Local Versus Landscape-Scale Effects of Savanna Trees on Grasses

Authors: RIGINOS, C - UNIVERSITY OF CALIFORNIA; GRACE, J - USGS; Augustine, David; YOUNG, T - UNIVERSITY OF CALIFORNIA

Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/28/2009
Publication Date: 10/1/2010
Citation: Riginos, C., Grace, J.B., Augustine, D.J., Young, T.P. 2010. Local Versus Landscape-Scale Effects of Savanna Trees on Grasses. Journal of Ecology. 25(5):310-318.

Interpretive Summary: Savanna trees can have both positive and negative effects on the grasses with which they share the ecosystem. At the scale of individual trees, a number of studies have found net positive effects of trees on sub-canopy grass nutrient concentrations and biomass. At the landscape scale, others have found negative effects of high tree densities on grass productivity. These disparate approaches have led to different conclusions about the effects of trees on forage quality and ungulate nutrition in savanna rangelands and conservation areas. Here we integrate these approaches by examining the effects of trees on grasses at both spatial scales and across a range of landscape-scale tree densities. We quantified grass biomass, species composition, and nutrient concentrations in a savanna dominated by the whistling thorn acacia (Acacia drepanolobium)in Laikipia, Kenya. Individual trees had positive effects on the amount of grass under their canopies, most likely because trees enrich soil nitrogen. Grass leaf phosphorus in sub-canopy areas, however, was depressed relative to grass phosphorus in inter-canopy areas. The abundance of two of the dominant grass species, including the species with the highest leaf protein content, appeared to be influenced by the canopies of individual trees. The abundance of the remaining two species, however, appeared to be driven by other factors operating at the landscape scale. Grass leaf nitrogen was positively related to tree cover. The negative effects of trees on phosphorus levels in grass leaves, however, combined with depressed grass productivity in areas of high tree cover, suggest the ungulate nutrition may be compromised in areas with many trees. We conclude that isolated trees may have positive effects on forage, but at high tree densities, the negative landscape-scale effects are likely to outweigh the positive local effects of trees on grass biomass.

Technical Abstract: Savanna trees can have a variety of effects, both positive and negative, on the grasses with which they share the ecosystem. Studies of the effects of trees on grasses have focused on these effects at two different spatial scales. At the scale of individual trees, a number of studies have net positive effects of trees on sub-canopy grass nutrient concentrations and biomass. At the landscape scale, others have negative effects of high tree densities on grass productivity. These disparate approaches have led to different conclusions about the effects of trees on forage quality and ungulate nutrition in savanna rangelands and conservation areas. Here we integrate these approaches by examining the effects of trees on grasses at both spatial scales and across a range of landscape-scale tree densities. We quantified grass biomass, species composition, and nutrient concentrations in these different contexts in an Acacia drepanolobium savanna in Laikipia, Kenya. Individual trees were found to have positive effects on grass biomass, most likely because trees enrich soil nitrogen. Grass leaf phosphorus in sub-canopy areas, however, was depressed relative to grass phosphorus in inter-canopy areas. The effects of increasing landscape-scale tree cover were not always predictable based on individual tree effects. The abundance of two of the dominant grass species, including the species with the highest leaf protein content, appeared to be explained by individual tree effects; their biomass in the landscape was directly related to overall canopy cover. The abundance of the remaining two species, however, appeared to be driven by other factors operating at the landscape scale. Grass leaf nitrogen was positively related to tree cover. The negative effects of trees on phosphorus levels in grass leaves, however, combined with depressed grass productivity in areas of high tree cover, suggest the ungulate nutrition may be compromised in areas with many trees. We conclude that few, isolated trees may have positive effects on forage, but at high tree densities, the negative landscape-scale effects are likely to outweigh the positive local effects of trees on grass biomass.