Climate Change and Vegetation Evolution During the Holocene: Editorial

1. Introduction

The study of plant micro- and macrofossil evidence as proxy indicators of past environmental conditions and the influence of factors such as climate change has a long history, originally in Europe but subsequently much more widely [1,2]. Plant communities are adapted to their environment, having evolved to exist within tolerance ranges, and are sensitive to environmental change, so that internal or external changes in environmental factors initiate responses and community changes within the vegetation that are reflected in subfossil assemblages. This is particularly so in preserved pollen assemblages that provide spatial data regarding vegetation history. As well as internal autogenic successional change, Holocene vegetation has also been influenced by various environmental factors, including climate, pedology, hydrology and disturbance by fire or fauna, such as human activity. Of these factors, climate has been one of the most influential, particularly in the first half of the Holocene before major vegetation disturbance and transformation by human activity, but also in more recent millennia. Of non-anthropogenic factors, whether operating gradually on longer time scales or rapidly during short-term climatic ‘events’, climate change has mostly been the major instigator and control of vegetation community change. The analysis of assemblages of plant remains, especially pollen microfossils but also macrofossils and sedaDNA, enables the reconstruction of past plant communities and therefore also of past environments and the factors influencing their development. The twelve papers published in this Special Issue cover a wide variety of Holocene palaeoenvironments and time periods from the end of the Late Glacial to the present and employ a wide range of palaeoecological research techniques. Sites are mainly from Europe and North America, but with important contributions from South America and southern Asia. They have a varied geographical distribution that ranges from sea level to high altitude in arid, maritime, montane, and sub-polar regions, and so provide diverse records of vegetation history under the influence of climate, as well as other environmental factors.

2. Summary of the Special Issue

This Special Issue opens with two papers from North America by Albert, who has studied vegetation history in areas around the Gulf of Mexico, in the southeastern USA and in northeastern Mexico. His first paper [3] examines the vegetation history of the Texas Gulf coastal plain using a multi-profile methodology that allows the spatial reconstruction of both terrestrial and marine environmental change. Mid-Holocene changes in forest limits in the southeastern USA under the influence of climate change are revealed, with a southerly expansion of woodland around 6 ka BP under increased tropical influence and a warming climate, but major forest retreat northward around 4 ka BP at the time of the transition to the later Holocene Neoglacial and climatic deterioration. The paper’s microfossil and lithostratigraphic analyses also record sea-level movements along the Texas coast, with marine transgression phases after 4 ka BP and related changes in coastal morphology, stabilising after 3 ka BP. The mid- and later Holocene periods of environmental stability coincided with increased human settlement and activity in this coastal zone.

Albert’s second paper [4] investigates vegetation history in the Chihuahuan Desert region of northeast Mexico, an area currently very arid with seasonal precipitation. A long sediment core yielded pollen and alluvial sediment data that covered almost the whole Holocene, allowing the long-term history of the process of desertification in this area to be elucidated. While several influencing factors were considered, it became clear that climate was a principal agent of vegetation change throughout the early and mid-Holocene, with other factors such as soil development and hydrology playing very secondary roles. After an early Holocene mesic woodland phase, the repeated expansion and retraction of a xerophytic desert-type vegetation occurred, with a tendency towards increased desertification as the Holocene progressed, particularly so in the last two millennia. This periodic expansion of desert vegetation over several millennia has led to a high level of species diversity in the region, including some local floral endemism. Before modern times, there was no human impact on vegetation development to deflect natural, climate-driven successions in the region.

The paper by Lageard [5] utilises plant macrofossils in the form of Pinus tree stumps that colonised a raised bog surface in northern England in the mid-Holocene, and which provided tree ring-width chronologies and were dated by radiocarbon. The Pinus colonisation and occupation of the bog surface occurred over a period of about 200 years in the millennium preceding the widely recognised ‘Pine Decline’ event, when pine pollen frequencies declined sharply in the northern and western British Isles around 4.2 cal. BP, recording a substantial fall in pine populations in those areas at that time and coinciding with severe climatic deterioration. Many other similar sites of Pinus bog surface growth occur in northern England, but with considerable chronological variability, with some bog pine woods colonising and persisting well after the ‘Pine Decline’. This raises the possibility of pine survival on bog surfaces in this region throughout the later Holocene, despite climatic fluctuations, providing possible unsuspected refugia for native Pinus sylvestris in northern England up to the present.

Quamar et al.’s [6] study of a palaeo lake sedimentary sequence in the Lesser Himalaya of northwest India addresses the mid-Holocene variability in Monsoon precipitation in this region, its effects on forest development, and its use as a proxy for climate change. Dating and high-resolution palynology showed that there was considerable climatic fluctuation in the mid-Holocene from the start of the Holocene climatic optimum about 7500 cal. BP until the end of the pollen record about 5800 cal. BP, with resulting changes in forest composition. Alternation between colder and dry conditions and warmer and wetter conditions favoured the expansion and woodland domination of conifer and broad-leaved forest, respectively. The Holocene climatic optimum as recognised here can be correlated with similar mid-Holocene records in many other regions of India, where the climatic effects of the South Asian Monsoon have also regulated vegetation patterns.

In the far north of Norway, Elliott et al. [7] reconstructed vegetation history throughout the Holocene from a lake core situated near a small local ice-cap that drains into it. They employed the novel palaeoecological technique of sedimentary ancient DNA (sedaDNA) analysis rather than pollen analysis, as plant sedaDNA is deposited close to its point of origin, thus avoiding the spatial uncertainty inherent in the taphonomy of pollen assemblages, which comprise a spatially composite signal derived from several different vegetation units at various distances from the sampling site. Four distinct local vegetation zones were recognised, with the transitions between them corresponding to major Holocene climatic shifts that are also recorded in the activity of the nearby glacier. The local presence of the glacier and its climatic responses appears to have made the local vegetation particularly sensitive to climate change throughout the middle and late Holocene. The results show that vegetation in such high latitude areas will be affected both directly and indirectly by future climate change.

Mighall et al. [8] used a range of palaeoecological techniques to reconstruct the vegetation history of most of the Holocene at an ombrotrophic mire in the mountains of northwest Iberia, with pollen, non-pollen palynomorph, microscopic charcoal, and geochemical data providing a high-resolution, multi-proxy record of vegetation change. They found that a combination of factors influenced vegetation development during that time, with climate, fire, and human activity all having an impact. In the early to mid-Holocene, fire and climate were the main drivers of vegetation change, with maximum extension of forests during the climatic optimum, but clear woodland recession during the widely recognised cold climate events of 8.2 and 4.2 cal. BP. Fire was more important during dry climate phases. Human disturbance of vegetation became important during the later Holocene, gradually assuming the major role, with extensive forest decline in recent millennia. Climate remained influential but increasingly less so as human impacts became more severe.

The paper of Chiverrell et al. [9] investigates the early and mid-Holocene vegetation history of a large island in the middle of the Irish Sea between Ireland and Britain, with special interest in the effects of its insularity and climate on the timing of the immigration and colonisation of the main postglacial forest trees. With pollen data from multiple sites and radiocarbon dates on the rational pollen limit of the main tree taxa, when they expanded to become a significant component of the island’s woodland, a good understanding of the island’s forest history was achieved. A comparison of this insular vegetation history with that of Ireland and Britain showed that the immigration of the pioneer trees Juniperus and Betula in the first Holocene millennium was significantly delayed on the island. The timing of the expansion of the rest of the main postglacial trees, however, was very similar to the British and Irish ages for those events and was governed by regional climatic factors. The timing of the Elm Decline, an important mid-Holocene pollen stratigraphic event, was also comparable. Insularity appears not to have been a significant factor in forest history after the first Holocene millennium.

Selby et al. [10] used pollen, spores, microcharcoal, particle size, and loss on ignition analyses in a multi-proxy investigation of early to mid-Holocene vegetation change at two topographically contrasting sites on islands off the west coast of Scotland, with the aim of reconstructing climate changes and any possible hunter–gatherer impacts. Some degree of woodland recession at the time of the 8.2 cal. BP cold event was recognised at both sites, while low-intensity impacts occurred during periods of stable climate which could therefore perhaps be attributed to a human origin. Low-scale woodland recession occurred periodically and at increasing scales after 5 ka BP and can more likely be associated with the activities of agricultural societies. Climate, however, was an important driver of vegetation change throughout the Holocene in these exposed island locations on the Atlantic fringe of Britain.

Simmons et al. [11] have conducted a multi-profile pollen investigation of the early and mid-Holocene vegetation history of palaeo lake Flixton in northeast Yorkshire, UK, which existed from Late Glacial to mid-Holocene times before its infilling by sediment, and was the location of several centuries of major Early Mesolithic settlement. A spatial reconstruction of the effects of postglacial climate change and human activity was achieved, with the former driving the successive immigration of pioneer, conifer, and then mixed deciduous woodland in the environs of the lake. Probable human impacts were recognised in several pollen profiles, primarily through the use of fire during the drier climate phases, but it was mainly climate change that brought about changes in woodland composition in the longer term. The spatial study showed that the timings of the establishment and the relative abundance of the major postglacial tree taxa varied considerably around the lake margins, probably because of local factors. Woodland successions away from the lake were similar to those in the wider northern English landscape as closed mixed forest developed.

Using changes in vegetation communities as a proxy, Marcos et al. [12] investigated the vegetation history of the last three millennia in highland Patagonia, in the far south of South America, with the aim of helping to elucidate the palaeoclimate of the region. A combination of modern pollen studies and a radiocarbon-dated pollen profile allowed the reconstruction of past plant communities, showing that various combinations of grass/shrub steppe taxa with patches of open woodland dominated the vegetation at the start of the study period, governed by the climatic influence of westerly wind systems, their relative strength and rainfall variability. Climate was shown to have been very variable in the later Holocene with periods of great aridity, when fire became an ecological factor, resulting in alternation between grass steppe and shrub steppe conditions. The area became very arid at the start of the last millennium, conditions that have lasted to the present.

The paper by Goutiers and Carcaillet [13] presents the results of a biogeochemistry and sedimentology study of a shallow minerotrophic peat in upland southeast France, with the aim of identifying the main influences on local ecosystem history. Multiple AMS radiocarbon dates on plant macrofossils revealed the peat to have accumulated within the last two millennia, consistent with the age of similar fen peats in the region. Charcoal of any grain size was not present in the sediment, indicating the absence of local wildfires during this lengthy recent time period, unusually for this Mediterranean region, and perhaps indicating a grassland rather than woodland cover with little available potential fuel. Although variable during this recent period, climate does not seem to have had a major impact on the local ecosystem. The peat’s chemical composition was dominated by atmospheric input, which influenced the peat’s trophic status through time. Metallic trace element studies showed a heavy imprint of regional human activities such as mining and agriculture, as human influence on the ecosystem steadily increased throughout the period of peat deposition.

The final paper in this Special Issue returns to North America, where McAuliffe et al. [14] investigated the environmental record of a presently semi-arid site in the eastern Mojave Desert of the southwestern United States. Modern vegetation units in the study area were mapped in detail and distinct species associations identified, mainly comprising xeric grassland and low shrubs and succulents. Although vegetation development after the start of the Holocene was mainly driven by climate change, other factors played major roles in subsequent changes, particularly geomorphic and soil processes, including hydrology, colluviation, and erosion. Soil profiles in the study area, mainly of aeolian origin, were dated using OSL combined with previously published radiocarbon ages and range from Late Glacial to the late Holocene. Plant macrofossils recovered from ancient packrat middens within soils allow the reconstruction of local Holocene vegetation and record the changing relative altitudinal limits of pine and juniper shrub woodland through time. A progressive retreat of this woodland vegetation to higher altitudes can be traced through the Holocene, caused by steadily increasing aridity from its start, although there was a time-lag between climate change and vegetation response mainly controlled by changes in the other, more local, factors.