Validation of non-pollen palynomorphs as paleoenvironmental indicators in tropical Africa: Contrasting ~ 200-year paleolimnological records of climate change and human impact

Abstract

Multi-proxy investigation of high-resolution, ~ 200-year sediment records from two shallow, climate-sensitive crater lakes in western Uganda, surrounded by either presently undisturbed (Lake Chibwera) or severely disturbed (Lake Kanyamukali) catchments, provides insights into lake ecosystem and vegetation response to short-term natural climate variability, as separate from local human impact. Here, we present fossil non-pollen palynomorph (NPP) assemblages, mainly including spores of saprotrophic, coprophilous (dung-colonizing), and/or mycorrhizal fungi, to assess and validate their paleoecological significance in a tropical African context. At both sites, high abundances of the saprotrophic Coniochaeta spp. broadly coincide with documented decade-scale episodes of increased humidity (early- to mid-19th century, ~ 1870–1900 AD and late 20th century). Severe drought in the late 18th and early 19th century reduced both lakes to ephemeral ponds, which were intensively visited by large wild and/or domestic herbivores, as indicated by abundant Sordaria spp. and Delitschia spp., obligately growing on herbivore dung. From the mid-19th century onwards, large herbivore presence at Chibwera declined, and remained low as the site became incorporated in a game reserve in the early 20th century, and was added to Queen Elizabeth National Park in 1965. In contrast, herbivore presence remained strong at Kanyamukali, where grazing and subsistence farming intensified from the mid-20th century onwards. The location of Lake Kanyamukali near an ancient cattle trail and/or trading route underscores its historical linkage with transhumance. Grazing activity at Kanyamukali is indicated mainly by slightly increased abundances of coprophilous Sordaria spp. and of Glomus sp., indicative of soil erosion. Only this modest change in the NPP assemblage can be uniquely attributed to relatively intense recent human impact at Kanyamukali, contrasting with the strong NPP response at both lakes to the shifts in large herbivore behavior associated with late 18th- and early 19th century climatic drought. Comparison of the two records therefore allows us to conclude that in African lakes, (i) only high densities of domestic large herbivores create a paleoenvironmental NPP signature that can be unambiguously linked to past pastoralist activity, and (ii) facultative coprophilous fungal spores are overwhelmingly controlled by natural, climate-driven fluctuations in the local availability of preferred substrates for the fungi.

Introduction

Natural resources and ecosystems in tropical Africa are increasingly vulnerable to human impact and climate change, threatening the services which these ecosystems provide to human society (e.g., Conway et al., 2005, Boko et al., 2007). However, the role of human activity in ongoing ecosystem degradation is controversial, as it must be disentangled from large natural climate and ecosystem dynamics on short and longer time scales (e.g., Dearing, 2006, Costanza et al., 2007, Pongratz et al., 2008). Studies of historical land use tend to assume that pre-20th century human impact on natural ecosystems in East Africa was very low, due to the low density of indigenous people (Ramankutty and Foley, 1999, Klein Goldewijk, 2001). Yet, paleoecological (e.g., Taylor, 1990, Jolly et al., 1997, Kiage and Liu, 2006) and archeological (e.g., Robertshaw and Taylor, 2000, Killick, 2009) data seem to indicate significant anthropogenic deforestation from at least 3000 years ago. Reflecting a lack of solid evidence, the notable discrepancy between both scenarios must be resolved to properly assess the resilience and recovery potential of those natural ecosystems.

High-resolution paleolimnological records of Ugandan crater lakes spanning the last 200 to ~ 1000 years (Ssemmanda et al., 2005, Russell et al., 2007, Bessems et al., 2008, Ryves et al., 2011) have provided strong evidence for the regional complexity of both natural climate variability and the timing and trajectory of historical human impact. These lakes are particularly sensitive to short-term (decadal to century-scale) rainfall variability (Russell et al., 2007, Bessems et al., 2008), so that their records complement and fine-tune the climate change recorded over longer timescales in the sediments of the nearby large rift lakes Albert, George, Edward and Victoria (e.g., Beuning et al., 1997, Stager et al., 1997, Stager et al., 2003, Williams et al., 2006). Human impact on lakes and their catchments can also create strong paleoenvironmental signatures (Russell et al., 2009, Ryves et al., 2011), however these are often modified by simultaneous system response to climate fluctuation. This is mainly because the responses of vegetation proxies (pollen, charcoal and phytoliths) to land-use change are significantly mediated by basin-specific factors (e.g., hydrology, lake morphometry, catchment size) (Ryves et al., 2011) and proxy-specific problems such as low pollen visibility of crop cultivation, variation in pollen and spore influxes, ambiguous pollen signals of increased Poaceae caused either by drought or agriculture, and uncertainty about whether charcoal signatures reflect natural or human-induced fire (e.g., Lamb et al., 2003, Lejju et al., 2005, Ssemmanda et al., 2005, Gillson and Ekblom, 2009). Multi-proxy investigation of paired or multiple lake-sediment records, however, may provide the comparison and replication needed to detect environmental changes more accurately (e.g., Fritz, 2008), and thus provide the means to more clearly separate local human impacts from regional-scale climatic impacts.

We investigate high-quality sediment records of two shallow crater lakes in western Uganda with a contrasting history of human impact (presently undisturbed Chibwera and severely disturbed Kanyamukali) to unravel lake-ecosystem and vegetation response to climatic fluctuations and human impact over the past 200 years. Multi-proxy analyses of lithological parameters (bulk sediment composition, % sand and coarse organic detritus, C/N ratio, and magnetic susceptibility) revealed distinct signatures of decade-scale rainfall variability over the whole time span covered by both sediment sequences (Bessems et al., 2008). In the sediment record of Lake Kanyamukali, appearance of the diatoms Amphora pediculus and Sellaphora pupula also indicated increased aquatic productivity during the past 60–70 years, probably reflecting eutrophication linked to intensified usage of the crater basin by pastoralists and farmers since the mid-20th century (Audenaert et al., unpubl. data). Increased human impact was also suggested by the pollen record, which shows regional presence of cereals and exotic plantation trees (eucalyptus and pine) in the past 50 years (Ssemmanda et al., unpubl. data).

In this particular study, we exploit the contrasting histories of these two lakes to test and validate fossil non-pollen palynomorphs (NPPs) as proxies for paleoenvironmental reconstruction in a tropical African context. The NPPs mainly include spores of saprotrophic, coprophilous and/or mycorrhizal fungi. During the last two decades, NPPs have received increased attention in paleoecological studies worldwide (e.g., special issues of the Review of Palaeobotany and Palynology: van Geel, 2006, and Vegetation History and Archaeobotany: Haas, 2010), but only recently comprehensive surveys of their diversity have become available (Gelorini et al., 2011, van Geel et al., 2011) which permit their firm introduction in the African paleoecological research field. These microfossils are common in all sorts of habitats (e.g., wetlands, grasslands, tropical and temperate forests) and may provide complementary insights into climate and/or human-driven soil and hydrological processes (e.g., moisture balance, erosion, etc.) and vegetation shifts (e.g., crop cultivation, pasture and fire frequency). Here, we directly compare the fossil NPP assemblages extracted from the Chibwera and Kanyamukali sediment records to assess and validate their role as independent ecological indicators for unraveling past climatic and human impacts on tropical African ecosystems. Strictly pollen-analytical data are thus not included, however some selected pollen taxa from aquatics and cultivated plants, and spores from ferns and mosses are briefly discussed to support NPP evidence for local habitat changes. More specifically, this study aims to critically test the purported link between spores of coprophilous fungi and pastoralism in a tropical African context.