Miombo Ecoregion Vision Report - Biodiversity Foundation for Africa

Miombo Ecoregion Report, page 30
4. ECOLOGICAL DETERMINANTS AND PROCESSES
4.1 Ecological Determinants
The major ecological determinants in the Miombo Ecoregion are climate, soil moisture, soil
nutrients, herbivory, fire and human use. Ecosystem diversity in the Miombo Ecoregion has
evolved from the interaction between geology and the subhumid climate with seasonal rainfall.
This interaction has given rise to a generally flat topography and sluggish drainage. The
hydrology is therefore characterised by a mosaic of dambos and seasonally flooded grasslands,
especially in the headwaters of the major rivers. The ecoregion exhibits landscape heterogeneity
both at large and small scale. Consequently, it has higher between-habitat (beta) diversity than
within-habitat (alpha) diversity. This implies that conservation of biodiversity is better achieved
by conserving landscapes than isolated habitats.
4.2 Biophysical Processes
Whereas the broader ecoregion boundaries are controlled by climatic factors, the internal spatial
variability is controlled more by edaphic factors, including soil moisture, soil nutrients and soil
texture. However, the shallow plateau soils with a lateritic zone imply rapid soil moisture
saturation during the rainy season which can potentially generate much surface and subsurface
run-off. In many parts of the ecoregion, the vegetation cover that allows rainwater to infiltrate
and recharge deep soil water also regulates this soil-water relationship. Lateral tree roots occupy
a large part of the soil volume in Brachystegia woodland, which in turn protects the soil and
conducts subsurface lateral flow. Run-on areas also reduce erosion as they act as sediment sinks.
Widespread clearing of woodland cover on run-off areas can therefore disrupt the normal
functioning of these hydrological processes through increased run-off, erosion and accelerated
sedimentation of run-on areas with considerable impacts on wetland biodiversity. The
understanding of these crucial linkages and feedback processes between physical and biological
processes is important for conservation.
The linkage between seasonality in rainfall and primary production is well known in the Miombo
Ecoregion. But landscape heterogeneity creates temporal and spatial differences in primary
production, especially between run-off and run-on areas. For example, run-on areas, such as
dambos, floodplains and swamps, sustain production even during the dry season, thereby freeing
such areas from the constraint of rainfall seasonality. Hydrological processes that sustain this
heterogeneity in soil moisture are themselves influenced by vegetation cover.
Seasonality in rainfall confines production by shallow-rooted plants to the rainy season,
especially on run-off landscapes. However, deep-rooting plants, especially trees, have access to
soil moisture stored at depth during most of the dry season. In this case, one of the key
determinants of primary production is temperature. For example, Brachystegia woodland is
deciduous for only a short period in the dry season, and may even be semi-evergreen in the
wetter parts of the ecoregion. Annual growth often starts during spring (September) with the new
colourful leaf flush, and stem expansion continues until May in the following year. Cessation of
growth appears to be triggered by low temperatures (