Shade, nutrients, and grass competition are important for tree sapling establishment in a humid savanna

Abstract

The structure and function of savannas is contingent upon tree sapling establishment into the ecosystem. Frequent rainfall, canopy shade and nutrient availability are the main factors influencing recruitment of tree saplings. Grass competition has potential to change the tree establishment dynamics by interacting with these factors. The factors affecting tree sapling establishment have been well studied and tested in arid and semi‐arid environments (approx. 0.350–0.750 m mean annual precipitation [MAP]) compared to humid (>1.000 m MAP) savanna ecosystems. We studied the effects of water (frequent irrigation vs. rainfall), shade (presence vs. absence), nutrients (addition vs. no addition) and grass competition (presence vs. absence) on sapling survival and functional traits related to growth and size of four dominant humid savanna species and four dominant mesic savanna species (approx. 0.750 m MAP), under controlled field conditions in a humid South African savanna. None of the treatments had an effect on sapling survival. Growth of the saplings did not depend on the frequency of watering, probably due to high rainfall received in humid savannas. Shade had a strong negative effect on sapling growth, suppressing the beneficial effects of nutrient availability and absence of grass competition. Net positive effects of nutrient availability had less influence over sapling growth than the negative effects exerted by grass competition. Overall shoot growth increased with nutrient addition while investment in leaves drastically declined with grass competition. We conclude that canopy shade is a strong potential driver of structure and function of humid savanna ecosystems, although we recognize that nutrient limitation and grass competition additionally influence tree sapling establishment.