Shade is a team effort in a South African forest‐savanna mosaic

Forests and mesic savannas can both exist under the same climatic conditions across the tropics and subtropics but differ greatly in ecosystem function. Grass fuels are one key difference: flammable grasses promote fire in savanna but are shade intolerant and thus excluded from forest. While fire suppression via shading is widespread in forests, the extent to which shade results from the traits of individual forest trees versus from a high density of trees remains poorly understood.

We examined whether individual or ecosystem‐level traits are responsible for excluding shade‐intolerant grasses (and hence fire) in forest understory in southern Africa using a simple mathematical model linking tree traits to ecosystem function via their impact on shade and grass exclusion.

To parameterize the model, we used trait data collected along a Leaf Area Index (LAI) gradient spanning across a forest‐savanna mosaic in Hluhluwe‐iMfolozi Park, South Africa. We contrasted two versions of the model, one based on individual tree functional traits and another that also incorporated tree neighbour density and evaluated which model better predicted measured LAI.

We found that forest trees had higher crown fill than savanna trees but nonetheless found that a single forest tree would not produce sufficient shade to exclude grass. However, our model also predicted that a dense collection of savanna trees would not shade enough to exclude grasses. Thus, we conclude that grass (and fire) exclusion from forests at the African forest‐savanna ecotone is a combination of individual and ecosystem‐level function.

Synthesis . This work highlights the importance of both individual‐ and community‐level traits when considering savanna and forest biome function, with implications for biome responses to global change. For example, woody encroachment into savannas without colonization by fire‐suppressing trees may not exclude fires, nor would forest degradation without colonization by savanna trees result in forest transition to savanna.