LANDSCAPE AND CLIMATIC CONTROL OF WOODY VEGETATION IN A DRY TROPICAL ECOSYSTEM - TURKANA DISTRICT, KENYA

The spatial organization of a dry woodland/savanna/shrub-steppe ecosys tem in a 9000 km2 region of arid Northern Kenya was explored by analys ing the abundance and distribution of woody vegetation in relation to landscape gradients and gradients in rainfall. Woody species assemblag es were clustered into four major groups. Three of these (and most of the sites) were dominated by species of Acacia. Acacia tortilis was th e community dominant in riparian and riverine zones, A. senegal on hil ly and rocky sites, and A. reficiens on non-riparian sites with fine s oils. The fourth group, found at highest elevations, was distinguished by a very low abundance of Acacia. These soil/landform associations w ere systematically distributed along land system gradients (mountains, pediment, valley, bajada), thus relating woody species groups to larg e-scale landscape characteristics. Tree woody canopy cover ranged from < 1.0% to 100% over the region as a whole. Cover was directly related to precipitation when the effects of water concentration in riparian zones were removed. However, canopy cover was not greatly influenced b y either species composition or landscape characteristics. Fire reduce d woody canopy cover, however, fires appeared to affect a relatively s mall portion of the ecosystem. Recent anthropogenic disturbances such as wood-cutting and livestock corralling were encountered in patches, but the cumulative long-term effects of patch scale disturbances could not be discerned in vegetation patterns at the regional scale. Vegeta tion physiognomy (woodland, bush, bushed grassland, etc.) was controll ed by both water availability and landscape pattern. Woodland and fore st occurred almost exclusively in riparian and riverine situations whi le the driest parts of the region supported dwarf shrub grassland with few trees. Over the region as a whole, climate and landscape gradient s converged in diverse ways, giving rise to structurally variable asso ciations of woody plant species. Vegetation structure in tropical sava nnas and dry woodlands is often interpreted in terms of competition be tween woody and herbaceous life forms for soil moisture. The outcome o f this competitive interaction is thought to be influenced by disturba nces that shift the system from one stable state to another. However, our findings suggest that dry tropical ecosystem structure is hierarch ically constrained by physical factors: by climate at regional to cont inental scales; by topographic effects on rainfall and landscape water redistribution, and geomorphic effects on soil and plant available wa ter at the landscape to regional scales; and finally by water redistri bution and disturbance at local and patch scales.