Steal the light: shade vs fire adapted vegetation in forest–savanna mosaics
2018
Charles‐Dominique, Tristan | Midgley, Guy F. | Tomlinson, Kyle W. | Bond, William J.
Shade cast by trees, which suppresses grass growth, and fire fuelled by grass biomass, which prevents tree sapling establishment, are mutually exclusive and self‐reinforcing drivers of biome distribution in savanna–forest mosaics. We investigated how shade depth, represented by canopy leaf area index (LAI), is generated by adult trees across savanna–forest boundaries and how a shade gradient filters tree functioning, and grass composition and biomass. Forest trees exerted greater shading through increased stem density and greater light interception per unit biomass. A critical transition at LAI c. 1.5 was linked to tree shifts from savanna to forest species, functional shifts from fire‐tolerant to light‐competitive species, and grass composition shifts from C₄ to C₃ pathways. A second transition to grass fuel loads too low to support fires, occurred at a lower canopy density (LAI > 0.5), accompanied by shifts in C₄ subtype dominance. This pattern suggests that shade suppression of grass biomass is an essential first step for the maintenance of alternative stable states.
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