scale of this study.
The results add to the increasing body of literature demonstrating the ecological importance of local genetic differentiation in a wide range of plant species (Linhart & Grant 1996). Similar findings have been published for native and continental provenances of other British woody species, including Pinus sylvestris Scots pine (Worrell 1992; Perks & Ennos 1999).
More specifically in relation to hawthorn, the results confirm the findings of Jones & Evans (1994), also at an upland site, that a hawthorn of local provenance was superior to a continental hawthorn. These results go further, demonstrating a wide range of responses of both native and continental hawthorn to the different sites and treatments, and demonstrating that the local provenance is superior to a broad range of continental and indeed native British material, in fact to every provenance included in the experiment.
There were large differences among the five native British as well as among the four continental provenances. Some of them appeared visually distinctive, indicating that biometrical analysis of a range of characters might provide a basis for discriminating between provenances. However, there was no clear distinction between native and continental forms, so that separating material from the two regions on morphological criteria alone may be problematic. Three of the continental provenances originated from northern European areas with late springs and relatively cool summers similar to the UK and would therefore be expected to behave like the British material.
The results suggest that for greater establishment success and hence cost benefits in hedge planting, as well as for greater environmental benefits, there should be closer matching of hawthorn provenance to the planting site. It is not sufficient merely to advocate the use of ‘native species/material’, as is commonly stated in guidelines for hedgerow replanting schemes (e.g. Welsh Office Agriculture Department 1995; Countryside Council for Wales 1995). Rather, the use of locally collected material should be made a requirement.
Reviews of forestry research suggest that this should be a common precaution for the planting of all species where conservation is the main objective (Ennos et al. 2000). It is of particular concern for long-lived features such as managed hedgerows and woodland. However, such measures will require native sources of seed to be identified and their use in planting schemes to be administered under a sophisticated source certification scheme. An example of such a regulated scheme exists in France (Règlement Forestier administered by the research institute CEMAGREF; Cemagref 1995), where material planted for different tree species in specific localities must have been produced from seed collected from the same locality.
Together with regulation, it is important that sufficient supplies of local material are available for planting, and it is important that conservation organizations are able to accredit locally native stands and supply site information to the horticulture industry for seed collection purposes. A voluntary code of practice (Flora Locale 2000) may encourage nurseries to grow natives if they see a financial benefit in becoming an accredited supplier. Alongside this there could be ‘genetic fingerprinting’ of hawthorn provenances to aid in the identification of origin, as for example in work by Scheepers, Eloy & Briquet (1997) on clone verification in Picea abies Norway spruce. If successful, and in combination with morphological attributes, this would enable the identification and rejection of non-local material in line with planting requirements for grant aid and the policing of certification schemes.
As with many common species, hawthorn has been somewhat overlooked in terms of its conservation biology (i.e. how to retain patterns of regional forms and ecotypes) but is planted widely to support other wildlife. Conservation efforts focus primarily on rare plant species, for example through individual Species Action Plans (DoE 1995), but even for such species there may be a lack of emphasis on genetic variation (Kay & John 1993; Kay 1993). Possible dangers include out-breeding depression when flowering time coincides between indigenous provenances and planted non-natives such that hybridization takes place (Keller, Kollmann & Edwards 2000). We suggest future refinement of gene conservation would be genetic screening and strategic conservation of common plant species (which may be endangered at a subspecific level because of local extinction, erosion of geographical pattern in variation or hybridization with other species or subspecies), particularly in the operation of national ecological restoration programmes such as with hawthorn in grant-aided hedge planting and renovation schemes. Genetic variation in common, as well as rare, species may contain a valuable record of evolution in the British Isles and elsewhere that can also be measured in terms of local adaptation (Worrell 1992; Linhart & Grant 1996) and that holds the potential for continued evolution.
Apart from the local provenance, this study used only commercially available material. There is little quality control over data on the provenance of commercial hawthorn. Precise details of their original collection sites were not available, and it was not always possible to guarantee the accuracy of the data that were available. For example, one British provenance from the Forest of Dean was aberrant compared both with other British provenances and with northern continental provenances. It had very similar characteristics to the Hungarian provenance, and may have been incorrectly sourced. Therefore, care must be exercised in interpreting the results as a pure ecological study analysing the natural distribution of ecotypic diversity within C. monogyna. On the other hand, the same feature adds essential extra value as an applied ecological study on hedge renovation. Only by using commercial material has it been possible to reach two vital conclusions: locally provenanced material can be superior to any commercially available material; and the current state of the commercial sector is insufficient to enforce the necessary controls over provenance of material used for hedge renovation.
In terms of the practical requirements of hedge planting, fencing of establishing hawthorn plants was shown to be essential in the upland site where animals grazed the hedge. In the second year the local provenance retained the most branched structure in the face of heavy grazing at the upland site. There was no evidence that even the thorniest plants could survive the high levels of grazing at this site, possibly because grazing occurred before the secondary thickening of the new growth. However, with lower grazing pressures, the local provenance may be able to survive in the absence of fencing, particularly following the secondary thickening of growth. In contrast, in the lowlands there was some browsing in unfenced plots but little mortality, and conditions were suitable for greater growth recovery. Hedges here showed the potential to survive unfenced, but not necessarily to be stock-proof. The addition of mulch during establishment was highly cost effective, giving c. 320% greater height increment for an increase of approximately 50% in additional material and planting costs, and without the extra costs involved in weeding, confirming the results from work in a similar situation by Wildig, Griffiths & Milsom (1994).
In conclusion, planting locally native hawthorn provenances may have potential for increasing efficiency of establishment and performance of hedges, especially in more extreme climates or under high levels of grazing. Such material may also be better adapted for disease avoidance and have potential wildlife benefits in the longer term. Even without these practical advantages, planting of local native material should be strongly urged on gene conservation grounds alone.
We would like to express our appreciation and thanks to L. Powell and J. Wildig of ADAS Pwllpeiran Research Centre for their collaboration on managing the experimental hedge sites and support over the years. We thank S. Hughes and S. Jones for technical assistance. We would also like to acknowledge the Ministry of Agriculture, Fisheries and Food for financial support for this work. We thank A.R. Watkinson and anonymous referees for valuable comments on the draft.
Received 14 January 2000; revision received 1 May 2000