Lead Research Organisation: University of Leeds
Department Name: Inst of Integrative & Comparative Biolog


Savannas cover ~33 million km2 of the Earth's surface and support a fifth of its human population and most of its rangeland, livestock and wild herbivore biomass. Because of their large spatial extent - they cover an area greater than that occupied by boreal or temperate forests / they play a significant role in influencing local, regional and global climate and biogeochemical cycles, contributing ~15% to the annual global carbon sink. Climate change, particularly altered precipitation regimes, is predicted to be a major threat to the ecological future of these biomes. Given their significance to human welfare and economy, understanding the impacts of future environmental change on savannas is critical to their effective management, be it for their sustainability or their role as global carbon sinks. Despite decades of research, factors regulating the mix of trees and grasses that characterize savannas remains poorly understood, with many proposed solutions but no definitive answers. Traditionally, ecologists have tended to emphasize tree-grass competition for water, fire-induced bottlenecks to tree establishment, and large mammal herbivory as being the key determinants of savanna structure. Yet, savanna models that explicitly account for these have failed to recreate patterns observed in nature, leading ecologists to seek additional mechanisms for the same. An important factor which has thus far been overlooked is the role of tree-tree competition. There is a growing recognition that such interactions might be equally important, if not more so, in regulating savanna dynamics, highlighting the need for explicitly including such processes in models if they are to be useful in predicting savanna responses to future environmental changes. Little is currently known about the nature and importance of competitive interactions between trees in savannas. In large part this is due to impediments imposed by the 'event-driven' nature of their dynamics, where most establishment and growth occurs only following episodic rainfall events, such as during rare wet years. Few field studies in arid and semi-arid regions have been long enough to encapsulate such effects. However, long-term aerial photos exist for many savanna regions which provide a convenient way around the time-scale problem. In the proposed study, we will use a combination of field work, image analysis and modeling to investigate long-term tree demography in selected large-scale experimental plots in the savannas of Kruger National Park, South Africa where fires have been manipulated for over 50 years. Importantly, the experiments have been replicated in sites underlain by coarse and fine-textured soils in both high and low rainfall areas, with recent herbivore exclusion treatments included, making it one of the most thorough long-term data sources presently available for isolating the effects of rainfall, soil properties and disturbance on savanna tree demography. Specifically, we will investigate how vegetation spatial structure and local neighborhoods influence tree recruitment, growth and mortality, and how the nature of such interactions changes across gradients of rainfall, fire and geomorphology. We will use these data to develop a spatially explicit model of savannas that integrates the effects of spatial structure and disturbance on tree demography. We will use the model to explore how future changes in precipitation, as is predicted by many climate models, might influence the structure and above-ground carbon sequestration potential of different savannas, and to evaluate how the inclusion of spatially explicit processes influences model outcomes and predictions. This work will provide new insights into the importance of spatial pattern for savanna ecology, and will help define which processes must be included in comprehensive models of savanna vegetation dynamics if we are to successfully predict their responses to future environmental change.
Description One of the primary findings of the work involved the role of small herbivores in regulating tree dynamics in arid savannas. Small herbivores in arid savannas play a similar role to fire in wetter savannas by creating strong bottlenecks to recruitment through browsing. Overall, herbivores significantly impact all aspects of savanna tree demography, reducing recruitment into larger size classes 7-fold and increasing adult mortality 2.5 fold.
In addition, our results also suggest that woody plants in semi-arid savannas are aggregated in clumps despite significant competition from other trees that result in decreased growth rates when in close proximity to neighbors. These results indicate that the spatial distribution of shrubs in these savannas depends on dispersal and establishment processes rather than on competitive, density-dependent mortality.
Exploitation Route Our results demonstrate the critical role that native mammalian herbivores play in preventing woody encroachment in savannas, with implications for management of these systems in arid and semi-arid areas.
Sectors Environment,Other

Description Our findings have implications for the management of arid and semi-arid savannas. We have communicated these results to land managers in Africa where we work.
Sector Environment,Other
Impact Types Policy & public services

Title Tree canopy spatial maps in Experimental Burn Plots, Kruger National Park 
Description Spatially explicit time series maps of tree canopies in the experimental burn plots of the Kruger National Park, South Africa 
Type Of Material Database/Collection of data 
Year Produced 2014 
Provided To Others? Yes  
Impact The classified maps, generated from aerial photos, were provided to the Scientific Services, South African National Parks. The data are now available to other researchers working in the Kruger National Park. 
Description Collaboration on data synthesis with Colorado State University 
Organisation Colorado State University
Country United States 
Sector Academic/University 
PI Contribution Provided data for a synthesis paper on the role of facilitative versus competitive effects of trees on understory communities in savannas
Collaborator Contribution Provided additional data from other sites and was responsible for data analysis and manuscript write-up
Impact One manuscript published: Dohn, J., Dembélé, F., Karembé, M., Moustakas, A., Amévor, K. A., & Hanan, N. P. (2013). Tree effects on grass growth in savannas: competition, facilitation and the stressgradient hypothesis. Journal of Ecology, 101(1), 202-209. Another manuscript in preparation.
Start Year 2009
Description Collaboration with USDA-ARS 
Organisation U.S. Department of Agriculture USDA
Country United States 
Sector Public 
PI Contribution Collected and analysed data on herbivore effects on savanna tree communities, and wrote up a manuscript.
Collaborator Contribution Access to the site, and to earlier data from the site, to analyze growth, recruitment and mortality of savanna woody plants in the presence and absence of herbivory.
Impact One paper published from the collaboration: Sankaran M., D. J. Augustine, and J. Ratnam. 2013.Native ungulates of diverse body sizes collectively regulate long-term woody plant demography and structure of a semi-arid savanna. Journal of Ecology 101:1389-1399. Another manuscript in preparation
Start Year 2009
Description Collaboration with the Scientific Services, Kruger National Park 
Organisation SANParks
Country South Africa 
Sector Charity/Non Profit 
PI Contribution Worked in conjunction with the Scientific Services Kruger National Park on the role of fire and competition in regulating savanna tree community dynamics.
Collaborator Contribution Access to long-term experimental burn plots and logistical support in the field.
Impact Strengthening of partnerships between scientists; improved understanding of savanna community dynamics with implications for managers
Start Year 2007