Topo-edaphic controls over woody plant biomass in South African savannas

[摘要] The distribution of woody biomass in savannas reflects spatial patternsfundamental to ecosystem processes, such as water flow, competition, andherbivory, and is a key contributor to savanna ecosystem services, such asfuelwood supply. While total precipitation sets an upper bound on savannawoody biomass, the extent to which substrate and terrain constrain trees andshrubs below this maximum remains poorly understood, often occluded bylocal-scale disturbances such as fire and trampling. Here we investigate therole of hillslope topography and soil properties in controlling woody plantaboveground biomass (AGB) in Kruger National Park, South Africa. Large-areasampling with airborne Light Detection and Ranging (LiDAR) provided a meansto average across local-scale disturbances, revealing an unexpectedly linearrelationship between AGB and hillslope-position on basalts, where biomasslevels were lowest on crests, and linearly increased toward streams (R² = 0.91). The observed pattern was different on granite substrates, where AGBexhibited a strongly non-linear relationship with hillslope position: AGBwas high on crests, decreased midslope, and then increased near streamchannels (R² = 0.87). Overall, we observed 5-to-8-fold lower AGB onclayey, basalt-derived soil than on granites, and we suggest this is due toherbivore-fire interactions rather than lower hydraulic conductivity or clayshrinkage/swelling, as previously hypothesized. By mapping AGB within andoutside fire and herbivore exclosures, we found that basalt-derived soilssupport tenfold higher AGB in the absence of fire and herbivory, suggestinghigh clay content alone is not a proximal limitation on AGB. Understandinghow fire and herbivory contribute to AGB heterogeneity is critical topredicting future savanna carbon storage under a changing climate.

[学科分类] 地球化学与岩石