Size-class distribution of Acacia erioloba mortality: the middle class shall die!.
Moustakas, Aristides *,1, Guenther, Matthias2, Wiegand, Kerstin1, Mueller, Karl Heinz2, Ward, David3, Meyer, Katrin1, 1 Institute of Ecology, Jena 07743, Germany2 Department of Geography, Research Lab GIS & Remote Sensing, 35037 Marburg, Germany3 School of Botany & Zoology, Scottsville 3209, South Africa
ABSTRACT- Understanding of population dynamics of long-lived organisms such as savanna trees rely on information on mortality. Such data are scarce as they are hard to obtain because trees often outlive researchers and tree ring data are unreliable for non-temperate trees. We investigate the size-frequency distribution of Acacia erioloba mortality in a South African savanna using remote sensing techniques that combined analyses of individual mortality derived from a recent satellite photograph and a long-term series of aerial photographs. Overall, the tree population within our study plots was increasing. However, it is not known if increased levels of CO2 have caused such change. Our results showed that the size-frequency distribution of dead trees was significantly different from the size-frequency distribution of total trees. Overall mortality was size dependent. Mortality was low in small trees, reaching a peak value when tree canopy surface area is about 30 m2 and usually declined thereafter. Even though the causes of this result are unknown to us, we hypothesize that it can be the result of synchronous episodic recruitment and intra-specific competition and/or evolutionary strategies developed by large-sized trees. The mortality distribution that we found is an innovative result compared to the mortality distributions recorded in other environments.
Key words: tree, competition, size dependent mortality, long term data
All materials copyright The Ecological Society of America (ESA), and may not be used without written permission.