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.
Publisher
University of Nairobi
Rights
Attribution-NonCommercial-NoDerivs 3.0 United StatesUsage Rights
http://creativecommons.org/licenses/by-nc-nd/3.0/us/Collections
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