Effects of Post-fire Browsing on Mortality and Growth of Vachellia Drepanolobium Trees in Semi-arid Savanna Ecosystem at Mpala Research Centre in Laikipia, Kenya

Abstract

Fire and herbivory have been identified as the key top-down drivers of vegetation in savannas ecosystems. There has been a long-standing interest in understanding how interactions between fire and herbivory influence woody vegetation dynamics in savanna. Effects of both fire and herbivory on woody vegetation are quite diverse, including direct mortality, reducing on growth and limited rates, as well as reproductive success. While there is extensive studies on how both fire and herbivory may affect trees in virtually all height classes, vulnerability of trees to ground fires is remarkably higher for coppices and saplings, which are within the height where temperatures from grassy fires are the highest and also because they have less below ground storage, their interaction and implications on tree cover are poorly understood, yet most management decisions in savanna ecosystem revolve around fire and herbivory prescription. This study represents results from controlled, replicated experiments examining how different fire regimes interact with different herbivore groups are rare. This study investigated the effects of single and repeated burns, crossed with six replicated herbivore treatments on mortality and growth of Vachellia drepanolobium at Mpala Research Centre (MRC) within the Kenya Long-term Exclosure Experiment (KLEE) plots located in a semi-arid savanna system in Laikipia Kenya. The study examined mortality rates of Vachellia drepanolobium coppices and saplings in a controlled burned area, tested the influence of tree height on survival and compared growth rates of coppices and saplings in burned and unburned plots that are either protected or exposed to different herbivores combinations. Data was collected by locating individual trees that were previously marked with an aluminum tag before burning to assess mortality, examine regrowth rates, located survivors were measured, survival class size determined and canopy volume as an index of growth. Data was analyzed using generalized linear mixed models (GLMMs) to assess mortality rate and influence of tree height on survival while linear mixed models (LMMs) were used to test for growth rate using function ANOVA. Burned plots experienced higher tree mortality overall, but differences between burns and non-burns were only significant in plots excluding all wild herbivores and in plots accessible to megaherbivore. Additionally, mortality was lower in subplots burned for the second time than in subplots burned once but in herbivore treatment accessible by cattle (C, WC, MWC). Cattle ameliorated the negative effects of repeat burns on tree mortality, perhaps by suppressing fuel load accumulation. Across all herbivore treatments taller trees (>100cm) demonstrated a higher survival probability than shorter individuals (<100cm). Yearly height variations showed trees experienced a significant reduction in height within the first two years after fire (top-kill), which was followed by a gradual recovery across all herbivore regimes. Saplings and coppices (here trees <100cm) subjected to repeated burns regrew faster than those that were burned once, except in presence of megaherbivores. Findings from this study provide evidence that fire and herbivory interactively influence woody vegetation dynamics. Specifically, this study highlights strong context-dependent interactions between fire and different herbivore groups, and extends previous approaches to understanding fire-herbivory interactions, which have tended to lump effects of different herbivore groups, or study them separately. Therefore, this study provides significant theoretical and conservation implications.