Vegetation history in western Uganda during the last 1200 years: a sedimentbased reconstruction from two crater lakes

Abstract

Pollen, plant macrofossil and charcoal records from two neighbouring crater lakes (Lake Wandakara and Lake Kasenda) in lower montane (altitude-1200m) western Uganda (0.5DN, 30) reveal major changes in local and regional vegetation over the last 1200 years, which can be related to regional variations in climate (especially effective precipitation) and human impact. The biological signals are supplemented by evidence from physical sedimentary analyses (magnetic susceptibility, loss-on-ignition, dry mass accumulation rate) for major catchment changes in a wider regional context. From AD 750 to 900, medium-altitude moist forest existed around Lake Kasenda, which was largely replaced by grassland by AD 1000, coincident E _ = with evidence for increased catchment erosion and fire events, linked to human impact during a brief arid phase. Despite wetter conditions from AD 1000 to 1200 and in the 1400s, grassland prevailed around both sites until c. AD 1700-1750, with increased catchment disturbance from AD 1600 to 1700, shortly before the abandonment of major regional settlements and a switch to dispersed homesteads more reliant on pastoralism. A Both sites show the return of semi-deciduous and swamp forest from c. AD 1700 to 1800, with tree and shrub HOLOCENE pollen remaining high until the most recent times. Pollen assemblages reveal increasingly clear human impact RESEARCH on local and regional vegetation in the twentieth century, which obscures any climate signal. Pollen signals may PAPER not be sensitive to all climatic fluctuations (e.g., late eighteenth-early nineteenth-century aridity), but appear to record the varying impacts on vegetation communities of both climate and human activity over long timescales. Other proxies should be analysed to disentangle further the relative importance, and interactions, of these two major drivers of environmental change in tropical Africa in recent millennia.

Environmental change over the last millennium recorded in two contrasting crater lakes in western Uganda, eastern Africa (Lakes Kasenda and Wandakara)

“…Wandakara, ~½º N, 30º E) span the last 700 (Wandakara) and 1200 years (Kasenda), and have provided a regional history of vegetation and catchment change (Ssemmanda et al, 2005). Here we present diatom, chironomid, aquatic macrofossil, stable isotopic (δ 13 C org from organic matter; δ 13 C carb and δ 18 O carb from authigenic carbonates) and C/N records from these sequences to provide a multiproxy reconstruction of aquatic environmental changes over the last 1200 years, and set these in the context of late Holocene environmental change across eastern Africa.…”

“…There is little variation in the diurnal and monthly temperatures (daily variation 10 -15°C, mean monthly maxima 24 -27°C; Atlas of Uganda, 1962; recent data available from the Ugandan Government Department of Meteorology; www.meteo-uganda.net/). The natural vegetation of the region is low-altitude montane forest (Langdale-Brown et al, 1964;Ssemmanda et al, 2005;Taylor et al, 1999), which, outside of the national parks, has been subjected to widespread clearance to make way for small-scale agriculture and banana plantations.…”

“…regional climate change: Stager and Mayewski, 1997), may smooth out some of the inherent regional complexity by integrating responses over large areas, and, depending on their residence times, over longer timescales (Fritz, 2004). Recently, palaeolimnological studies from smaller lakes are yielding high resolution (10 0 -10 2 yr) records of environmental and climatic change (Bessems et al, 2008;Ssemmanda et al, 2005), often highlighting smaller-scale, shorter-term changes. Such studies can both complement, and challenge, the regional view afforded by palaeolimnological records of larger lakes.…”

“…Lamb et al, 2007b;Russell and Johnson, 2007;Stager et al, 2005;Verschuren et al, 2000). For example, questions remain over the spatial extent and synchroneity of climatic changes in eastern Africa and their relationships to events at higher latitudes, such as the extent of aridity in the 15 th -18 th centuries (coincident with the Little Ice Age; Ssemmanda et al, 2005;. Research focused on short-term, regionally specific archives of natural climate variability (cf.…”

“…Research focused on short-term, regionally specific archives of natural climate variability (cf. Ssemmanda et al, 2005;Bessems et al, 2008;Mills, 2009;Russell et al, 2009) have shown that, while there is great potential for extracting high quality, high resolution archives from the smaller lakes in western Uganda which can address such issues, these archives remain underexploited.…”