Abstract
The eastern communal conservancies are situated along the western fringe of the Kalahari basin. Under a very short rainfall gradient, the vegetation abruptly changes from microphyllous Acacia-dominated savannas to mesophyll savannas, dominated by Terminalia sericea and Combretum spp. We hypothesise that this is caused by changes in soil moisture availability brought about by changes in soil texture from loamy soils to deep sands (the ‘inverse texture effect’). For this analysis, we used vegetation and soils data derived from a recognisance survey of the natural resources of the study area. As the sites in the soil and vegetation surveys did not overlap, it was decided to use only synoptic data for the plant associations in the analysis. Non-metric multidimesional scaling ordination was utilised as ordination technique of the vegetation data and various environmental parameters, including soil texture, soil hydraulic parameters, climatic and fire regime parameters, were overlaid as biplots onto the resulting graph, as were various plant functional attributes particularly related to climatic conditions. The main environmental gradient identified within the study area is the rainfall gradient. This relatively short gradient, however, does not explain the marked change in vegetation observed within the study area. This change is attributed to the change in soil type, in particular, the soil texture and the associated soil hydraulic parameters of the soil. This gradient is closely correlated to leaf size, explaining the change from microphyll savannas to mesophyll savannas along the change from loamy to sandy soils. One of the lesser understood mechanisms for the survival of these mesophyll plants on sandy soils seems to be a deep root system, which is actively involved in water redistribution within the soil profile – by hydraulic lift, inverse hydraulic lift and stem flow. Conservation implications: Understanding these mechanisms will greatly assist in understanding savanna dynamics. With the threat of global climate change, we postulate that the vegetation will gradually change from the present mesophyll to a microphyll savanna, but that the grass sward will probably not develop very well. Shrub and tree removal (‘bush harvesting’) is likely to speed up the desertification process within this area.
Highlights
In the first article of this series (Strohbach 2014), the vegetation of eight communal conservancies, as well as two farming areas in east-central Namibia, are described
Under similar climatic conditions, the vegetation abruptly changes from a microphyll, Acacia-dominated savanna to a mesophyll savanna dominated by Terminalia sericea and various Combretum species, as the soils change from relative fine-grained soils of the central plateau to deep, coarse sands of the Kalahari basin (Leser 1972)
On an aridity gradient stretching over roughly 1100 km from north-northwest to south-southwest within the Kalahari Desert of Botswana, macrophyllous trees are gradually replaced by microphyllous trees as the rainfall decreases and the aridity increases (Skarpe 1996)
Summary
In the first article of this series (Strohbach 2014), the vegetation of eight communal conservancies, as well as two farming areas in east-central Namibia, are described These conservancies are situated within the Camelthorn savanna in the south and the Tree savanna and woodlands described by Giess (1998) in the north. On an aridity gradient stretching over roughly 1100 km from north-northwest to south-southwest within the Kalahari Desert of Botswana (on uniformly sandy soils), macrophyllous trees are gradually replaced by microphyllous trees as the rainfall decreases and the aridity increases (Skarpe 1996). More abrupt changes in the composition of moist savannas of central Mozambique (ranging from grasslands to savannas, to woodlands, forests and swamp forests) have, in detail, been demonstrated by Tinley (1982) to be related to differences in soil profiles, especially with regard to soil texture, soil depth and the presence and depth of constricting layers within the profile, despite a uniform climate
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