Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Accueil du site → Doctorat → États-Unis → 2012 → Tree biomass in South African savannas : flying over, hugging, and destroying trees to save them

Stanford University

Tree biomass in South African savannas : flying over, hugging, and destroying trees to save them

Colgan, Matthew Stephen.

Titre : Tree biomass in South African savannas : flying over, hugging, and destroying trees to save them

Auteur : Colgan, Matthew Stephen.

Université de soutenance : Stanford University

Grade : Doctor of Philosophy (Ph.D.) 2012

Résumé partiel
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.

Présentation

Version intégrale (3,7 Mb)

Page publiée le 14 septembre 2012, mise à jour le 28 août 2017