Spatial variation in functional traits and grazing value of savanna grasses at the community and species level

Abstract

Compositional and functional plant community characteristics can strongly govern some ecosystem processes. These community characteristics, may, in turn, be related to multiple environmental variables, including edaphic, climatic, and topographic conditions. However, there is still an incomplete understanding of how environmental conditions affect plant community composition and functioning and, therefore, how species composition and trait expression potentially link habitat conditions to ecosystem processes. This is particularly true for grasses, despite this taxon having considerable ecological and economic importance. The grass family (Poaceae) is a species-rich taxon, with close to 10 000 species occurring worldwide, and is chiefly responsible for providing the key ecosystem service of grazing provisioning since grass species make up the bulk of the herbaceous plant growth in grasslands and savannas. The grazing quality of grasses is assumed to vary strongly between species, with intra-specific variation in grazing quality often ignored, resulting in grass species often uniformly being categorized as being of high or low grazing quality. There is, however, an increasing debate about the validity of this approach since many grass characteristics (including plant functional traits; PFTs) vary intra-specifically along environmental gradients. Therefore, this study examined the relationship between 19 environmental factors and community composition and cover, community weighted mean (CWM) trait values, intra-specific trait variation and the grazing quality in two C4- dominated savanna grass assemblages. This study demonstrated that the relationships between grazing quality and PFTs and environmental variables are typically weak and highly idiosyncratic at both the community- and species-level. Cover and soil (particularly soil nutrients) variables were most consistently influential environmental variables. Grazing quality (i.e. nutritional value) differed significantly between grass species, with large proportions of the variation in all grazing quality components being explained by the identity of species. However, within species, few environmental variables explained intra-specific variation in grazing quality. Additionally, plant functional traits were also weakly related to intra-specific variation in grazing quality. Therefore, these findings suggest that grass grazing quality and PFTs do not respond consistently to environmental variables frequently quantified in ecological studies and that, contrary to results from C3-dominated temperate grasslands, leaf dry matter content, specific leaf area and force to tear are not useful proxies of grazing quality. More broadly, this study highlights that there may be considerable intra-specific variation in grass grazing quality, but that this within-species variation is not related to commonly recorded environmental conditions or easily measured plant traits, and, therefore, remains challenging to predict.