Grassland structural heterogeneity in a savanna is driven more by productivity differences than by consumption differences between lawn and bunch grasses.

Michiel P Veldhuis,Heleen F Fakkert,Han Olff,Matty P Berg

doi:10.1007/s00442-016-3698-y

Michiel P Veldhuis, Heleen F Fakkert + Show 2 more

Open Access

https://doi.org/10.1007/s00442-016-3698-y

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Journal: Oecologia	Publication Date: Aug 13, 2016
Citations: 50	License type: CC BY 4.0

Affiliation: University of Groningen, Vrije Universiteit Amsterdam

Abstract

Savanna grasslands are characterized by high spatial heterogeneity in vegetation structure, aboveground biomass and nutritional quality, with high quality short-grass grazing lawns forming mosaics with patches of tall bunch grasses of lower quality. This heterogeneity can arise because of local differences in consumption, because of differences in productivity, or because both processes enforce each other (more production and consumption). However, the relative importance of both processes in maintaining mosaics of lawn and bunch grassland types has not been measured. Also their interplay been not been assessed across landscape gradients. In a South African savanna, we, therefore, measured the seasonal changes in primary production, nutritional quality and herbivore consumption (amount and percentage) of grazing lawns and adjacent bunch grass patches across a rainfall gradient. We found both higher amounts of primary production and, to a smaller extent, consumption for bunch grass patches. In addition, for bunch grasses primary production increased towards higher rainfall while foliar nitrogen concentrations decreased. Foliar nitrogen concentrations of lawn grasses decreased much less with increasing rainfall. Consequently, large herbivores targeted the biomass produced on grazing lawns with on average 75 % of the produced biomass consumed. We conclude that heterogeneity in vegetation structure in this savanna ecosystem is better explained by small-scale differences in productivity between lawn and bunch grass vegetation types than by local differences in consumption rates. Nevertheless, the high nutritional quality of grazing lawns is highly attractive and, therefore, important for the maintenance of the heterogeneity in species composition (i.e. grazing lawn maintenance).Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-016-3698-y) contains supplementary material, which is available to authorized users.

Highlights

Savanna grasslands are characterized by high spatial heterogeneity, with a diverse species assemblage that exhibits a wide variety of plant traits
Periodic primary productivity of both lawn and bunch grasses was strongly positively related to periodic rainfall (Table 1; Fig. 2b)
Closer investigation on separate models per vegetation type (Table 2) shows that annual production in bunch grasses was positively related to annual rainfall, but leveled off with increasing amounts of rainfall towards a threshold of ca. 1000 g m−2 (Fig. 2a, c; Table 2, significant negative quadratic term)

Summary

Introduction

Savanna grasslands are characterized by high spatial heterogeneity, with a diverse species assemblage that exhibits a wide variety of plant traits. Grazing lawn patches, existing of short (0–20 cm) stoloniferous grass species with high foliar nutrient concentrations (McNaughton 1984; Stock et al 2010; Hempson et al 2014) and bunch grassland patches, consisting of medium/ tall (>30 cm) and generally nutrient-poor grass species. This differentiation results in lawn-bunch mosaics that exhibit high spatial heterogeneity in both food quantity and quality for herbivores and have important implications for other trophic levels. Good understanding of the determinants of this type of spatial heterogeneity in vegetation structure is needed

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