Immaculate Ssemmanda

Study of the modern pollen rain in Western Uganda with a numerical approach

Modern soil samples from Western Uganda, from a range of ten plant communities belonging to five African phytogeographical regions and distributed along an altitudinal gradient from 650 m (grass savannas) to 4400 m (Afroalpine moorland) were analyzed for pollen content to define modern pollen/vegetation relationships. Correspondence analysis applied to the pollen counts (100 sites and 167 taxa) indicates four distinctive vegetation types arranged along an altitudinal gradient and thus a temperature one with respect to axis 1(contrast between montane and lowland vegetations), and along a physiognomical gradient (from densely structured to open vegetations) defined by axis 3. These results confirm the empirical interpretation proposed on the initial pollen data set and are in agreement with those previously obtained on modern or fossil pollen spectra from other African regions.

Contrasting long-term records of biomass burning in wet and dry savannas of equatorial East Africa

Rainfall controls fire in tropical savanna ecosystems through impacting both the amount and flammability of plant biomass, and consequently, predicted changes in tropical precipitation over the next century are likely to have contrasting effects on the fire regimes of wet and dry savannas. We reconstructed the long-term dynamics of biomass burning in equatorial East Africa, using fossil charcoal particles from two well-dated lake-sediment records in western Uganda and central Kenya. We compared these high-resolution (5 years/sample) time series of biomass burning, spanning the last 3800 and 1200 years, with independent data on past hydroclimatic variability and vegetation dynamics. In western Uganda, a rapid (<100 years) and permanent increase in burning occurred around 2170 years ago, when climatic drying replaced semideciduous forest by wooded grassland. At the century time scale, biomass burning was inversely related to moisture balance for much of the next two millennia until ca. 1750 ad, when burning increased strongly despite regional climate becoming wetter. A sustained decrease in burning since the mid20th century reflects the intensified modern-day landscape conversion into cropland and plantations. In contrast, in semiarid central Kenya, biomass burning peaked at intermediate moisture-balance levels, whereas it was lower both during the wettest and driest multidecadal periods of the last 1200 years. Here, burning steadily increased since the mid20th century, presumably due to more frequent deliberate ignitions for bush clearing and cattle ranching. Both the observed historical trends and regional contrasts in biomass burning are consistent with spatial variability in fire regimes across the African savanna biome today. They demonstrate the strong dependence of East African fire regimes on both climatic moisture balance and vegetation, and the extent to which this dependence is now being overridden by anthropogenic activity.

Vegetation Changes and Their Climatic Implications for the Lake Victoria Region during the Late Holocene

The pollen sequence of the core V95-2P (00°58.67’ S, 33°27.32’ E, 67 m depth) from Lake Victoria mirrors larger extension of forests between ca 6500 and ca. 4100 yr BP, implying a wetter climatic conditions than during the later period. Highest humidity was experienced in the region prior to ca. 6500 yr BP, before the semi-deciduous forest formations became widespread in the region. From ca. 5000 yr BP, the forests around Lake Victoria were mainly of semidecideous character with increasing abundance of celtis associated with Noloptelea grandis, mixed with some Guineo-Congolian elements such as Tetrorchidium The period ca. 4100 to ca. 3000 yr BP shows a progressive decline of the semideciduous forest formations and the establishment of open vegetation with Capparidaceae and Gramineae attesting to a dry climate. After ca. 3000 yr BP, the pollen data particulary that from high altitude, evidence an amelioration of climate. The dry montane forest with Podocarpus and Juniperus procera underwent significant extension which reached a maximum at ca. 1700 yr BP. At low altitude, the extension of the semideciduous forests in relation to this improvement of climate, is evidenced mainly until ca. 2200 yr BP. During this sub-humid climatic phase, either the amount of precipitation was inadequate or the dry season was too long for a large development of evergreen forests in the Lake Victoria region. Pollen data from this core and from the other cores in the region indicates that the increase in rainfall during this period was larger and lasted for a longer duration in the high altitude sites than at low altitude. From ca. 1700 yr BP, the significant decrease in the abundance of the pollen of the regional taxa indicates a decline of the Juniperus-Podocarpus dry montane forest or a reduction in precipitation at high altitude. During this dry period, both the montane and the semideciduous forests decline at the profit of the open grass dominated formations: open woodlands and probably savannas.