Determining patch size

Abstract

Shrub encroachment, i.e. the increase in density of woodyspecies, is threatening tree-grass coexistence in savannasworldwide (see e.g. Smit, 2004; Wiegand, Ward & Saltz,2005; Wiegand, Saltz & Ward, 2006). In addition to eco-logical problems, shrub encroachment creates economicproblems, because it reduces the extent of areas suitablefor grazing of livestock. In recognition of the importance ofspatial and temporal scales for savannas, Wiegand et al.(2005, 2006) proposed patch-dynamics as the drivingmechanism of tree-grass coexistence in savannas includinga naturally shrub encroached phase. In patch-dynamiclandscapes, patches are asynchronously cycling betweenwoody and grassy dominance. Evidence for patch-dynamicsavannas is accumulating (e.g. Gillson, 2004; Wiegandet al., 2006), but simple methods for the determination ofthe spatial scale of patches are still lacking. In the presentstudy, we propose a method for estimating patch sizesbased on the canopy diameter and the spatial location ofindividuals and apply it to an example data set from asemi-arid savanna in South Africa.