Research paperValidation of non-pollen palynomorphs as paleoenvironmental indicators in tropical Africa: Contrasting ~ 200-year paleolimnological records of climate change and human impact
Highlights
► NPPs are validated as paleoenvironmental indicators in tropical Africa. ► Changes in fungal assemblages are related to climate variability and human impact. ► Only high densities of herbivores create a paleoenvironmental NPP signature. ► Local substrate controls the distributions of facultative coprophilous fungal spores.
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.
Section snippets
The study lakes and their environmental setting
Our two study lakes are located in southwestern Uganda, in the Edward–George branch of the East African Rift System southeast of the Ruwenzori Mountains (Fig. 1). This tropical sub-humid region experiences a seasonally bimodal rainfall regime, with long rains during spring (March–May) and short rains during autumn (October–December). The wet seasons are strongly driven by monsoonal wind systems from the Indian Ocean, modulated by the passage of the Intertropical Convergence Zone (ITCZ) and the
Sediment cores, lithostratigraphy and chronology
Sediment cores were collected in 2001 and 2002 from the deepest parts of Lake Chibwera (CHIB02-1P-I and -II) and Lake Kanyamukali (KANYA01-1P and KANYA02-1P-I and -II) using a rod-operated single-drive piston corer (Wright, 1980). The uppermost, unconsolidated sediments (~ 25–30 cm) near the sediment–water interface were retrieved and sectioned upright with a fixed-interval sectioning device (Verschuren, 1993) in 1-cm increments into Whirl-Pack™ bags. The deeper, more consolidated sediments were
Lake Chibwera
CONISS stratigraphic zonation of NPP assemblages in the Chibwera sediment record defines five non-pollen palynomorph assemblage zones (NPPZ, Ch-1 to Ch-5), with Ch-2 and Ch-5 subdivided in two distinct sub-zones (Fig. 5). The fungal spore abundances varied from 21 to 187% of the non-local pollen sum, with the highest values (> 125%) in zone Ch-1 and Ch-2a and lowest values (< 43%) in zone Ch-4.
Zone Ch-1 (126–112 cm, late 18th century) corresponds stratigraphically with the dry, mostly low-organic
Environmental similarities and differences between Lakes Chibwera and Kanyamukali
Lakes Chibwera and Kanyamukali have experienced similar major environmental trends during the past 200 years. Given the proximity of these lakes, the similar natural landscape in which they are located, and the combined evidence of multi-proxy paleoenvironmental data (Bessems et al., 2008; Audenaert et al., Ssemmanda et al., unpublished data), they can be expected to have shared a similar climate-driven environmental history.
During the late 18th-century regional drought, shallow lake conditions
Conclusion
Paired study of fossil non-pollen palynomorph records from two small, relatively shallow crater lakes in western Uganda allowed us to separate the environmental responses of both lakes to historical climate variability and local human impact over the past 200 years. The NPP assemblages of Lakes Chibwera and Kanyamukali were clearly affected by habitat change associated with lake-level fluctuation and moisture changes in the surrounding lake catchments. The marked presence of the obligately
Acknowledgments
This research was sponsored by the ‘Science for a Sustainable Development’ programme of the Belgian Federal Science Policy (project SD/BD/03 CLANIMAE). We thank Bas van Geel and Annemarie Philip (Institute for Biodiversity and Ecosystem dynamics, University of Amsterdam) for sample preparation. This paper is dedicated to Bas van Geel on the occasion of his long years of devotion to the study of non-pollen palynomorphs. Bas enthusiastically supported many palynologists (including the first
References (92)
- et al.
Palaeolimnological evidence for widespread late 18th century drought across equatorial East Africa
Palaeogeography, Palaeoclimatology, Palaeoecology
(2008) - et al.
A revised 30,000-year paleoclimatic and paleohydrologic history of Lake Albert, East Africa
Palaeogeography, Palaeoclimatology, Palaeoecology
(1997) - et al.
Sporormiella fungal spores, a palynological means of detecting herbivore density
Palaeogeography, Palaeoclimatology, Palaeoecology
(2006) - et al.
Modern non-pollen palynomorphs from East African lake sediments
Review of Palaeobotany and Palynology
(2011) CONISS: a FORTRAN 77 program for stratigraphically constrained cluster analysis by the method of incremental sum of squares
Computers & Geosciences
(1987)- et al.
Palynological evidence of vegetation change and land degradation in the Lake Baringo area, Kenya, East Africa during the Late Holocene
Palaeogeography, Palaeoclimatology, Palaeoecology
(2009) - et al.
Coprophilous fungi
- et al.
A new species of Coniochaeta with a key to the species known in Argentina
Mycological Research
(1999) - et al.
Human impacts, climate change, and aquatic ecosystem response during the past 2000 yr at lake Wandakara, Uganda
Quaternary Research
(2009) - et al.
Environmental change over the last millennium recorded in two contrasting crater lakes in western Uganda, eastern Africa (Lakes Kasenda and Wandakara)
Quaternary Science Reviews
(2011)