Bark investment is key to forest expansion into African savannas by conferring resistance to fire and seasonal drought

Abstract

BACKGROUND AND AIMS : Forest expansion into savannas is widespread even though fire and seasonal drought provide environmental conditions against encroachment by forest specialists. A distinct suite of species can establish under savanna trees, forming bush clumps and facilitating forest establishment. Understanding the functional traits of clump-forming species is crucial for uncovering encroachment mechanisms and devising management strategies. Bark likely plays a key role in enabling clump initiation. Fire resistance can be achieved by accumulation of outer bark thickness (OBT), height and/or stem diameter (SD), while drought resistance may be enhanced by greater inner bark thickness (IBT), associated with water and carbohydrate storage. METHODS : We selected representative savanna, clump-forming and closed-canopy species (ecological categories) at two South African sites experiencing forest expansion and differing in rainfall and fire frequency. We compared OBT–SD and IBT–SD allometries across ecological categories and sites and examined whether categories separated along axes reflecting fire/drought resistance (OBT and IBT) and resource allocation strategy (density and water content, leaf size). KEY RESULTS : OBT–SD scaling of clump-forming species was more similar to savanna than forest species, and savanna species at the more fire-prone savanna had steeper OBT–SD scaling, consistent with high OBT providing fire protection in early clump formation. Similar IBT–SD slope across groups was consistent with similar metabolic needs, while higher intercepts in savanna and clump-forming species indicated higher water storage. ‘Cheap’ low-density tissues in savanna species allow fast accumulation of SD and OBT and resistance to fire topkill. Closed-canopy species had denser tissues and thin stems and bark for a given height, while the clump-forming species were intermediate. CONCLUSIONS : Bark and probably other traits are key in the capacity of some species to form bush clumps. Identifying these traits and the mechanisms underlying clump formation is essential for managing encroached savannas and grasslands.