Influence of competition and rainfall manipulation on the growth responses of savanna trees and grasses

Abstract

In this study, we explored how rainfall manipulation influenced competitive interactions between grasses and juvenile trees (small nonreproductive trees capable of resprouting) in savanna. To do this, we manipulated rainfall amount in the field using an incomplete factorial experiment that determined the effects of rainfall reduction, no manipulation, rainfall addition, and competition between grasses and trees on grass and tree growth. As response variables, we focused on several measures of tree growth and Disc Pasture Meter settling height as an estimate of grass aboveground biomass. We conducted the study over four years, at two sites in the Kruger National Park, South Africa. Our results show that rainfall manipulation did not have substantial effects on any of the measures of tree growth we considered. However, trees at plots where grasses had been removed grew on average 15 cm more in height and 1.3-1.7 times more in basal area per year than those in plots with grasses. Grass biomass was not influenced by the presence of trees but was significantly and positively influenced by rainfall addition. These findings were not fundamentally influenced by soil type or by prevailing precipitation, suggesting applicability of our results to a wide range of savannas. Our results suggest that, in savannas, increasing rainfall serves to increase the competitive pressure exerted by grasses on trees. The implication is that recruitment into the adult tree stage from the juvenile stage is most likely in drought years when there is little competition from grass for resources and grass fuel loads are low.

A dominance shift in arid savanna: An herbaceous legume outcompetes local C4 grasses

“…The impact of Crotalaria on Stipagrostis in contrast was consistently negative. Contrary to other studies on seedling competition (February, Higgins, Bond, & Swemmer, 2013), Crotalaria significantly reduced the growth rate of Stipagrostis . This negative effect of Crotalaria on Stipagrostis extends over the later growth stages of Crotalaria, increases with higher legume density, and is, other than the recruitment of Crotalaria , not compensated for by higher rainfalls and water availability.…”

“…Thereby, our interaction experiments support the findings of Maestre, Bautista, and Cortina (2003) who found seedlings of legumes to grow largely unaffected in living grass tussocks under stressful conditions similar to those of our study region. In the case of Crotalaria, the C 4 grass Stipagrostis is not, or probably no longer able to exert its supposed dominance (February et al., 2013; Riginos, 2009; Sankaran et al., 2004; Scholes & Archer, 1997) over the legume and suppress its recruitment. This clearly contradicts the widespread opinion that grasses are able to outcompete legumes and encroachers due to their competitive advantage in water‐limited environments (O’Connor et al., 2014; Riginos, 2009).…”

Trade-off analyses of multiple ecosystem services by plantations along a precipitation gradient across Loess Plateau landscapes

“…In contrast, the significantly higher trade-offs between UPD and AGC in the semiarid than those in the semi-humid areas may be caused by a weak competitive relationship. This may be partly due to the constraint of shading effects of trees on the understory vegetation (Barbier et al 2008;February et al 2013). Therefore, AGC sequestration outcompeted the development of UPD in the dry ecosystems.…”

Identifying drivers that influence the spatial distribution of woody vegetation in Kruger National Park, South Africa

“…Competition between trees and grasses in savannas is a complex topic. Rainfall and nutrient manipulation studies are becoming more common to increase understanding of tree-grass coexistence (e.g., Skarpe 1991, Jeltsch et al 1998, February et al 2013. Recently, February et al (2013) showed that additional rainfall tends to increase the competitive pressure exerted by grasses on trees to the extent that trees will only recruit into the adult stage during drought years.…”