Root trait (multi)functionality in savanna trees: Progress and challenges

Savanna trees experience various ecological pressures, requiring their roots to serve multiple functions. However, savanna root trait studies have focused on easily measurable traits, emphasizing a single axis of function: resource acquisition in competition with grasses, resilience to episodic droughts or resource storage for recovery following fire or herbivory. This narrow focus limits understanding of trait–function relationships and factors driving trait variation. Our synthesis evaluates a few key hypotheses with varying support: (1) tree-grass niche partitioning is stronger in drier, sandier savannas, supported by studies on rooting depth and biomass distribution; (2) drought stress is more severe in drier, clay-rich savannas, although studies on drought traits are limited; (3) fire and herbivory constraints are expected in semi-arid to mesic savannas, but empirical evidence is mixed and not specific to these functions. The multifunctionality of common root traits complicates hypothesis testing across environmental or disturbance gradients. More focused evaluations would be useful to identify traits tied to specific functions, which could clarify the drivers of root trait variation. Synthesis. Overall, integrated and mechanistic studies of root trait variation across environmental and disturbance gradients remain scarce, despite clear evidence that roots are crucial to savanna tree responses to climate change and disturbances.