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Grasslands and savannas are experiencing intensive land-cover change due to woody plant encroachment. This change in land cover is thought to alter soil carbon (C) and nitrogen (N) storage in these ecosystems. Some studies have reported a negative correlation between soil C and N and mean annual precipitation while others have indicated that there is no relationship with mean annual precipitation. We quantified the changes in C and N pools and δ13C and δ15N values to a depth of 1 m in pairs of encroached and adjacent open grassland sites along a precipitation gradient from 300 mm to 1500 mm per annum in South Africa. Our study showed a negative correlation between changes in soil organic C stocks in the 0–100 cm soil layer and mean annual precipitation (MAP). The most humid site (1500 mm MAP) had less C in shrub-encroached sites while the drier sites (300–350 mm MAP) had more C than their paired open grasslands. This study generally showed soil organic C gains in low precipitation areas, with a threshold value between 750 mm and 900 mm. Our threshold value was higher than that found in North America, suggesting that one cannot extrapolate across continents.

Questions What is the effect of neighbour competition on the survival, growth and biomass of mesic and humid savanna tree species? Can competitive effect and response be linked to plant functional traits? Location Neil Tainton Arboretum, University of KwaZulu‐Natal, Pietermaritzburg, ZA. Methods Using a target–neighbour design, all combinations of four humid and four mesic savanna tree seedlings were tested in a greenhouse experiment, to establish the effects of neighbourhood competition on target plant performance. The competitive response and effect of each species was quantified, and regressed against several functional traits to determine which traits are predictive of competitive ability. Results We found that neighbour density negatively affected the survival of mesic tree seedlings but not humid tree seedlings. Mesic plants were able to maintain their relative growth rates (RGRs) despite increasing neighbourhood competition. The RGRs of humid species significantly decreased as the density of neighbours increased, indicating that competition may be a factor affecting plants in savannas receiving more than 650 mm mean annual rainfall. We used the quantile regression method to show that the biomass of humid tree seedlings decreased with increased neighbour biomass, when considering competitive response only. We also found that resource‐acquiring traits (such as maximum RGR, plant height and root biomass) were positively correlated with the competitive effect of mesic savanna seedlings, while specific leaf area captured the competitive response of humid savanna seedlings to neighbour competition. Conclusions Competitive interactions between savanna trees negatively affect survival and growth rate, although the competitive effect or response of a particular species depends on its adaptation to either high‐ or low‐resource environments. Our results show significant relationships between plant traits and competitive ability, which may be used to predict competitive interactions between tree seedlings from humid and mesic savannas.