@article{MoustakasGuentherWiegandetal.2006,
  author    = {Moustakas, Aristides and G{\"u}nther, Matthias and Wiegand, Kerstin and M{\"u}ller, Karl-Heinz and Ward, David and Meyer, Katrin M. and Jeltsch, Florian},
  title     = {Long-term mortality patterns of the deep-rooted Acacia erioloba},
  series = {Journal of vegetation science},
  volume    = {17},
  journal   = {Journal of vegetation science},
  publisher = {Blackwell},
  address   = {Malden},
  issn      = {1100-9233},
  doi       = {10.1111/j.1654-1103.2006.tb02468.x},
  pages     = {473 -- 480},
  year      = {2006},
  abstract  = {Question: Is there a relationship between size and death in the Iona-lived, deep-rooted tree, Acacia erioloba, in a semi-arid savanna? What is the size-class distribution of A. erioloba mortality? Does the mortality distribution differ from total tree size distribution? Does A. erioloba mortality distribution match the mortality distributions recorded thus far in other environments? Location: Dronfield Ranch, near Kimberley, Kalahari, South Africa. Methods: A combination of aerial photographs and a satellite image covering 61 year was used to provide long-term spatial data on mortality. We used aerial photographs of the study area from 1940, 1964, 1984, 1993 and a satellite image from 2001 to follow three plots covering 510 ha. We were able to identify and individually follow ca. 3000 individual trees from 1940 till 2001. Results: The total number of trees increased over time. No relationship between total number of trees and mean tree size was detected. There were no trends over time in total number of deaths per plot or in size distributions of dead trees. Kolmogorov-Smirnov tests showed no differences in size class distributions for living trees through time. The size distribution of dead trees was significantly different from the size distribution of all trees present on the plots. Overall, the number of dead trees was low in small size classes, reached a peak value when canopy area was 20 - 30 m(2), and declined in lamer size-classes. Mortality as a ratio of dead vs. total trees peaked at intermediate canopy sizes too. Conclusion: A. erioloba mortality was size-dependent, peaking at intermediate sizes. The mortality distribution differs from all other tree mortality distributions recorded thus far. We suggest that a possible mechanism for this unusual mortality distribution is intraspecific competition for water in this semi-arid environment.},
  language  = {en}
}
@article{SchleicherMeyerWiegandetal.2011,
  author    = {Schleicher, Jana and Meyer, Katrin M. and Wiegand, Kerstin and Schurr, Frank Martin and Ward, David},
  title     = {Disentangling facilitation and seed dispersal from environmental heterogeneity as mechanisms generating associations between savanna plants},
  series = {Journal of vegetation science},
  volume    = {22},
  journal   = {Journal of vegetation science},
  number    = {6},
  publisher = {Wiley-Blackwell},
  address   = {Malden},
  issn      = {1100-9233},
  doi       = {10.1111/j.1654-1103.2011.01310.x},
  pages     = {1038 -- 1048},
  year      = {2011},
  abstract  = {Question: How can we disentangle facilitation and seed dispersal from environmental heterogeneity as mechanisms causing spatial associations of plant species? Location: Semi-arid savanna in the Kimberley Thorn Bushveld, South Africa. Methods: We developed a two-step protocol for the statistical differentiation of association-promoting mechanisms in plants based on the Acacia erioloba-Grewia flava association. Individuals of the savanna shrub G. flava and the tree A. erioloba were mapped on four study plots. Disentangling the mechanism causing the association of G. flava and A. erioloba involved tests of three spatial and one non-spatial null model. The spatial null models include homogeneous and heterogeneous Poisson processes for spatial randomness based on the bivariate spatial point patterns of the four plots. With the non-spatial analysis, we determined the relationship between the canopy diameter of A. erioloba trees and presence or absence of G. flava shrubs in the tree understorey to find whether shrub presence requires a minimum tree canopy diameter. Results: We first showed a significant positive spatial association of the two species. Thereafter, the non-spatial analysis supported an exclusion of environmental heterogeneity as the sole cause of this positive association. We found a minimum tree size under which no G. flava shrubs occurred. Conclusions: Our two-step analysis showed that it is unlikely that heterogeneous environmental conditions caused the spatial association of A. erioloba and G. flava. Instead, this association may have been caused by seed dispersal and/or facilitation (e.g. caused by hydraulic lift and/or nitrogen fixation by the host tree).},
  language  = {en}
}