Spatial vegetation patterns and neighborhood competition among woody plants in an East African savanna

Authors: DOHN, JUSTIN - Colorado State University; Augustine, David; HANAN, NIALL - South Dakota State University; RATNAM, JAYASHREE - The Tata Institute Of Fundamental Research; SANKARAN, MAHESH - The Tata Institute Of Fundamental Research

Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2016
Publication Date: 4/25/2017
Citation: Dohn, J., Augustine, D.J., Hanan, N.P., Ratnam, J., Sankaran, M. 2017. Spatial vegetation patterns and neighborhood competition among woody plants in an East African savanna. Ecology. 98(2):478-488. doi:10.1002/ecy.1659.

Interpretive Summary: Savannas are characterized by a sparse layer of woody plants (trees and shrubs) that coexist with and understory of grasses. Ecologists have traditionally thought that woody plants in savannas are strongly affected by competition with the grasses, with relatively little consideration given to whether woody plants compete with one another. However, woody plants can often occur in a clumped pattern in savannas, and completion within those clumps could influence the species composition of these communities. We present data from a 10-year study of growth patterns of woody vegetation in an East African savanna where each individual plant was mapped. This design allowed us to examine how the neighborhood surrounding any given woody plant (i.e. the density and size of other plants growing near it) influences its rate of growth. We found that woody plants in this semi-arid savanna exhibit strongly clumped spatial distributions, with most plants occurring in clumps that were 3 – 15 feet in diameter. However, we also found clear evidence that the rate at which a given woody plant grows is strongly reduced when growing in dense clusters. In other words, most woody plants occur in clusters even though they experience significantly decreased growth rates when growing in that distribution. Species that increased substantially over the study period (notably Acacia etbaica and A. mellifera) were associated with high growth rates and low levels of neighborhood competition, suggesting increased establishment success away from existing shrub clumps. Our processes such as seed dispersal, seed germination and seedling establishment can be more important than competition among established shrubs in determining the distribution of woody plants in this ecosystem. Our documentation of suppressive effects of woody plants on neighbors in the absence of fire and herbivory has significant implications for our understanding of plant community dynamics across global savannas, particularly in savannas where frequent disturbances limit how often woody plants can grow into large size classes.

Technical Abstract: The majority of research on savanna vegetation dynamics has focused on the coexistence of woody and herbaceous vegetation; interactions among woody plants in savannas are relatively poorly understood. We present data from a 10-year longitudinal study of spatially explicit growth patterns of woody vegetation in an East African savanna in the absence of fire and herbivory. We quantified the degree of competition among woody individuals and discuss the importance of inter-plant interactions for the spatial distribution of woody plants in the landscape. We found that woody plants in this semi-arid savanna exhibit strongly clumped spatial distributions at scales of 1 – 5 m. However, analysis of shrub growth rates relative to their conspecific and heterospecific neighbors revealed evidence for strong competitive interactions at neighborhood scales out to 5 m for most woody plant species. Thus, woody plants are aggregated despite significantly decreased growth rates in close proximity to neighbors. Species that increased substantially over the study period (notably Acacia etbaica and A. mellifera) were associated with high growth rates and low levels of neighborhood competition, suggesting increased establishment success away from existing shrub clumps. Our findings indicate that processes governing seed dispersal, seed germination and seedling establishment can be more important than competition among established shrubs in determining landscape-scale shrub distributions. However, strong neighborhood effects at a scale of ~3 m suggest that belowground competition has an important influence on shrub growth rates. Our documentation of suppressive effects of woody plants on neighbors in the absence of fire and herbivory has significant implications for our understanding of plant community dynamics across global savannas, particularly in systems subjected to frequent disturbances where escape from demographic bottlenecks is paramount.