Browsing by Author "Archibald, Sally"

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Dataset from : Browsing is a strong filter for savanna tree seedlings in their first growing season
(ARCHIBALD ECOLOGY LAB, 2021-06-15) Archibald, Sally; Twine, Wayne; Mthabini, Craddock; Stevens, Nicola
1: Newly germinated seedlings are vulnerable to biomass removal but usually have at least six months to grow before they are exposed to dry-season fires, a major disturbance in savannas. In contrast, plants are exposed to browsers from the time they germinate, making browsing potentially a very powerful bottleneck for establishing seedlings. 2: Here we assess the resilience of seedlings of 10 savanna tree species to top-kill during the first 6 months of growth. Newly-germinated seeds from four dominant African genera from across the rainfall gradient were planted in a common garden experiment at the Wits Rural Facility and clipped at 1 cm when they were ~2, 3, 4, and 5 months old. Survival, growth, and key plant traits were monitored for the following 2.5 years. 3: Seedlings from environments with high herbivory pressure survived top-kill at a younger age than those from low-herbivore environments, and more palatable genera had higher herbivore-tolerance. Most individuals that survived were able to recover lost biomass within 12 months, but the clipping treatment affected root mass fraction and branching patterns. 4: Synthesis: The impact of early browsing as a demographic bottleneck can be predicted by integrating information on the probability of being browsed and the probability of surviving a browse event. Establishment limitation through early-browsing is an under-recognised constraint on savanna tree species distributions. Data may be used without requesting permission after the J Ecology paper is published and in the public domain (estimated after October 2021).
Fire regimes in southern Africa - determinants, drivers and feedbacks
(2010-08-24) Archibald, Sally
The spatial and temporal patterns of fire in Africa south of the equator are described and investigated to test hypotheses on the main drivers of fire. In particular, the relative importance of climate, vegetation, and human activities are assessed and related to current theory on savanna fire regimes and their response to global change. This thesis makes use of newly-released burned area and fire radiative power data derived from satellite imagery. This information was integrated with data on climate, vegetation, and human activities, as well as long-term field data on fire, to test hypothe- ses about savanna fire regimes. Regression tree techniques and mixed effects models were used to determine the main drivers of spatial and temporal variability in fire, and feedbacks between fire and tree cover were explored at a range of spatial scales. New datasets on fire size and fire number were also developed to test theories on the effect of human activities on ignition probability and fire spread. The results show that fire in these grassy ecosystems is very responsive to climatic factors such as rainfall but there are large areas where human activities play an equally significant role. The effect of people was always to reduce the total area burned, although the number of individual fires increased with population density up to about 10 people per km2. Areas inhabited by people were also less susceptible to year-to-year fluctuations in annual burned area caused by climatic variability, possibly because of the reduced probability of large, runaway fires in these highly-impacted landscapes. Both of these findings are important for modelling of fire and carbon emissions in Africa. Evidence of negative feedbacks between tree cover and fire were found. When the percentage tree cover is less than 40% a range of factors control the extent of fire in a system; but above 40% tree cover fire is never a large component of the landscape. On the other hand, tree cover was shown to have a bi-modal distribution, and examples of intermediate tree cover (40-70%) were rare. Over a wide range of mean annual rainfall (1000-2000mm) vegetation can exist either in a "high tree-cover, low fire" or a "high fire, low tree cover" state. This research challenges some assumptions about how savanna fires will respond to global change, and provides important clues to help disentangle the mechanisms driving fire in Southern Africa. Importantly, it highlights that fire in southern Africa, and in other highly-utilised savanna systems, can not be understood outside the context of human use.