Modelling the impact of small-scale heterogeneities on tree-grass coexistence in semi-arid savannas

Abstract

1 Savanna ecosystems show codominance of trees and grasses, and the mechanisms involved in their coexistence remain unresolved. We investigated the possible influence of small-scale heterogeneities and disturbances in determining tree spacing and tree-grass coexistence in semi-arid savannas, using a spatially explicit, grid-based simulation model. 2 We added factors such as seed clumping, locally improved moisture conditions, vegetation clearing and combinations of all three of these variables to a previously published model. We also examined the influence of changing the size and spariotemporal correlation among individual heterogeneities. 3 Increased seed availability in localized clumps, especially in combination with the other heterogeneity types, had the strongest impact on the long-term tree-grass coexistence in the model. Localized deposition of tree seeds in herbivore dung and underground seed caches built by seed-collecting rodents may act as a determinant of the distribution of trees in savannas. 4 Spatially autocorrelated small-scale heterogeneities led to a more stable coexistence of trees and grasses than uncorrelated heterogeneities did, and at high levels of autocorrelation runs with all three variables led to long-term coexistence in up to 60% of the rainfall scenarios tested. 5 The size of individual heterogeneities also played a significant role in determining whether trees would establish as lone individuals or in clumps. In simulations that included small-scale heterogeneities, the number of isolated trees scattered throughout the landscape increased, whereas large heterogeneity patches and high spatio-temporal autocorrelation tended to promote tree clustering. 6 Simulations based on rainfall data from the southern Kalahari produced a realistic density and spatial distribution for trees in this semi-arid savanna for a realistic range of small-scale heterogeneities. 7 Small-scale disturbances and heterogeneities, at least those that furnish better establishment conditions for tree seedlings, therefore act to increase the range of environmental conditions under which trees and grasses can coexist as savanna.