Root trait variation in African savannas

Abstract

While patterns of variation in woody plant aboveground traits related to disturbance and resource availability in savanna ecosystems are fairly well understood, dimensions of variation in belowground traits remain poorly understood. We investigate how sapling coarse root (>2 mm diameter) traits and belowground storage organs vary with respect to differing disturbance regimes (i.e. fire or herbivore dominated) and soil fertility in Southern African savannas. We examined how sapling rooting strategies (type of rooting system, storage organs, root allocation) as well as root total non-structural carbohydrate concentrations [TNC], root N and C:N of 69 common woody savanna and forest species at 16 sites vary with respect to differing disturbance regimes and soil fertility. We found that root [TNC] and the occurrence of storage organs were highest while root N was lowest for species growing at fire-driven sites, compared to herbivore-driven sites and competition-controlled forest species. Allocation to belowground biomass was higher at fire-dominated sites but root volume:stem basal area did not differ between disturbance regimes. None of the measured traits were found to differ between nutrient rich clayey soils and nutrient poor sandy soils. Our results suggest that disturbance related controls are important drivers of savanna belowground traits.