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Climate of the Past An interactive open-access journal of the European Geosciences Union
Clim. Past, 10, 2067-2080, 2014
https://doi.org/10.5194/cp-10-2067-2014
© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
28 Nov 2014
Sensitivity of East African savannah vegetation to historical moisture-balance variation
I. Ssemmanda1, V. Gelorini2, and D. Verschuren2 1Department of Geology, Makerere University, P.O. Box 7062, Kampala, Uganda
2Limnology Unit, Department of Biology, Ghent University, K.L. Ledeganckstraat 35, 9000 Gent, Belgium
Abstract. Fossil pollen records provide key insight into the sensitivity of terrestrial ecosystems to climate change. However, tracing vegetation response to relatively modest historical climate fluctuations is often complicated by the overriding signature of anthropogenic landscape disturbance. Here we use high-resolution pollen data from a ~200-year lake-sediment record in open wooded savannah of Queen Elizabeth National Park (southwestern Uganda) to assess the sensitivity of the tropical lowland grassland–forest transition to historical, decade-scale moisture-balance fluctuations. Specifically we trace vegetation response to three episodes of higher average rainfall dated to the 1820s–1830s, ca. 1865–1890 and from 1962 to around 2000. Our pollen data indeed reveal a sequence of three wet periods, separated by two drier periods. During the inferred wetter episodes we find increases in the percent pollen abundance of trees and shrubs from moist semi-deciduous forest (Allophylus, Macaranga, Alchornea, Celtis), riparian forest (Phoenix reclinata) and wooded savannah (Acalypha, Rhus-type vulgaris, Combretaceae/Melastomataceae) as well as taxa common in the local rift-valley grasslands (Acacia, Ficus), together creating strong temporary reductions in Poaceae pollen (to 45–55% of the terrestrial pollen sum). During intervening dry periods, Poaceae pollen attained values of 65–75%, and dryland herbs such as Commelina, Justicia-type odora and Chenopodiaceae expanded at the expense of Asteraceae, Solanum-type, Swertia usambarensis-type, and (modestly so) Urticaceae. Noting that the overall richness of arboreal taxa remained high but their combined abundance low, we conclude that the landscape surrounding Lake Chibwera has been an open wooded savannah throughout the past 200 years, with historical moisture-balance variation exerting modest effects on local tree cover (mostly the abundance of Acacia and Ficus) and the occurrence of damp soil areas promoting Phoenix reclinata. The strong apparent expansion of true forest trees during wet episodes can be explained partly by enhanced pollen input via a temporarily activated upland stream. Pollen from exotic trees and cultural indicators appears from the 1970s onwards, but their combined influence fails to mask the signature of natural vegetation dynamics in the pollen record.

Citation: Ssemmanda, I., Gelorini, V., and Verschuren, D.: Sensitivity of East African savannah vegetation to historical moisture-balance variation, Clim. Past, 10, 2067-2080, https://doi.org/10.5194/cp-10-2067-2014, 2014.
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Short summary
Using fossil pollen assemblages to trace vegetation response to relatively modest historical climate fluctuations is difficult. In this study, pollen data from a lake-sediment record in Queen Elizabeth National Park (Uganda) show that its landscape has been an open wooded savannah throughout the past 200 years, with known historical moisture-balance variation exerting modest effects on local tree cover (mostly Acacia and Ficus) and the occurrence of damp soil areas (promoting Phoenix palms).
Using fossil pollen assemblages to trace vegetation response to relatively modest historical...
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