Resolving complexities of pollen data to improve interpretation of past human activity and natural processes

Pollen analysis provides a powerful tool for understanding past human activity and its impact on the environment. This is due to pollen being preserved in a wide range of sedimentary environments and pollen being derived from, and therefore providing a record of, vegetation beyond the sampling location. While the basic premise of pollen analysis has remained constant since the pioneering work of Lennart von Post 100 years ago, methodological approaches for interpretation of pollen records have continued to evolve. Large datasets can now be compiled for identifying and exploring the complexities of pollen data temporally and spatially. Here two Holocene pollen stratigraphic changes in the British Isles are focused upon: the Ulmus and Tilia declines. Methodologies for examining the chronological controls on the timing of these changes and disentangling the processes recorded in pollen data are presented. Of particular note are the complexities of separating human impacts from natural processes in coastal wetland records which have been one of the main sources of pollen data from lowland England. We argue that it is only by unravelling the complexities of both the chronological and pollen data that extant theories on the interaction between past human activity and vegetation change can be rigorously tested

Holocene vegetation changes in the Sahelian zone of NE Nigeria: The detection of anthropogenic activity

The relative importance of climate change and anthropogenic activity in the vegetation history of the Sahel has been the subject of much recent discussion. Pollen diagrams from the Manga Grasslands (NE Nigeria) indicate a Holocene vegetation history primarily controlled by climate. During the relatively humid early and mid-Holocene the interdune depressions of the Mangas were occupied by swamp forest with Guinean affinities. Savanna, with Sudanian and Sahelian arboreal elements, occurred on the surrounding dunefields. The modem Sahelian vegetation of the region became established c.3300 yr BP as a result of drier conditions. Although the archaeological record indicates that the Manga Grasslands themselves have been occupied since at least c.3700 yr BP, there is little evidence of human activity in the pollen diagrams. The number of herb taxa recorded declines after c.3300 yr BP and unambiguous indicators of human activity are absent even from a diagram which covers the recent past. The drier post c.3300 yr BP conditions are probably masking human activity. In addition, nomadic pastoralism (which is still the major economic system) appears to be palynologically undetectable, the major effect of this activity today being the replacement of perennial grasses and herbs with more xeromorphic and less palatable species

Sediment supply and barrier dynamics as driving mechanisms of Holocene coastal change for the southern North Sea basin

The combined effects of climate change and human impact lead to regional and local coastal responses that pose major challenges for the future resilience of coastal landscapes, increasing the vulnerability of communities, infrastructure and nature conservation interests. Using the Suffolk coast, southeast England, as a case study, we investigate the importance of sediment supply and barrier dynamics as driving mechanisms of coastal change throughout the Holocene. Litho-, bio- and chronostratigraphic methods are used to decipher the mechanisms of coastal change from the record preserved within coastal stratigraphy. Results suggest that local coastal configuration and sediment supply were the most influential in determining coastal change during the mid- and late Holocene, against a background control of sea-level rise. The importance of sedimentological and morphological factors in shaping Holocene coastal changes in the southern North Sea basin must therefore be considered when using the database of evidence from this region as an analogue for future change under accelerated sea-level rise

Leaf traits interact with management and water table to modulate ecosystem properties in fen peatlands

Aims Trade-offs between slow and fast nutrient turnover rates among plants may affect soil properties and biomass production. We examined how plant traits interact with abiotic variables to modulate ecosystem properties (soil C, soil C/N ratio, aboveground biomass) in peatlands. Methods We determined the interacting effects of abiotic variables (vegetation management, water table height) and leaf traits (specific leaf area, leaf dry-matter content, leaf C/N ratio) on ecosystem properties in two lowland fens in East Anglia, UK using structural equation modelling. Results Our models explained between 21% and 95% of the variability in ecosystem properties. Leaf traits directly influenced soil nutrient content and plant biomass and mediated the effects of abiotic variables on ecosystem properties. Abiotic variables exerted larger effects on ecosystem properties among herbaceous communities, but leaf traits were equally important when modelling all communities in combination. Conclusions The expected trade-offs between exploitative and conservative life strategies among species scaled-up to changes in soil properties and biomass production, even in fen habitats where abiotic variables play an important role through marked seasonal variations. Our findings suggest an important role of leaf economics in the functioning of fens, but their effects on ecosystems may be highly dependent on local conditions

Disentangling the pollen signal from fen systems : modern and Holocene studies from southern and eastern England

Thick deposits of peat derived from fen environments accumulated in the coastal lowland areas adjacent to the North Sea during the middle and late Holocene. These sediments are frequently used in pollen-based reconstructions of in situ and more distant vegetation. However, discriminating between wetland and dry land originating pollen signals, and between the potential fen communities present in the wetland, is complex. In this study, a suite of analytical approaches are used to explore the pollen signal of modern fen communities and compare them against Holocene pollen assemblages. At two sites in eastern England, Woodwalton Fen and Upton Broad, vegetation composition was recorded around a series of moss polster sampling points. The communities investigated included herbaceous fen communities under different cutting regimes, a grazed area, glades, and woodland with canopies dominated by Alnus glutinosa and Betula. Cluster analysis is used to provide an overview of, and compare the structure within, the datasets consisting of the vegetation, the vegetation converted to palynological equivalents, and the pollen data. It is demonstrated that any loss of taxonomic precision in pollen identifications does not pose particular problems when attempting to identify fen communities, including tall-herbaceous vegetation, in the pollen record. Indices of Association imply pollen presence can be interpreted as indicating the local presence for some taxa, though few of these are confined to a particular community. Herbaceous fen vegetation subject to different management regimes are, however, shown to produce distinctive pollen signatures. Middle and late Holocene pollen assemblages from eastern (Fenland) and southern (Romney Marsh) England, interpreted as derived from fen vegetation, are compared against the modern pollen dataset using ordination. Most of the fossil samples plot out within or adjacent to the groupings produced by the modern samples in the ordinations. While these investigations demonstrate that modern pollen work can help improve the interpretation of Holocene assemblages they also call attention to a number of limitations including the restricted range of communities from which modern samples are currently available and the potential for non-analogous modern vegetation. The paper concludes with ideas to aid the interpretation of pollen data collected from fen peats and suggestions for future work

Changes in the West African landscape at the end of the African Humid Period

Existing pollen datasets from northern Africa stored in the African Pollen Database were used to assess changes in landscape physiognomy at the end of the African Humid Period (AHP) from 5000 cal yr BP to the present using arboreal pollen percentages. The thirty-six sites available were used to map changes in arboreal cover at a sub-continental scale. Based on their location in present-day forested and non-forested areas and their relatively higher temporal resolution eight of them were selected to examine the timing and amplitude of the vegetation response in more detail, and particularly in the Sahel. In spite of low pollen production and dispersal of many tropical plants, which lead to the under representation of most of the trees relative to their abundance in the landscape, we were able to distinguish the geographical pattern and timing of vegetation changes. The landscape response to the end of the AHP was far from homogeneous particularly in the Sahel where a clear east-west gradient of changing tree cover is indicated with the central Sahel being notably species poor. In areas where forests were well developed during the AHP, i.e. in the south and west, the establishment of the modern landscape was abrupt with a threshold crossed between 3300 and 2500 cal yr BP according to local conditions. Elsewhere in northern Africa the switch from tree (C3) to grass (C4) dominated landscapes occurred more gradually during the same period. This review shows that the timing of the ecosystem response at the end of the AHP was remarkably synchronous throughout northern Africa

A method for reconstructing temporal changes in vegetation functional trait composition using Holocene pollen assemblages

Methods of reconstructing changes in plant traits over long time scales are needed to understand the impact of changing environmental conditions on ecosystem processes and services. Although Holocene pollen have been extensively used to provide records of vegetation history, few studies have adopted a functional trait approach that is pertinent to changes in ecosystem processes. Here, for woody and herbaceous fen peatland communities, we use modern pollen and vegetation data combined with pollen records from Holocene deposits to reconstruct vegetation functional dynamics. The six traits chosen (measures of leaf area-to-mass ratio and leaf nutrient content) are known to modulate species’ fitness and to vary with changes in ecosystem processes. We fitted linear mixed effects models between community weighted mean (CWM) trait values of the modern pollen and vegetation to determine whether traits assigned to pollen types could be used to reconstruct traits found in the vegetation from pollen assemblages. We used relative pollen productivity (RPP) correction factors in an attempt to improve this relationship. For traits showing the best fit between modern pollen and vegetation, we applied the model to dated Holocene pollen sequences from Fenland and Romney Marsh in eastern and southern England and reconstructed temporal changes in trait composition. RPP adjustment did not improve the linear relationship between modern pollen and vegetation. Leaf nutrient traits (leaf C and N) were generally more predictable from pollen data than mass-area traits. We show that inferences about biomass accumulation and decomposition rates can be made using Holocene trait reconstructions. While it is possible to reconstruct community-level trends for some leaf traits from pollen assemblages preserved in sedimentary archives in wetlands, we show the importance of testing methods in modern systems first and encourage further development of this approach to address issues concerning the pollen-plant abundance relationship and pollen source area