Most studies of density‐dependent demography in plants consider the density only of the single focal species being studied. However, density‐dependent regulation in plants may frequently occur at the level of the entire community, rather than only within particular species. In addition, because density dependence may differ considerably (even in direction) among demographic parameters, generalizing about patterns of density dependence and extrapolating to lifetime fitness and to population dynamics require comparisons among life history stages, as well as among types of species and physical environments. We constructed seminatural communities of desert annuals composed of all the constituent species in the same relative proportions as found in the natural habitat. These experimental communities were planted at a range of densities that extended far above and below mean natural field density. We compared among physical environments (irrigation treatments), among communities from different physical environments, and among growth forms (dicot and graminoid) to search for generalizations about the magnitude and direction of density dependence. Strong evidence of community‐level density dependence was detected at all three life history stages studied in these desert annuals: emergence, survival, and final size. However, both the direction and degree of consistency of this density dependence varied considerably among the stages. The strongest and most consistent competitive effects were experienced at the emergence stage, where the mechanism is most likely a form of interference competition rather than exploitation competition. At the survival stage, the magnitude of effects was highly variable among physical environments and source communities, but negative effects were relatively rare, with either positive or no significant effects of increasing density. Thus, exploitation competition was also unimportant at the survival stage. In contrast, for growth, exploitation competition appeared to be the primary mechanism of interaction influencing growth. This variation in mechanism, direction, and magnitude of interactions among life history stages suggests that current models of plant community structure that are based largely on exploitation competition as it influences growth (with mortality a simple function of growth) are inadequate for even this simple annual plant community. We also compared growth forms and found that graminoids were superior competitors to dicots at the emergence and survival stages; they also had higher emergence and survival, regardless of density. Consistent with this result, grasses are always the numerical dominants in the source communities. In contrast, the two growth forms did not differ in competitive ability for growth, and dicots were consistently larger individuals, independent of density, even though grasses were also usually the biomass dominants in the source communities. These results suggest the importance of nontrophic mechanisms of interaction in controlling community structure an...

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Three strip transects, each ca 100 contiguous 0.5 X l m 2 quadrats, were sampled during the spring bloom of March 1981 across four surface structural units of a Negev Desert research watershed at Sede Boqer, Israel. Presence of all vascular plants was recorded. Data were subjected to detrended correspondence analysis (DCA ordination), and resulting spatial patterns of species distribution and abundance were compared. Large-scale gradients of vegetation were related to differences in soil moisture availability among the four structural units. Where micro-scale vegetation patterns were important, these correlated with rock and crevice microtopography. Species richness was influenced by high numbers of therophytes on the dry upper slope of the watershed and their reduced importance on the lower three units. Relationships between vegetational patterns and known ecosystem properties of the watershed are discussed.

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We tested whether increasing seed density results in a change in the timing of emergence in two communities of sand dune annual plants in Israel. Specifically we tested (i) if emergence is accelerated or delayed due to high seed density. We also tested two predictions about the consequences of changes in the timing of emergence; (ii) seedlings emerging earlier will have higher survival and growth; (iii) the advantage of earlier emergence increases as seed density increases. We examined these predictions for both monocots and dicots growing under different irrigation regimes and using species from a desert and a semi-arid community of sand dune annual plants. Dicots showed increasing negative density-dependent emergence in later cohorts, consistent with the prediction of delayed emergence. In contrast, grasses showed no shift in timing of emergence, although they did experience strongly negative densitydependence in the large intermediate cohort, with the first and the last-emerging cohort ARTICLE IN PRESS www.elsevier.com/locate/jnlabr/yjare 0140-1963/$ -see front matter rshowing weaker and less significant negative density-dependence. Cohort had no impact on survival with earlier emerging seedlings being no more or less likely to survive to the end of the growing season than later emerging seedlings. For dicots, earlier emerging seedlings tended to become larger adults, especially for plants from the desert site. Our results differ from those of other field studies where timing of emergence seems to have a larger effect on components of fitness. We suggest that most other field studies have been conducted in more productive habitats where asymmetric competition through light limitation is much more likely and therefore small differences in timing are expected to have larger cumulative effects. r