The effect of long-term fire frequencies on soil hydraulic properties in semi-arid savannas in Kruger National Park.

M. Sc. University of KwaZulu-Natal, Pietermaritzburg 2013.

Soils are vital in supporting healthy and functioning ecosystems. Thus when soils are degraded, important ecosystem services are affected. In African savannas where fire is a key driver controlling ecosystem composition and structure, there is a lack in current understanding regarding the impacts of long-term fire management on soil hydrology. The Experimental Burn Plots (EBPs) in Kruger National Park (KNP), a long-term fire experiment initiated in the 1950‟s, offered a unique opportunity to determine the effects of long-term fire treatments (i.e. annual burn vs. no burn) compared to a “variable” fire regime (VFR) outside of the EBPs on soil hydraulic properties in semi-arid savannas. This study was conducted during October 2012- May 2013 on different soil types stemming from the two dominant geologies in KNP, i.e. granites and basalts. This study revealed that it is rather the time following a fire and not necessarily frequency which resulted in decreased soil infiltration, with slowest infiltration rates immediately after the fire. Findings suggested that fire only affected infiltration rates at the soil surface and that these fire effects would dissipate within approximately two years— suggesting the soil‟s ability to recover; at least in terms of their hydrological function. Soil compaction, which is recognized for impeding soil infiltration, was measured. The research presented in this thesis indicates that surface compaction may be due to soil processes such as raindrop impact and splash but deeper compaction is attributed to high herbivore concentrations trampling the soil. Interestingly, the extent of soil compaction caused by high densities of herbivores does not result in significantly reduced soil infiltration rates. In addition, long-term fire management effects on soil organic matter content and soil water retention was investigated. Besides promoting soil fertility, soil organic matter is considered hydrophilic and aids in soil water retention. Although alluding to greater organic matter on the fire-suppressed plot on the granitic EBPs, there were no statistically-significant differences found across the varying fire frequencies. However on the basaltic EBPs, organic matter content varied between the various fire frequencies. Unlike the granitic plots where it is believed that fire intensities are not substantial enough to transfer heat deep into the soil and consume organic matter, it is thought that the huge contrast in above-ground biomass between the basaltic burn plots is in fact responsible for the contrast in organic matter contents. Consequently, soil water retention was found to be greatest on the fire-suppressed no burn plots. The ability of the soil to retain moisture, especially at low water contents, is crucial in a post-fire environment in order to facilitate re-establishment of vegetation. Fire impacts on soil hydraulic processes ultimately influence soil water balances. These impacts may have cascading effects on large-scale catchment processes. This study provides valuable insight not only into the relationship between water and fire but also how other factors such as soil, vegetation and herbivores all interact within a water-controlled savanna landscape.