Tree dynamics in African savannas
Savanna trees evolved under frequent and interacting disturbances, including fire, drought, and herbivory by large mammals. Each of these pressures independently exert selective pressures on how plants allocate carbon among growth, defense against herbivory, and carbon storage. Tradeoffs in C allocation across these pools can influence competitive dynamics among species across rainfall and disturbance gradients. In this project, we are characterizing the physiological differences of the dominant Acacia species at Mpala Research Centre, which collectively account for ~60% of tree cover across the site. Our goal is to link species-specific physiological strategies to changes in tree growth and abundance in response to herbivore exclusion and drought, using data from a long-term herbivore exclusion experiment.
Impacts of herbivore exclusion on savanna tree physiology
Mpala Research Centre, Kenya
Collaborators: Dr. Tyler Coverdale (Notre Dame) and Dr. Jason McLachlan (Notre Dame)
Impacts of rainfall regimes on savanna tree-grass interactions
Savanna tree cover generally increases with mean annual rainfall (MAR), but we lack a mechanistic understanding of whether this increase is driven by rainfall intensity, frequency, or longer growing seasons associated with MAR. To disentangle these factors, we are conducting an in situ rainfall manipulation experiment to isolate the effects of rainfall intensity, deep soil water availability, and growing season length on the growth of tree saplings and grasses. We aim to (1) identify the key resource axes that differentiate trees from grasses and (2) predict changes in tree cover under novel rainfall regimes. Our results suggest that differences in rainfall regimes take time to manifest but drive long-term impacts on savanna structure by reducing the time required for trees to escape from disturbance-induced bottlenecks. Furthermore, species-specific physiological responses to rainfall determine which tree species escape and which do not, shaping the long-term composition of savanna communities.
Wits Rural Facility, South Africa
PIs and collaborators: Dr. Ricardo Holdo (PI; UGA), Dr. Jesse Nippert (PI; KSU), Dr. Michael Belovitch, Jules NeSmith (project manager), Wayne Twine (WRF), Justice Ndhala (on-site manager)
Repeated tree clearing to mitigate bush encroachment
Mthimkhulu Game Reserve, South Africa
The increase of woody plants in semi-arid savannas has reduced herbaceous biomass and diversity, altered habitat structure, and modified savanna ecohydrology. The goal of this project is to assess whether repeated shrub clearing is a viable and effective management practice to reduce shrub cover and increase herbaceous biomass and herbivore presence in a mopane-dominated savanna. Three 60 x 60 m plots have been repeatedly cleared all mopane trees greater than 4 m tall since 2015 at Mthimkhulu Game Reserve. It took 6 years of repeated clearing to reach 50% tree mortality, but soil moisture, grass biomass, and herbivore visitation increased after three years. Our results suggested repeated and targeted management is needed for long-term reductions in woody cover but restoring a grass-dominated ecosystem is possible. This project is in collaboration with the South African Environmental Observation Network and the Mthimkhulu Community.
PIs and collaborators: Dr. Tony Swemmer (SAEON), Dr. Jesse Nippert (KSU), and
Peace Nkuna (project manager)
Control plot
