Tree biomass in South African savannas [electronic resource] : flying over, hugging, and destroying trees to save them
Keywords: Environmental Earth System Science

The abstracts for the three primary chapters (after the introduction in Chapter 1) are: Chapter 2. The distribution of woody biomass in savannas reflects spatial patterns fundamental to ecosystem processes, such as water flow, competition, and herbivory, and is a key contributor to savanna ecosystem services, such as fuelwood supply. While total precipitation sets an upper bound on savanna woody biomass, the extent to which substrate and terrain constrain trees and shrubs below this maximum remains poorly understood, often occluded by local-scale disturbances such as fire and trampling. Here we investigate the role of hillslope topography and soil properties in controlling woody plant aboveground biomass (AGB) in Kruger National Park, South Africa. Large-area sampling with airborne Light Detection and Ranging (LiDAR) provided a means to average across local-scale disturbances, revealing an unexpectedly linear relationship between AGB and hillslope-position on basalts, where biomass levels were lowest on crests, and linearly increased toward streams (R2 = 0.91). The observed pattern was different on granite substrates, where AGB exhibited a strongly non-linear relationship with hillslope position: AGB was high on crests, decreased midslope, and then increased near stream channels (R2 = 0.87). Overall, we observed 5-to-8-fold lower AGB on clayey, basalt-derived soil than on granites, and we suggest this is due to herbivore-fire interactions rather than lower hydraulic conductivity or clay shrinkage/swelling, as previously hypothesized. By mapping AGB within and outside fire and herbivore exclosures, we found that basalt-derived soils support tenfold higher AGB in the absence of fire and herbivory, suggesting high clay content alone is not a proximal limitation on AGB. Understanding how fire and herbivory contribute to AGB heterogeneity is critical to predicting future savanna carbon storage under a changing climate. Chapter 3. Tree biomass is an integrated measure of net growth and is critical for understanding, monitoring and modeling ecosystem functions. Despite the importance of accurately measuring tree biomass, several fundamental barriers preclude direct measurement at large spatial scales, including the fact that trees must be felled to be weighed, and that even modestly sized trees are challenging to maneuver once felled. Allometric methods allow for estimation of tree mass using structural characteristics, such as trunk diameter. Savanna trees present additional challenges, including limited available allometry and a prevalence of multi-stemmed trees. Here we collected airborne LiDAR data over a pristine semi-arid savanna adjacent to the Kruger National Park, South Africa and then harvested and weighed woody plant biomass at the plot scale to provide a standard against which field and airborne estimation methods could be compared. We found for an existing airborne LiDAR method that half of the total error was due to averaging canopy height at the plot scale. This error was eliminated by instead measuring maximum height and crown area of individual trees from LiDAR data using an object-based method to identify individual tree crowns and estimate their biomass. The object-based method approached the accuracy of field allometry at both the tree and plot levels, and roughly doubled the accuracy compared to existing airborne methods. We found allometric error accounted for 22-30% of the total error in airborne biomass estimates at the plot scale. Airborne methods also gave more accurate predictions at the plot-level than field methods based on diameter-only allometry. These results provide a novel comparison of field and airborne biomass estimates using harvested plots and advance the role of LiDAR remote sensing in savanna ecosystems. Chapter 4. Tree biomass is both a fundamental state variable of ecosystems and the critical parameter in effectively monitoring aboveground carbon stocks. Destructively harvesting and weighing biomass is labor intensive, expensive, and prohibited in protected areas. Thus one of the largest sources of error in non-destructively estimating forest biomass is the selection of an allometric model. This is particularly problematic in African savanna woodlands, where limited availability of allometry, small sample sizes, and lack of harvested large trees make it unclear whether species-specific allometries are more accurate than generic models or significantly different from each other. It also remains unclear whether variation in growth form or wood specific gravity ([rho]) is the principal cause of biomass differences among savanna species. Here we constructed ten species-specific allometric models from 714 destructively harvested stems in savanna woodland near Kruger National Park, South Africa. Three of the four most common woody species had fits significantly different (p < 0.05) from one another, with the highest species curve (Acacia nigrescens) more than twice the biomass of the lowest (Sclerocarya birrea) even for moderately sized trees (D = 10 cm). We then compared the form factors (taper) of each species by dividing mass by [rho] and cylindrical volume, showing most species are in the range of F = 0.78-0.80 without significant variation between most species. These results suggest there are both statistically significant and ecologically substantial differences between the allometries of common savanna species, and that variation in wood density is the primary source of biomass differences between these species.