Competition with trees does not influence root characteristics of perennial grasses in semi-arid and arid savannas in South Africa

•We investigate root characteristics of perrenial grasses.•This was to examine the affect of competition and water limitation on root reserves.•Trees and rainfall did not affect any of the root characteristics except nitrogen.•Trees and rainfall positively influenced root nitrogen content in the three sites.•Tree-grass interactions were likely conforming to the stress-gradient hypothesis.

Savannas support mixed tree-grass communities and interactions between these are typically viewed as being competitive based on studies that focused on grass aboveground production. However, an important plant response to competition and resource limitation is an increase in root reserves. We investigated root characteristics of perennial grasses in the presence and absence of trees as a proxy of competition in South African savannas in three sites that differed in rainfall. We based our study on the hypothesis that competition from trees and water limitation will result in increased storage in roots of grasses under trees. Results indicate no significant effect of variation in rainfall of the different study locations on root characteristics of grasses. Furthermore, trees did not significantly influence most grass root characteristics that we measured. The only exception was nitrogen-content that showed an increase with rainfall and tree presence through potentially higher mineralization rates and nitrogen availability in the under-tree canopy environment. As the study sites are in the drier rainfall range in South Africa, it is likely that trees and grasses in these dry savannas may have a positive relationship conforming to the stress-gradient hypothesis. Alternatively, grasses and trees may be using complementary water and nutritional resources.