title: The Consequences of Precipitation Seasonality for Mediterranean-Ecosystem Vegetation of South Africa.
abstract: Globally, mediterranean-climate ecosystem vegetation has converged on an evergreen, sclerophyllous and shrubby growth form. The particular aspects of mediterranean-climate regions that contribute to this convergence include summer droughts and relatively nutrient-poor soils. We hypothesised that winter-precipitation implies stressful summer droughts and leaches soils due to greater water availability (i.e. balance between precipitation and potential evapotranspiration; P-PET) during cold periods. We conducted a comparative analysis of normalised difference vegetation indices (NDVI) and edaphic and climate properties across the biomes of South Africa. NDVI was strongly correlated with both precipitation and P-PET (r2 = 0.8). There was no evidence, however, that winter-precipitation reduces NDVI in comparison to similar amounts of summer-precipitation. Base saturation (BS), a measure of soil leaching was, however, negatively related to P-PET (r2 = 0.64). This led to an interaction between P-PET and BS in determining NDVI, indicating the existence of a trade-off between water availability and soil nutrients that enables NDVI to increase with precipitation, despite negative consequences for soil nutrient availability. The mechanism of this trade-off is suggested to be that water increases nutrient accessibility. This implies that along with nutrient-depauperate geologies and long periods of time since glaciation, the winter-precipitation may have contributed to the highly leached status of the soils. Since many of the ecophysiological characteristics of mediterranean-ecosystem flora are associated with low nutrient availabilities (e.g. evergreen foliage, sclerophylly, cluster roots), we conclude that mediterranean-climates promote convergence of growth-forms in these regions through high leaching capacity.
authors.affiliation: Department of Biological Sciences, University of Cape Town, Cape Town, South Africa.
authors.forename: Michael D
authors.initials: MD
authors.lastname: Cramer
authors.affiliation: Department of Biological Sciences, University of Cape Town, Cape Town, South Africa.
authors.forename: M Timm
authors.initials: MT
authors.lastname: Hoffman
mesh_terms: D002980:Climate; D055864:Droughts; D017753:Ecosystem; D011891:Rain; D012621:Seasons; D012987:Soil; D013019:South Africa
chemical_list: D012987:Soil