Tree (Prosopis glandulosa) effects on grass growth: An experimental assessment of above- and belowground interactions in a temperate savanna

Savanna trees can impose above- and belowground effects on the herbaceous layer by changing water, nutrients and microclimate. Proposed mechanisms governing savanna tree-on-grass interactions include: (1) improved fertility and structure of soils below tree crowns; (2) improved water relations of shaded plants; and (3) increased competition for light, soil moisture and nutrients. To assess the relative importance and outcome of these interacting positive and negative factors, we conducted a series of field experiments that altered the presence and absence of tree canopy and tree roots at locations both immediately under trees and in interstitial locations in a mesquite (Prosopis glandulosa) savanna. Basal area, tiller density and production of the dominant C3 grass, Nassella leucotricha, and herbaceous layer annual net primary production (ANPP) were quantified in 1998 and 1999. Annual rainfall during these 2 years was substantially below normal and most responses to treatments were neutral. However, a significant reduction in herbaceous ANPP, largely annual C3 grasses, indicated that belowground competition rather than facilitation was the mechanism controlling tree effects on grass in this savanna. Lower than average rainfall was a potentially overriding factor. Hence, it is possible that other tree-on-grass mechanisms might operate under average or above-average rainfall years.