The spatial pattern of grasses in relation to tree effects in an arid savanna community: Inferring the relative importance of canopy and root effect

Both aboveground and belowground processes play important roles in tree-grass interactions in savannas. Little consideration has been given to within-site heterogeneity in the strengths of co-occurring canopy and root effects of trees on grasses in savanna communities. Here, we attempted to correlate the spatial pattern of grass morphological traits with the strengths of canopy and root effects. The results from a spatial analysis suggested that the grass traits had lower variability within the operating domain of the root effect than within that of the canopy effect in sub-canopy areas; in contrast, the operating domain of the root effect presented higher variability of grass traits than that of the canopy effect in inter-canopy areas. Combined with root investigations on vertical distribution patterns, these results suggested that the root effect appeared to outweigh the canopy effect in the sub-canopy areas, where apparent vertical root separation between trees and grasses was shown; while the canopy effect could outweigh the root effect in the inter-canopy areas, where root separation was not observed. This study could provide correlative information on the relative importance of canopy and root effects, and has some useful implications on within-site heterogeneity in terms of aboveground and belowground components in savannas.