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.