Grass competition is more important than fire for suppressing encroachment of Acacia sieberiana seedlings

Abstract

Understanding the factors controlling tree seedling recruitment is an integral research priority for savanna ecosystems, particularly for the management of woody plant encroachment. We investigated the effects of increased nutrients, fire and grazing on the survival, growth and biomass of Acacia sieberiana using a completely randomized experimental design in the greenhouse. We simulated a typical savanna microhabitat by planting a common savanna grass species, Eragrostis curvula, with seedlings of A. sieberiana. The trees and grasses were allowed to establish for 12 weeks. Thereafter, treatments of fertilizer (100 kg N/ha), simulated grazing and fire were applied. Weekly measurements of relative growth rate (RGRheight and RGRdiameter) were recorded. At the end of 24 weeks, survival, total above- and below-ground biomass, and key functional traits of the trees (specific leaf area and average thorn length) were recorded to assess differences in tree performance. We found that fertilization was beneficial to the grass only, increasing their total biomass. While, the direct removal of grass biomass by grazing in our study revealed no main (statistical) effect on tree seedling growth, we observed decreased survival of tree seedlings with higher grass biomass. Fire increased the tree seedlings’ investment in shoot growth and stem diameter. However, in the absence of grass competition, there were no differences in the post-fire allocation of the resprouting tree seedlings. Higher grass biomass was found to result in decreased specific leaf area of tree seedlings. We conclude that grass competition is important for controlling the encroachment of A. sieberiana.