Long-term ecological study (palaeoecology) to chronicle habitat degradation and inform conservation ecology: an exemplar from the Brecon Beacons, South Wales

For ecosystems perceived as degraded, but for which the causal factors or timescales for the degradation are disputed or not known, long-term (palaeo-)ecological records may aid understanding and lead to more meaningful conservation approaches. To help ‘bridge the gap’ between (very) long-term ecology and contemporary ecology for practical application, there have been calls for working relationships to be established between palaeoecologists and conservation ecologists. One environment in which this has been attempted is blanket mire. Many blanket mires in Europe are degraded and contain few sphagna. In South Wales, almost all exhibit symptoms of degradation, with dominance by purple moor grass (Molinia caerulea) widespread. We used palaeoecological techniques on three peat profiles in the Brecon Beacons to investigate vegetation history of high-altitude blanket mire to help assess the relative contribution of various factors in mire degradation and to inform strategies for mire conservation and restoration management. We found that declines in sphagna preceded the rise to dominance of monocotyledons. Macrofossil records showed that although Molinia was already present on the Beacons before the start of the industrial revolution, its major rise to dominance in one profile was within the 20th Century, coincident with evidence for local fire. In another profile, it was out-competed by Eriophorum vaginatum after the start of the industrial revolution; there is circumstantial evidence to suggest that a reduction in burning contributed to the rise in E. vaginatum. Conservation management to reduce the current local dominance of both Eriophorum and Molinia is supported by the palaeoecological data, but severe erosion and hagging of peat will constrain practical methods for achieving this on the Beacons until the peat is stabilised. We suggest that palaeoecological techniques have wider applicability in conservation.     

Vegetation transition following drainage in a high‐latitude hyper‐oceanic ecosystem

Questions: How does draining affect the composition of vegetation? Are certain functional groups favoured? Can soil parameters explain these differences? Location: Central Faroe Islands, treeless islands in the northern boreal vegetation zone. Since 1987, an area of 21 km² at 100–200 m a.s.l. was drained in order to provide water for hydro‐electric production. Method: Vegetation and soil of a drained area and a control, undrained neighbouring area of approximately the same size were sampled in 2007. Six sites were sampled in each area. The vegetation was classified with cluster analysis. Results: Four plant communities were defined in the area: Calluna vulgaris–Empetrum nigrum–Vaccinium myrtillus heath, Scirpus cespitosus–Eriophorum angustifolium blanket mire, Carex bigelowii–Racomitrium lanuginosum moss‐heath, Narthecium ossifragum–Carex panacea mire. Heath was more extensively distributed within, and was the dominant community of the drained area, whereas moss‐heath was more extensive in the undrained area. Blanket mire and mire had approximately the same distribution in both areas. For the blanket mire, species composition indicated drier conditions in the drained than in the undrained area. The drained area had higher frequencies of woody species and lichens, grasses had finer roots and available soil phosphate was considerably higher, whereas the undrained area had higher frequencies of grasses and sedges. Conclusion: The dominant plant communities were different in the two areas, which indicated that the blanket mire was drying in the drained area. Higher concentration of soil phosphate in the drained area also indicated increased decomposition of organic soils owing to desiccation.     

Recent vegetation history of Drygarn Fawr (Elenydd SSSI), Cambrian Mountains,Wales: implications for conservation management of degraded blanket mires

Many areas of blanket mire in Britain display apparently degraded vegetation, having a limited range of ericaceous and Sphagnum species. Data are presented here from Wales from the upland locality of Drygarn Fawr (Elenydd SSSI), which is dominated overwhelmingly by Molinia caerulea. Palaeoecological techniques were used to chronicle vegetation history and to determine the nature and timing of vegetation changes, as an aid to devising conservation management and restoration strategies. Although for the past 2000 years the pollen and plant macrofossil data indicate some evidence for cyclic vegetation change, they demonstrate that here the major vegetation change post-dated the start of the industrial revolution. The palaeoecological data show a greater proportion of Sphagnum than currently. Local extinction of some species (e.g., Myrica gale) apparently took place in Medieval times, but most of the degradation and floral impoverishment apparently occurred during the 20th Century. The implications for conservation management are far-reaching. The overwhelming dominance of Molinia is clearly unprecedented. While it was locally present for hundreds of years, some factor(s)—possibly a change in grazer and grazing regime—encouraged its recent ascendancy in the 20th Century. Consequently, any management attempts to reduce the pre-eminence of Molinia would not be countering an ingrained, long-established dominance. It is suggested that investigation of degraded blanket mires elsewhere by historical and multi-proxy palaeoecological techniques—through multiple, dated cores to track species extinctions and directional vegetation changes—would help ascertain previous mire floras and so indicate a range of restoration targets for mire vegetation.     

Nitrogen deposition enhances moss growth,but leads to an overall decline in habitat condition of mountain moss‐sedge heath

Ecosystems are subject to multiple, natural and anthropogenic environmental influences, including nitrogen (N) deposition, land use and climate. Assessment of the relative importance of these influences on biodiversity and ecosystem functioning is crucial for guiding policy and management decisions to mitigate global change; yet, few studies consider multiple drivers. In the UK, ongoing loss of the internationally important arctic/alpine moss‐sedge community, Racomitrium heath, has been linked to elevated N deposition, high grazing pressures and their combination; however, the relative importance of these drivers remains unclear. We used environmental gradients across the habitat's European distribution (UK, Faroes, Norway and Iceland) to investigate the relative impact of N deposition and grazing pressure, as well as climate, on the condition of the dominant moss species, Racomitrium lanuginosum. Key variables including tissue chemistry, growth and cover were measured at 36 sites, and multiple linear regressions were used to examine the relative importance of the drivers across sites. Our results clearly show that regional variation in the condition of R. lanuginosum across Europe is primarily associated with the impacts of N deposition, with climate (air temperature) and grazing pressure playing secondary roles. In contrast to previous experimental studies, we found moss growth to be stimulated by elevated N deposition; this apparent discrepancy may result from the use of artificially high N concentrations in many experiments. Despite increased growth rates, we found that moss mat depth and cover declined in response to N deposition. Our results suggest that this is due to increased decomposition of material in the moss mat, which ultimately leads to loss of moss cover and habitat degradation. This study clearly demonstrates both the key role of N deposition in degradation of Racomitrium heath and the importance of observational studies along natural gradients for testing predictions from experimental studies in the real world.     

Using palaeoecology to support blanket peatland management

Many peatlands have a recent history of being degraded by extraction, drainage, burning, overgrazing and atmospheric pollution often leading to erosion and loss of peat mass. Restoration schemes have been implemented aimed at rewetting peatlands, encouraging revegetation of bare peat or shifting the present vegetation assemblage to an alternative. Here we demonstrate the use of palaeoecological techniques that allow reconstruction of the historical development of a blanket peatland and provide a historical context from which legitimate restoration targets can be determined and supported. We demonstrate the applicability of simple stratigraphic techniques to provide a catchment-wide peatland development history and reinforce this with a detailed macrofossil reconstruction from a central core. Analysis at Keighley Moor Reservoir Catchment in northern England showed that the present vegetation state was ‘atypical’ and has been characteristic for only the last c. 100 years. Sphagnum moss was an important historic contributor to the vegetation cover between 1500 years ago and the early 1900s. Until the early 1900s Sphagnum occurrence fluctuated with evidence of fire, routinely returning after fire demonstrating good resilience of the ecosystem. However, from the turn of the 20th century, Sphagnum levels declined severely, coincident initially with a wildfire event but remaining extremely diminished as the site regularly underwent managed burning to support grouse moor gun sports where practitioners prefer a dominant cover of heather. It is suggested that any intention to alter land management at the site to raise water tables and encourage greater Sphagnum abundance is in line with peatland development at the site over the past 1500 years. Similar palaeoecological studies providing historical context could provide support for restoration targets and changes to peatland management practice for sites globally.     

Vegetation regeneration on blanket mire after mechanized peat-cutting

• 1 Blanket mire in Northern Ireland is an ecologically threatened habitat in which land use for hand peat‐cutting, forestry and agriculture has had a major influence. A recent land use change is the introduction of tractor‐powered peat‐harvesting. In this paper, the effect of machine peat‐cutting on ombrotrophic blanket mire vegetation is assessed from a regional sample of cut and uncut plots.
• 2 Principal components analysis identified water‐table depth and grazing intensity as major factors influencing the species composition of uncut mire. A key variable affecting the composition of machine‐cut mire across the drainage gradient was the number of times cut, with multiple annual cutting causing progressive decreases in acrotelm depth, catotelm bulk density and plant cover. Ericaceous species and Sphagnum spp. were particularly sensitive to cutting, with Eriophorum angustifolium and Campylopus introflexus characteristic of multiple‐cut sites.
• 3 Redundancy analysis, with number of times cut partialled out, showed that recovery time accounted for a significant amount of variance in vegetation composition. Species that significantly increased in abundance with recovery time were Sphagnum spp., Odontoschisma sphagni , Erica tetralix and Drosera rotundifolia.
• 4 Sites cut frequently, or which were grazed, recovered more slowly. Recovery from cutting was partly dependent on the post‐cutting structure of the mire surface and the species that survive cutting. The rate of recovery on sites cut once, then abandoned, is relatively rapid compared with multiple‐cut sites where species colonization is constrained by bare compacted peat.

     

Mid- and late-Holocene vegetation history and tephra studies at Fenton Cottage, Lancashire, U.K.

The Over Wyre region of Lancashire, England, was formerly dominated by a complex of ombrotrophic intermediate raised mires, most of which have been severely damaged by agricultural activity during the last 250 years. The degree of truncation and desiccation of the remaining peats means that opportunities for studying environmental history in this part of lowland England are extremely rare. An exception occurs at Fenton Cottage where a relict, non-truncated portion of original mire survives and forms an environmental archive spanning the second half of the Holocene. The palaeoecological record shows that there has been a continual human influence on the development of the landscape since the mire began growing, with a particularly significant deforestation episode commencing in the late Iron Age. A major expansion of the mire system commenced in the late Bronze Age/early Iron Age and may have led to the abandonment of settlement in and around the peatlands during much of the Iron Age. The investigation also yielded the first tephra to be discovered in English peats. This took the form of a discrete layer of microscopic tephra, geochemically assigned to the Icelandic Hekla-4 eruption of ca. 2300 cal. B.C.. This was recorded inScheuchzeria palustris-dominated peat which formed during a period of freshwater flooding. The find represents the most southerly geographical location in the British Isles where Hekla-4 tephra has been detected to date. The study demonstrates the palaeoecological value of threatened relict lowland peat archives in agricultural landscapes and highlights the potential of tephrochronology as a tool for studying environmental changes and wetland archaeology in southern Britain.     

Secondary succession in a Swiss mire after a bog burst

Severe natural disturbances can lead to the recovery of the original vegetation or the shift to new vegetation types. While post-disturbance succession is well documented for regularly disturbed ecosystems, little is known about the pathways and rapidity of vegetation dynamics after rare events such as peat mass movements in bogs. We monitored the floristic changes in a mire subject to a bog burst in 1987 for two decades through the repeated sampling of permanent plots. The mean species number per plot increased continuously, while the evenness increased only in the first decade and then slightly decreased. Declining species were mostly mire species, while colonist species were mostly wet meadow species. Species turnover was higher in the first decade after the disturbance, and was also higher in the area of peat erosion than in the area of peat accumulation. Changes in plant species composition indicate a succession towards tall-forb vegetation (Filipendulion), acidic fen vegetation (Caricion fuscae) and swamp willow forest (Salicion). We conclude that the effects of the disturbance are still ongoing, and that the mire’s potential for recovery is therefore difficult to predict.     

Human and climatic impact on mires: a case study of Les Amburnex mire,Swiss Jura Mountains

Modern period long-term human and climatic impacts on a small mire in the Jura Mountains were assessed using testate amoebae, macrofossils and pollen. This multiproxy data analysis permitted detailed interpretations of local and regional environmental change and thus a partial disentanglement of the different variables that influence long-term mire development. From the Middle Ages until a.d. 1700 the mire vegetation was characterised by ferns, Caltha and Vaccinium, but then abruptly changed into the modern vegetation characterised by Cyperaceae, Potentilla and Sphagnum. The cause for this change was most probably deforestation, possibly enhanced by climatic cooling. A decrease in trampling intensity by domestic animals from a.d. 1950 onwards allowed Sphagnum growth and climatic warming in the a.d. 1980s and 1990s may have been responsible for considerable changes in the species composition. The mire investigated is an example of the rapid changes in mire vegetation and peat development that occurred throughout the central European mountain region during the past centuries as a result of changing climate and land-use practice. These processes are still active today and will determine the future development of high-altitude mires.     

Biodiversity in montane Britain: habitat variation,vegetation diversity and some objectives for conservation

The montane (low- to mid-alpine) zone in Great Britain (GB) lies above the potential tree-line (700–800 m, but descending to 200 m in the north). It is composed of moss and lichen heaths, snowbeds, blanket bog and dwarf-shrub (Ericaceae) health-covered solifluction/gelifluction terraces (38 communities/sub-communities). Approximately 3.0% of the land surface is covered by this- the most extensive predominantly near-natural terrestrial habitat in GB. Internationally distinctive features include oceanic and southern biotic outliers of arctic-alpine fellfield and mountain tundra, and plant communities that are either globally rare/localised or especially well represented in GB. The absence of extensive sub-alpineBetula spp. andSalix spp. scrub is striking.The main sources of habitat diversity are climate, regional variation in topography and geology, and regional modifications due to land-use impact. Over 50 examples are given. Five important gradients in Scottish Highland vegetation are described. Only some 15% of the sampled montane vegetation is anthropogenic; the rest is semi- or near-natural. The vegetation is divided into 5 functional groups: chionophobous (avoids snow), chionophilous (prefers snow), species-rich, mires (including springs and flushes), and anthropogenic. Chionophobous and then chionophilous communities contribute most to montane vegetation diversity (calculated here as the ShannonH diversity index).H diversity increases asymptotically with montane site area but linearly with the number of communities present. A more varied topography, geology and topo-climate gives the highestH diversity.Two examples of montane biodiversity reductions south of the Highlands are the loss of prostrateCalluna vulgaris heaths and modification ofRacomitrium lanuginosum healths. Five objectives for nature conservation are proposed, covering restoration of montaneR. lanuginosum healths, prostrate dwarf-shrub dominated heaths, sub-alpine scrub and upper treelines, and the extension of the breeding ranges of both ptarmigan (Lagopus mutus) and dotterel (Charadrius morinellus) south of the Scottish Highlands. International support for monitoring is sought.     

Raised peat bog development and possible responses to environmental changes during the mid- to late-Holocene. Can the palaeoecological record be used to predict the nature and response of raised peat bogs to future climate change?

Palaeoecological analyses of raised peat bog deposits in northwest Europe show the naturalness, antiquity and robust response of these ecosystems to environmental changes from c. 7800 years ago to the present. A review of the techniques used to identify these long-term features is presented and the role of climate change, autogenic change processes and human disturbance is discussed. Millennial records of vegetation changes recorded in peat deposits demonstrate the response (often rapid) of raised peat bog vegetation to climatic changes during the mid-Holocene, Bronze Age/Iron Age transition and the Little Ice Age. Greenhouse warming scenarios exceed the reconstructed Holocene record of climatic changes (c. the last 11, 500 years), and bog-water tables may fall considerably. A combination of centennial palaeoecological analyses of bogs affected by human disturbance and experimental manipulations have been used as analogues for the potential response of raised peat bog vegetation to these changes. These show that possible greenhouse gas climate forcing scenarios may exceed the ability of Sphagnum- dominated raised peat bogs to respond to projected increases in summer temperature and decreases in summer precipitation. In combination with increasing N deposition, a loss of their Sphagnum-rich vegetation and increases in the abundance of vascular plants could occur on decadal timescales.     

Vegetation change in Southeast Greenland? Tasiilaq revisited after 40 years

Questions: Are there changes in species composition of the oceanic, Low‐Arctic tundra vegetation after 40 years? Can possible changes be attributed to climate change? Location: Ammassalik Island near Tasiilaq, Southeast Greenland. Methods: Species composition and cover of 11 key vegetation types were recorded in 110 vegetation survey plots in 1968–1969 and in 11 permanent plots in 1981. Recording was repeated in 2007. Temporal changes in species composition and cover between the surveys were tested using permutation tests linked with constrained ordinations for vegetation types, and Mann–Whitney tests for individual species. Changes in vegetation were related to climate change. Results: Although climate became warmer over the studied period, most of the vegetation types showed minor changes. The changes were most conspicuous in mire and snowbed vegetation, such as the Carex rariflora mire and Hylocomium splendens snowbed. In the C. rariflora mire, species number and cover of vascular plants and cover of bryophytes increased, whereas in the H. splendens snowbed species numbers of vascular plants, bryophytes, and also lichens increased. Lichen richness increased in the Carex bigelowii snowbed and cover of bryophytes in the Salix herbacea snowbed. No such changes occurred in the Alchemilla glomerulans meadow, Alchemilla alpina snowbed and Phyllodoce coerulea heath. There was no change of species composition within the Salix glauca scrub, A. alpina snowbed, lichen grassland and the Empetrum nigrum and Phyllodoce coerulea heaths. Most changes resulted from increasing frequency or cover of some species; there were very few decreasing species. Most of the increasing species indicate drier substrate conditions. Conclusions: Only minor changes in species composition and cover were detected in the vegetation types studied. These changes were probably caused by milder winters and warmer summers during the years before the 2007 sampling. Climate warming may have reduced the duration of snow cover and soil moisture, particularly in snowbed and mire habitats, where species composition change was most pronounced. However, its magnitude was insufficient to cause a major change in species composition. Thus, on the level of plant community types, tundra vegetation near Tasiilaq was rather stable over the last 40 years.     

A 6500-year pollen record from the Polistovo-Lovatskaya Mire System (northwest European Russia). Vegetation dynamics and signs of human impact

New pollen and plant macrofossil evidence from the Polistovo-Lovatskaya Mire System is presented. The results show that local vegetation of the mire system was affected by various factors such as climate, hydrogeology and autochthonous processes in the peat bog. An important palaeoecological event took place around 6500 cal bp and led to a dramatic increase of paludification processes and lateral expansion of the mire. Forests of spruce and broad-leaved species in various combinations represented primary vegetation of uplands; they began to degrade around 1500 years ago and disappeared in historical times (400–500 cal bp). First signs of arable farming in the area occurred around 4200 cal bp, regular and mild human pressure established about 1000 years cal bp, and a significant impact at the regional level became evident around 400 years ago.     

Blanket peat in the Scottish Highlands: timing, cause, spread and the myth of environmental determinism

This contribution describes the geomorphic, stratigraphic, palaeoclimatic, palaeoecological and ¹⁴C dating evidence for the timing within the present interglacial of blanket peat initiation and extension (‘spread’) from five localities throughout the upland and northern regions of Scotland. The results suggest that blanket peat was common or abundant over much of the highland landscape within a few thousand years of the beginning of the Holocene period. Blanket peat developed either as an inevitable but rapid end-stage to soil development in this generally cold and wet climate or was promoted by climatic change. There is no evidence from this data-set that blanket peat developed as a result of anthropogenic activities. It is suggested, indeed, that farming communities successfully resisted the natural spread of peat across their fields.     

Short-Term Vegetation Dynamics of Alnus Dominated Peatlands: a High Resolution Palaeoecological Case Study from Western Pomerania (NE Germany)

Actual ecological research postulates for alder carrs a cyclic alternation of Alnus tree vegetation with open fen or Salix dominated vegetation. Such cycles are also indicated in palaeo-ecological studies, but normally the temporal resolution of these studies is insufficient to resolve the duration of short-term cycles in vegetation development. This paper presents a high resolution palaeoecological study (including pollen, macrofossils and non-pollen palynomorphs) of a Late Holocene wood peat section from the small, long-term Alnus dominated peatland ‘Heger Soll’ in the ‘Rodder Forst’ in Western Pomerania (NE Germany) to reconstruct short-term vegetation changes. During a time-span of ca. 800 years, sedge-dominated fen vegetation types alternated with two phases of Alnus carr and one phase of Salix shrubland. The alder carr decline coincided with the beginning of intensified human activity in the surroundings of the mire and was probably connected to increased water discharge resulting from large-scale deforestation, after which willow scrub and sedge fen became established. Growth of Alnus trees was associated with prolonged phases of reduced human impact and probably less water supply. This study shows that human impact on the uplands surrounding the mire and on the alder carr itself may explain the observed “cyclic” vegetation development of alder carrs, willow scrubs and sedge fens in Central Europe.     

Hydro-ecological analysis of the Biebrza mire (Poland)

Vegetation composition and structure of 58 sites along gradients in the valley mire of Biebrza, Poland, are related to physical and chemical variables of groundwater and peat. The three most prominent hydrochemical processes in the valley are (a) dissolution of calcite; (b) dissolution of iron, manganese and aluminium; and (c) enrichment with nitrogen and potassium. Major factors determining these processes are vertical flow of the groundwater and river flooding.Within the rheophilous zone of the mire, calcium-richness of the shallow groundwater and base-saturation of the peat are caused by upward seepage of groundwater originating from adjacent higher grounds. This groundwater movement keeps the larger part of the mire saturated with calcium.Good correlations exist between hydrochemistry and vegetation patterns. Groundwater-fed sites support a characteristic rich fen vegetation (Caricetum limoso-diandrae) with a low biomass production. The flood-plain vegetation consists of highly-productive communities of Glycerietum maximae and Caricetum elatae. In a belt in the Upper Basin where neither flooding nor upward seepage occurs, succession, probably caused by intensified drainage, leads to a dwarf-shrub vegetation (Betuletum humilis; poor fen).     

Variable sensitivity of plant communities in Iceland to experimental warming

Facing an increased threat of rapid climate change in cold‐climate regions, it is important to understand the sensitivity of plant communities both in terms of degree and direction of community change. We studied responses to 3–5 years of moderate experimental warming by open‐top chambers in two widespread but contrasting tundra communities in Iceland. In a species‐poor and nutrient‐deficient moss heath, dominated by Racomitrium lanuginosum, mean daily air temperatures at surface were 1–2°C higher in the warmed plots than the controls whereas soil temperatures tended to be lower in the warmed plots throughout the season. In a species‐rich dwarf shrub heath on relatively rich soils at a cooler site, dominated by Betula nana and R. lanuginosum, temperature changes were in the same direction although more moderate. In the moss heath, there were no detectable community changes while significant changes were detected in the dwarf shrub heath: the abundance of deciduous and evergreen dwarf shrubs significantly increased (>50%), bryophytes decreased (18%) and canopy height increased (100%). Contrary to some other studies of tundra communities, we detected no changes in species richness or other diversity measures in either community and the abundance of lichens did not change. It is concluded that the sensitivity of Icelandic tundra communities to climate warming varies greatly depending on initial conditions in terms of species diversity, dominant species, soil and climatic conditions as well as land‐use history.     

South Patagonian ombrotrophic bog vegetation reflects biogeochemical gradients at the landscape level

Question: Which environmental variables affect the floristic composition of south Patagonian bog vegetation along a gradient of climate and biogeochemical changes with increasing distance from the Pacific ocean? Location: Trans‐Andean transect (53° S), southern Patagonia Material and Methods: Floristic composition, peat characteristics (water level, decomposition, pH, total nitrogen, total carbon, ash content and plant available P, K, Na, Ca, Mg, Fe, Mn, Zn, and Al) and climatic constraints of ombrotrophic peat‐lands were measured at 82 plots along a gradient of increasing distance from the Pacific Ocean. Results: Climatic constraints and biogeochemical peat characteristics significantly change with increasing distance from the Pacific. Peatland vegetation shifted from hyperoceanic blanket bogs dominated by cushion forming vascular plants to the west to Sphagnum bogs to the east. Climatic and biogeochemical variables explained a large proportion of the floristic variation along the first DCA axis. The second axis represented a water level gradient. When ‘distance to the Pacific’ was defined as a covariable in partial CCA, the proportion of variance explained declined for most other variables, especially in the case of annual precipitation and exchangeable base cations and related traits. The differences in biogeochemical characteristics related to peat were mainly attributed to the input of sea‐borne cations. Conclusions: While variation in vegetation composition along a longitudinal gradient crossing the southern Andes was attributed to climatic constraints as expected, vegetation composition was also strongly affected by the biogeochemical characteristics of peat. Sea spray was of high ecological importance to peat chemistry and, consequently, to floristic composition. Presumably, south Patagonian peat bogs represent a glimpse of pre‐industrial environments, so that these peat bogs may act as reference systems with respect to atmospheric inputs in mire ecology research.     

Post-drainage changes in vegetation composition and carbon balance in Lakkasuo mire,Central Finland

The post-drainage changes in vegetation composition and carbon balance were studied on four site types (from minero- to ombrotrophic conditions) in Lakkasuo mire, central Finland, by directly comparing undrained and drained parts (30 years ago) of the mire. Drainage had drastically changed the species composition of the sites, especially at the minerotrophic sites, where almost all Sphagna had been replaced by forest mosses. On the ombrotrophic sites much of the mire vegetation still remained 30 years after drainage. Drainage had decreased the C stores in ground vegetation on the minerotrophic sites but increased them on the ombrotrophic sites. The changes were, however, very small compared to the changes in the tree stand, where the C stores had increased at all sites (increasing with nutrient level). The change in peat C balance over the 30-year post-drainage period was negative on the most nutrient-rich site, and positive on the others, increasing with lower nutrient levels. The decrease in the peat C balance on the most nutrient-rich site was compensated by the greater increase in the tree stand C stores and the changes in the total C balance (peat+tree stand+ground vegetation) remained positive on all sites. This revised version was published online in June 2006 with corrections to the Cover Date.     

Response of Amazonian forests to mid-Holocene drought: A model-data comparison

There is a major concern for the fate of Amazonia over the coming century in the face of anthropogenic climate change. A key area of uncertainty is the scale of rainforest dieback to be expected under a future, drier climate. In this study, we use the middle Holocene (ca. 6000 years before present) as an approximate analogue for a drier future, given that palaeoclimate data show much of Amazonia was significantly drier than present at this time. Here, we use an ensemble of climate and vegetation models to explore the sensitivity of Amazonian biomes to mid-Holocene climate change. For this, we employ three dynamic vegetation models (JULES, IBIS, and SDGVM) forced by the bias-corrected mid-Holocene climate simulations from seven models that participated in the Palaeoclimate Modelling Intercomparison Project 3 (PMIP3). These model outputs are compared with a multi-proxy palaeoecological dataset to gain a better understanding of where in Amazonia we have most confidence in the mid-Holocene vegetation simulations. A robust feature of all simulations and palaeodata is that the central Amazonian rainforest biome is unaffected by mid-Holocene drought. Greater divergence in mid-Holocene simulations exists in ecotonal eastern and southern Amazonia. Vegetation models driven with climate models that simulate a drier mid-Holocene (100–150 mm per year decrease) better capture the observed (palaeodata) tropical forest dieback in these areas. Based on the relationship between simulated rainfall decrease and vegetation change, we find indications that in southern Amazonia the rate of tropical forest dieback was ~125,000 km² per 100 mm rainfall decrease in the mid-Holocene. This provides a baseline sensitivity of tropical forests to drought for this region (without human-driven changes to greenhouse gases, fire, and deforestation). We highlight the need for more palaeoecological and palaeoclimate data across lowland Amazonia to constrain model responses.