Effects of Density on Timing of Emergence and its Consequences for Survival and Growth in Two Communities of Annual Plants

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 density-dependence in the large intermediate cohort, with the first and the last-emerging cohort showing 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