the development of crymlyn bog and models of hydroseral succession
The definition of vegetation succession has been debated for over 100 years (Cowles 1899; Walker 1970). Clements (1916) popularized the concept of succession as an autogenic, unidirectional and progressive turnover of species, resulting in the development of a stable climax community. Gleason (1926) argued from the opposite standpoint, stressing the role of chance disturbances to the community structure that could potentially deflect succession along any number of developmental pathways. He also suggested that the concept of a climax community might be erroneous and that succession may be continuous. Despite the efforts of Gleason and a mounting list of critics (e.g. Whittaker 1953; Walker 1970; Connel & Slatyer 1977; Finegan 1984), aspects of the simple model of autogenic succession offered by Clements (1916) have persisted in the literature, albeit in a somewhat altered form. For example, Walker (1970) examined the nature, direction and rate of succession in wetland communities, and followed the basic assumption that a set of mires could be identified that were relatively free of allogenic disturbances, so enabling a study of long-term autogenic succession. Although Walker’s study lacked the most direct form of palaeoecological data for tracking mire succession, namely plant macrofossil records, the work remains an influential and rare re-assessment of succession theory using palaeo-mire communities.
Recent studies of the fen–bog transition (Hughes 2000) in a number of the mires used by Walker (1970) suggest that the assumption of predominantly autogenic development is not necessarily a safe one, even when there are no obvious reversals in the succession. For example, Hughes (2000) suggests that the frequently observed Eriophorum/Calluna mire type (‘Pseudohochmoor’, Rybniček 1973) lying at the fen–bog transition (FBT) is a response to the peat surface becoming perched above the general water table. This could happen for several reasons, such as the autogenic development of tussocks and subsequent leaching of nutrients from their crowns by rainwater; or, alternatively, similar conditions could occur after a drop in the mire water table. A falling mire water table could be triggered by a number of factors (e.g. a change in the base level or a reduction in effective precipitation). Tregaron Bog is a good example of a bog that became ombrotrophic following a phase of water table oscillation, with the FBT coinciding with a period of low water levels (Godwin & Mitchell 1938).
The results of the macrofossil investigation at Crymlyn Bog (Figs 2 and 6) show at least two major reversals in the course of succession. As a result, the mire sequence contains two transitions towards raised bog communities, the first in zone CRBmd and the second in zone CRBmf. In between (zone CRBme), there is a return to more nutrient-rich conditions. The mire sequence can be used therefore to make a comparison of the pathways of succession towards raised mire in two contrasting periods of the Holocene at the same sample site. The first oligotrophic transitional mire community (the first FBT) developed on the mire at c. 5000 BP (just above radiocarbon date AA-28126, 5245 BP). The species present indicate that the water table of the mire at this point was either deep or fluctuating, laying down a layer of well-humified fibrous Eriophorum/Calluna peat, containing rather few Sphagna. Similar phases are found in many of the raised mire sequences of north and west Britain that became ombrotrophic in the early to mid-Holocene (Leah et al. 1998). The lowland Eriophorum/Calluna mire has no natural modern analogue in Britain and closely resembles the ‘Pseudohochmoor’ of central Europe described by Rybniček & Rybničkova (1968). A feature of this mire type is that it contains some moss species indicative of slight nutrient enrichment (e.g. Aulacomnium palustre), possibly resulting from peat mineralization during periods of desiccation. Cenococcum spp. soil fungi, abundant in aerated conditions, are also characteristic of this peat type.
The succeeding zone (CRBme) represents a period of reversal in the mire succession at Crymlyn Bog, with the reappearance of fen communities, indicating that the growing surface became reconnected to the nutrient-rich groundwater supply at c. 4435 BP. This reversal may have been a response to an allogenically driven increase in the mire water table. Disturbance to the catchment through woodland clearance, an alteration in the configuration of sand bars in the estuary, or base level change could all account for the observed mire development sequence (see Fig. 2, zone CRBme, and Fig. 6). Godwin (1941) recorded similar reversal events in mire succession at Shapwick Heath in the Somerset Levels.
The second phase of development towards ombrotrophy (the second FBT), registered at the beginning of zone CRBmf and dating to 2600 BP, contrasts markedly with that of the first phase of acidification (Fig. 6). Although Eriophorum vaginatum is still a major part of the flora, there is evidence that the tussocks were surrounded by wet mire surface conditions, as indicated by the arrival of, first Menyanthes trifoliata and then Rhynchospora alba and Erica tetralix. The stratigraphic transects in Figs 7 and 8 also show that large areas of the mire remained Phragmites swamp throughout the second acidification phase, although ombrotrophic communities can be traced for over 1 km between stratigraphic cores CRBa2 and CRBb3. The record outlined above contrasts markedly with many inland bogs, where ombrotrophic peats often permanently replace the majority of the preceding fen.
In sharp contrast to the lower FBT, the upper one appears to have developed from Eriophorum vaginatum tussocks whilst the water table remained near the surface of the mire. Leaching of nutrients from the tops of the tussocks and the cation absorption capacity of Eriophorum vaginatum may have been the dominant processes causing acidification. In these conditions, high levels of precipitation might aid the process of raised mire formation. The recorded date for the change to wet acidified mire (very poor fen) in zone CRBmf is 2729–2367 BP (2σ range), which may correspond to the widely recognized period of increased effective precipitation at the beginning of the Sub-Atlantic period (van Geel et al. 1996). There are problems associated with the calibration of radiocarbon dates in this period, caused by a plateau in the radiocarbon calibration curve. Consequently, the 2σ confidence interval for the date quoted above occupies an age range of c. 360 years. Despite the uncertainty associated with the date of the shift to wet poor fen, the most likely explanation for the major increase in mire water levels would appear to be climate change. Thus not only the character, but also the timing and the rate of development of both FBTs, could have been determined by the allogenic control of regional climatic change. This finding is important because it highlights the significance of allogenic factors in directing both forwards as well as reversed mire successions over long time-scales.
the role of eriophorum vaginatum and sphagnum in the two fbts
The traditional model of autogenic raised mire development (Walker 1970) stresses the importance of Sphagnum as an ‘ecosystem engineer’ in acidifying the mire (Bellamy 1968; Walker 1970). Although Sphagnum is present in the transitional community at the first FBT, it is not a major part of the second one until the establishment of full ombrotrophic conditions. Careful examination of the macrofossil diagram shows that Sphagnum sect. Acutifolia alone was recorded as a trace in the macrofossil assemblage of zone CRBmf. The lack of Sphagnum in the upper FBT is unlikely to be a result of differential preservation, as the few remains that were recorded were well-preserved delicate leaves. The pollen and spore data (Fig. 3) support this interpretation, as Sphagnum spores represent less than 10% of pollen and spores in zone CRBf. Many of the cores in the two transects also contained few ombrotrophic Sphagna at the upper FBT, suggesting that Sphagnum was then poorly represented across significant areas of the mire. The findings may indicate that whilst Sphagnum is important in some FBTs, the development of pioneer oligotrophic mire is not dependent upon the presence of this genus in the community. Studies of raised mires in Shropshire (Leah et al. 1998) are in agreement with this finding, indicating that the coastal site is not a special case. Further detailed macrofossil work will be required to examine the role of Sphagnum at the FBT. The present paper suggests that the arrival of Eriophorum vaginatum is an important precursor to the establishment of Sphagnum-dominated true raised mire, both in pathways developed from allogenically influenced dry oligotrophic mire (FBT1 at Crymlyn Bog) and from wet tussocky poor fen (FBT2 at Crymlyn Bog).
the demise of raised bog and development of phragmites fen at crymlyn bog
Walker’s model of the hydrosere suggests that mire sequences in Britain usually culminate in ombrotrophic communities that are rarely invaded by trees. Thus, raised bog has been proposed as the climax of the hydrosere (Walker 1970). The model is intended principally to describe autogenic processes, and assumes that allogenic controls have not been dominant during the course of succession unless major reversals are evident. Both the reversal in succession in zone CRBme and the second reversal in zone CRBmi demonstrate that on at least two occasions Crymlyn Bog was subjected to perturbations large enough to deflect the development pathway onto a completely new course. The final decline of ombrotrophic mire communities began in the Medieval period at c. 1045 BP (range 1130–960 BP). The full pollen diagram (Dumayne-Peaty, unpublished results) shows a clearance of dry land tree species before the expansion of Phragmites and Carex communities. Increased run-off from the steep slopes surrounding Crymlyn Bog may have triggered the observed changes.
The medieval date for the uppermost change from raised mire to poor fen in CRB93 refutes the view that the demise of raised bog was coincident with the Industrial Revolution and its consequent pollution (Meade unpublished data; Headley et al. 1992; Gilman 1994). However, the radiocarbon chronology pertains to one discrete location (CRB93), and the transition from ombrotrophic mire to fen could have been time-transgressive, perhaps explaining why historical and botanical accounts describe raised mire communities at Crymlyn Bog in the 18th century AD (Meade unpublished data). Nevertheless, the change began considerably earlier than the Industrial Revolution at CRB93.
Palaeoecological investigations of many other raised bogs (e.g. Barber et al. 1994; Leah et al. 1998; Mauquoy & Barber 1999) demonstrate that mire communities have changed dramatically in the last couple of centuries. For example, Molinia caerulea and Betula spp. have invaded numerous formerly Sphagnum-rich raised mires, altering the community structure significantly. Some of these changes may be a response to new allogenic triggers, such as increased nitrogen deposition. The continuous variation in allogenic factors highlighted by the analysis of core CRB93 at Crymlyn Bog, and the other palaeoecological records mentioned above, emphasize the problem with the definition of climatic climax communities sensuClements (1916). It can rarely be assumed that allogenic forces have remained unchanged over time-scales relevant to mire succession. The palaeoecological record, however, only demonstrates the mire’s response to forcing factors, and it is therefore often difficult to ascertain the exact cause and nature of external influences.