Dry Tropical Ecosystems Research Articles

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 ecosystem in a 9000 km2 region of arid Northern Kenya was explored by analysing the abundance and distribution of woody vegetation in relation to landscape gradients and gradients in rainfall. Woody species assemblages were clustered into four major groups. Three of these (and most of the sites) were dominated by species of Acacia. Acacia tortilis was the community dominant in riparian and riverine zones, A. senegal on hilly and rocky sites, and A. reficiens on non-riparian sites with fine soils. The fourth group, found at highest elevations, was distinguished by a very low abundance of Acacia. These soil/landform associations were systematically distributed along land system gradients (mountains, pediment, valley, bajada), thus relating woody species groups to large-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 by either species composition or landscape characteristics. Fire reduced woody canopy cover, however, fires appeared to affect a relatively small 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. Vegetation physiognomy (woodland, bush, bushed grassland, etc.) was controlled by both water availability and landscape pattern. Woodland and forest occurred almost exclusively in riparian and riverine situations while the driest parts of the region supported dwarf shrub grassland with few trees. Over the region as a whole, climate and landscape gradients converged in diverse ways, giving rise to structurally variable associations of woody plant species. Vegetation structure in tropical savannas and dry woodlands is often interpreted in terms of competition between woody and herbaceous life forms for soil moisture. The outcome of this competitive interaction is thought to be influenced by disturbances that shift the system from one stable state to another. However, our findings suggest that dry tropical ecosystem structure is hierarchically constrained by physical factors: by climate at regional to continental scales; by topographic effects on rainfall and landscape water redistribution, and geomorphic effects on soil and plant available water at the landscape to regional scales; and finally by water redistribution and disturbance at local and patch scales.

Influence of soil properties on microbial C, N, and P in dry tropical ecosystems

Relationships between soil physicochemical characteristics and soil microbial C, N, and P in Indian dry tropical ecosystems are discussed. The major ecosystem studies were on forest, savanna, cropped fields, and mine spoils. The highest microbial C, N, and P levels were recorded from the mixed forest and the lowest levels in 5-year-old mine spoil. Across the sites, microbial C ranged from 226 to 643 μg g-1, microbial N from 19 to 71 μg g-1, and microbial P from 9 to 28 μg g-1 soil. The proportion of soil organic C contained in the microbial biomass ranged from 2.2 to 5.0%. The microbial C: N ratio in these soils ranged from 7.4. to 13.1 and the microbial C: P ratio from 16.6 to 30.6. The concentrations of microbial C, N, and P were correlated with several soil properties and among themselves. The soil properties, in various linear combinations, explained 90–99% of the variability in the microbial nutrients. Grazing of the savanna had some effect on the level of microbial biomass, and as the mine spoil aged, the level of microbial C, N, and P also increased.

The role of seed banks in vegetation dynamics and restoration of dry tropical ecosystems

This paper reviews studies on seed banks in tropical dry vegetation and, for comparison, some wet tropical and dry subtropical vegetation. A first general conclusion is that tropi- cal seed banks are smaller than those in temperate ecosystems. Many studies are devoted to only one species, among which are several Acacia species, both tropical and subtropical, some of which reach densities in the seed bank of up to 10 000 seeds/ m2. The lesser importance of the seed bank strategy in tropical species may be related to higher risks of seed loss through higher mortality (fire, predation, pathogens etc.), or to inter- mittent germination occasions in relation to ephemeral favour- able conditions (e.g. rains during the dry season). Regarding vegetation dynamics, the importance of seed banks and regen- eration from seeds is most pronounced in larger forest gaps. Examples are discussed of rapid succession to forest of savan- nas after protection from fire, which is partly due to regenera- tion from the seed bank. The significance of seed banks in vegetation restoration projects is outlined and the need for additional sowing of seeds of important species underlined.

Termites, soil fertility and carbon cycling in dry tropical Africa: a hypothesis

ABSTRACTTermites, particularly the mound building, fungus growing Macrotermitinae, reach densities of up to 400 termites m−2in soils of dry tropical Africa. The influence of Macrotermi tinae in increasing certain soil nutrients in mounds compared to adjacent soils has been documented, but the links between litter harvesting by termites, soil fertility, and global C cycling have not been explored. This study reviews the evidence from soil science, ecology and atmos pheric chemistry and generates hypotheses to explain the role of termites in dry tropical eco systems. It is suggested that termite activity exhaustively partitions litterfall among adjacent com peting colonies, where it is so thoroughly decomposed that little or no organic C is incorporated into the soils. Associated N, P, and cations build up in the mounds, but C apparently is emitted as CO2and CH4from the mounds. While not adequate to calculate nutrient fluxes through termites, the data available support the argument that termites contribute significantly to atmospheric fluxes of CO2and CH4. Moreover, they suggest a coupling of regional soil forming processes and the global C budget.