Lesego M. Khomo

Regional insight into savanna hydrogeomorphology from termite mounds

Global vegetation models predict the spread of woody vegetation in African savannas and grasslands under future climate scenarios, but they operate too broadly to consider hillslope-scale variations in tree-grass distribution. Topographically linked hydrology-soil-vegetation sequences, or catenas, underpin a variety of ecological processes in savannas, including responses to climate change. In this study, we explore the three-dimensional structure of hillslopes and vegetation, using high-resolution airborne LiDAR (Light Detection And Ranging), to understand the long-term effects of mean annual precipitation (MAP) on catena pattern. Our results reveal that the presence and position of hillslope hydrological boundaries, or seeplines, vary as a function of MAP through its long-term influence on clay redistribution. We suggest that changes in climate will differentially alter the structure of savannas through hydrological changes to the seasonally saturated grasslands downslope of seeplines. The mechanisms underlying future woody encroachment are not simply physiological responses to elevated temperatures and CO(2) levels but also involve hydrogeomorphological processes at the hillslope scale.

Shaping post-orogenic landscapes by climate and chemical weathering

The spacing of hills and valleys reflects the competition between disturbance-driven (or diffusive) transport on hillslopes and concentrative (or advective) transport in valleys, although the underlying lithologic, tectonic, and climatic controls have not been untangled. Here, we measure geochemical and geomorphic properties of catchments in Kruger National Park, South Africa, where granitic lithology and erosion rates are invariant, enabling us to evaluate how varying mean annual precipitation(MAP = 470 mm, 550 mm, and 730 mm) impacts hill-valley spacing or landscape dissection. Catchment-averaged erosion rates, based on 10Be concentrations in river sands, are low (3-6 m/m.y.) and vary minimally across the three sites. Our lidar-derived slope-area analyses reveal that hillslopes in the dry site are gentle (3%) and short, such that the terrain is low relief and appears highly dissected. With increasing rainfall, hillslopes lengthen and increase in gradient (6%-8%), resulting in less-dissected, higher-relief catchments. The chemical depletion fraction of hilltop regoliths increases with rainfall, from 0.3 to 0.7, reflecting a climate-driven increase in chemical relative to physical erosion. Soil catenas also vary systematically with climate as we observe relatively uniform soil properties in the dry site that contrast with leached sandy crests and upper slopes coupled with downslope clay accumulation zones in the intermediate and wet sites. The geomorphic texture of this slow-eroding, granitic landscape appears to be set by climate-driven feedbacks among chemical weathering, regolith fabric differentiation, hydrological routing, and sediment transport that enhance the vigor of hillslope sediment transport relative to valley-forming processes for wetter climates.

Lithologic controls on biogenic silica cycling in South African savanna ecosystems

The efficacy of higher plants at mining Si from primary and secondary minerals in terrestrial ecosystems is now recognized as an important weathering mechanism. Grassland ecosystems are a particularly large reservoir of biogenic silica and are thus likely to be a key regulator of Si mobilization. Herein, we examine the effects of parent material (basaltic and granitic rocks) on the range and variability of biogenic silica pools in grass-dominated ecosystems along two precipitation gradients of Kruger National Park, South Africa. Four soil pedons and adjacent dominant plant species were characterized for biogenic silica content. Our results indicate that although soils derived from basalt had less total Si and dissolved Si than soils derived from granite, a greater proportion of the total Si was made up of biogenically derived silica. In general, plants and soils overlying basaltic versus granitic parent material stored greater quantities of biogenic silica and had longer turnover times of the biogenic silica pool in soils. Additionally, the relative abundance of biogenic silica was greater at the drier sites along the precipitation gradient regardless of parent material. These results suggest that the biogeochemical cycling of Si is strongly influenced by parent material and the hydrologic controls parent material imparts on soils. While soils derived from both basalt and granite are strongly regulated by biologic uptake, the former is a “tighter” system with less loss of Si than the latter which, although more dependent on biogenic silica dissolution, has greater losses of total Si. Lithologic discontinuities span beyond grasslands and are predicted to also influence biogenic silica cycling in other ecosystems.