Plant macrofossils

Ombrotrophic raised bogs receive all their nutrients and water from precipitation. As a result, surface conditions in these ecosystems are strongly governed by climate. As bogs accumulate over time the remains of plants that grow on the bog surface are frequently preserved, as plant macrofossils, in the acidic and anoxic conditions that develop. Different plant species thrive in different environmental conditions, some wetter, some drier, some cooler, some warmer and, as a result, they inhabit a variety different ecological niches in any given ecosystem. It has been shown that plants growing on the surface of bogs are often sensitive to changes in water table depth and, subsequently, bog surface wetness (BSW). By analysing the nature of vegetation changes down-core, through a peat sequence that has accumulated over time, it is possible to reconstruct past changes in BSW.

This climate-proxy technique has been widely practiced throughout the peatlands of northwest Europe and North America and our knowledge of the plants in these regions, in both their modern and sub-fossil forms, is very good. Conversely, relatively little plant macrofossil analyses have taken place in southern South America. Subsequently, part of the challenge facing PATAGON Research Fellow, Dr Tom Roland, will be identifying plant species, and their remains within the peat sequence, about which relatively little is known.

Peatland plants tend to group into different assemblages based on where they have grown within a bog’s microtopogtraphy (i.e. in a pool, lawn or hummock). For example, some Sphagnum species are extremely sensitive to desiccation and are only able to grow partially or entirely submerged in pool environments. Conversely, certain grasses and dwarf shrubs are able to survive on the tallest hummocks, far from the water table, owing to their long and/or dense root networks. A typical botanical assemblage may include a range of mosses, grasses, rushes, sedges and dwarf shrubs. It is sometimes possible to distinguish between the remains of different plant species at relatively low magnifications. However, when only small plant fragments of remain, or look superficially similar between species, it is necessary to examine the material at very high magnification to identify for subtle differences in cell structure, for example. Microscopic photographs of planet remains found at the PATAGON study sites can be found here.

Through statistical analysis, it is also possible to take raw plant macrofossil assemblage data and transform it into a single semi-quantitative index of changes in BSW. This is especially useful when looking to compare these data with similar climatic reconstructions from testate amoebae and stable isotopes, in multi-proxy studies, such as the PATAGON project.

More information on the use of plant macrofossils to reconstruct past climatic changes, can be found a paper co-authored by PATAGON team leader, Dr Dmitri Mauquoy. Dmitri was one of the first researchers to attempt plant macrofossil analysis in the peatlands of southern South America. His papers on the subject have been published in Quaternary Research and Earth and Planetary Science Letters.