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

Abstract

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-yr longitudinal study of spatially explicit growth patterns of woody vegetation in an East African savanna following exclusion of large herbivores and in the absence of fire. We examined plant spatial patterns and quantified the degree of competition among woody individuals. Woody plants in this semiarid savanna exhibit strongly clumped spatial distributions at scales of 1-5 m. However, analysis of woody plant growth rates relative to their conspecific and heterospecific neighbors revealed evidence for strong competitive interactions at neighborhood scales of up to 5 m for most woody plant species. Thus, woody plants were aggregated in clumps despite significantly decreased growth rates in close proximity to neighbors, indicating that the spatial distribution of woody plants in this region depends on dispersal and establishment processes rather than on competitive, density-dependent mortality. However, our documentation of suppressive effects of woody plants on neighbors also suggests a potentially important role for tree-tree competition in controlling vegetation structure and indicates that the balanced-competition hypothesis may contribute to well-known patterns in maximum tree cover across rainfall gradients in Africa.

Grasses continue to trump trees at soil carbon sequestration following herbivore exclusion in a semiarid African savanna

“…The most likely explanation is that the first 10 yr of herbivore exclusion resulted in rapid increases in recruitment rates overall, particularly of seedlings/saplings into larger size classes with associated substantial increases in canopy cover. After 10 yr of herbivore removal, however, competition appears to have come into play (e.g., theory of self‐thinning; Wiegand et al , Belay and Moe , Sea and Hanan , Dohn et al ), resulting in a decrease in tree density and a slight increase in average canopy size (see Appendix : Fig. ) and basal diameters (Fig.…”

Interaction of seed dispersal and environmental filtering affects woody encroachment patterns in coastal grassland

“…Once species successfully establish, they may create a range of stress gradients across a single habitat patch, potentially changing the effect from one of stress reduction to stress induction, with both competition and facilitation operating depending on spatiotemporal scale (Jurgens and Gaylord , Dohn et al. ). The balance between competition and facilitation is complex and can shift the outcome of biotic interactions.…”

Remotely sensed canopy height reveals three pantropical ecosystem states: a comment

“…, Dohn et al. ), all of which indicate that savanna ecosystems certainly do occur within this dry rainfall range (<600 mm) and thus constitute an alternative ecosystem state to the “treeless” one. Furthermore, treeless grasslands can also dominate in the rainfall range of 600–1,200 mm MAP (Sankaran et al.…”