Do spatial effects play a role in the spatial distribution of desert-dwelling Acacia raddiana?

We investigated the spatial pattern of A. raddiana in the Negev desert of I srael in order to gain insights into the factors and processes driving the dynamics of this species. Using a scale-dependent measure, the ring statist ic, we analysed both patterns observed in the field and time series of spat ial tree distributions produced by a simulation model. In the field, random spacing was the predominant pattern observed. However seedlings were clump ed on small scales. We ran the model under two contrasting scenarios repres enting hypotheses that explain the clumping of seedlings and the random dis tribution of trees. One hypothesis is that there is spatial heterogeneity i n seed distribution, germination and seedling mortality, but that these het erogeneities are not correlated with each other in space. The second hypoth esis assumes a correlation between these heterogeneities leading to areas s uitable for establishment. However, the suitability of the sites is tempora lly variable. Furthermore, the second hypothesis assumes density-dependent tree mortality due to competition. Both hypotheses lead to spatial distribu tions that are in qualitative agreement with the patterns observed in the f ield. Therefore, the classical view that a clumped seedling distribution an d a random pattern of older trees is due to clumped regeneration and densit y-dependent mortality may not hold for Acacia trees in the Negev.