Feedbacks between fire and patches of woody vegetation in tropical grasslands and savannas : abstract + powerpoint

In tropical grasslands and savannas, fire is used to reduce woody vegetation expansion. Woody vegetation in these biomes is often patchily distributed with unknown consequences for fire effects. We studied two feedbacks between fire and patches of woody vegetation: effects of fire on tree clustering and effects of tree clusters on fire effects. The first feedback was tested by measuring the extent of tree clustering in Kruger National Park. We found that fire frequency positively affects the clustering of
tree species that are not very abundant. We suggest that less abundant species are less resistant to fire and adopt a mechanism of clustering to exclude grass fires under their canopy. The second feedback was experimentally tested. We planted tree seedlings around wooded patches in a grassland and burnt these plots. We found that fire had lower temperature that prolonged for a shorter time period at the leeward side of wooded patches than at the windward side. Also, we found that the seedlings were less damaged at the leeward side. We conclude that fire can result in clustering of trees and that these patches of woody vegetation can have a large effect on the role of fire in tropical grasslands and savannas. These two feedbacks may lead to a ‘safe zone’ for seedlings around patches of woody vegetation, which consequently promotes woody vegetation expansion. We modelled these findings and illustrate the results using changing frequency distributions of patch sizes. Our study contributes to understanding of savanna functioning by showing which processes are relevant in the distribution of savanna trees.