Density experiment in Negev Desert Israel

1 Few experiments directly test the role of density dependence in natural plant communities.
2 We tested (i) whether different initial seed densities of the entire seed bank of an annual plant community affected performance (emergence, mean plant biomass and survival) and (ii) whether density-dependent processes were operating within the communities. We also tested whether life-stage, year and soil moisture (both on a topographic gradient and in an experimental manipulation) influenced the effects of seed density.

3 We considered two distinct phases: seed to emergent seedlings, and emergent seed- lings to established plants.
4 The seed bank was collected from a semistabilized sand dune in the Negev Desert, Israel. This was added to sieved sand in plots at the same site at four different initial seed densities: 1/16×, 1/4×, 1× (natural seed density) and 2×. The experiment was repeated for three consecutive growing seasons.

5 Emergence of seedlings was significantly influenced by initial seed density in all 3 years, with higher initial seed densities having lower rates of emergence.
6 Mean final plant size was negatively density dependent and consistently unaffected by the initial seed density sown.

7 In general, there were no strong interactions of topographic position with initial seed density, processes within the vegetation were not density dependent and the experi- mental addition of water did not influence any of the performance measures tested. Density-dependent processes vary from year to year, while moisture effects do not.

8 We conclude that seedling emergence and some processes in the established plant community are density dependent, but the established plant community is also affected by other processes, such as resource limitation.

1 鲜有实验直接检验密度依赖（density dependence）在自然植物群落中的作用。
2 本研究开展了两项检验：其一，一年生植物群落完整种子库（seed bank）的不同初始种子密度（initial seed density）是否会影响群落性能（出苗率（emergence）、植株平均生物量与存活率）；其二，群落内部是否存在密度依赖过程。此外，我们还检验了生活史阶段、年份以及土壤湿度（包含地形梯度（topographic gradient）下的自然变异与实验操控（experimental manipulation）处理两种场景）是否会调控种子密度的效应。
3 本研究设置了两个明确的阶段：种子至出苗幼苗阶段，以及出苗幼苗至定植植株（established plants）阶段。
4 种子库采集自以色列内盖夫沙漠一处半固定沙丘。将其加入同地点样地的过筛沙土中，设置4种初始种子密度梯度：1/16倍、1/4倍、1倍（自然种子密度）与2倍。实验连续重复开展了3个生长季（growing seasons）。
5 3个生长季中，幼苗出苗率均显著受初始种子密度调控：初始种子密度越高，出苗率越低。
6 植株最终平均个体大小表现为负密度依赖（negative density dependence）效应，且始终不受播种初始种子密度的影响。
7 总体而言，地形位置与初始种子密度之间未出现显著交互效应；植被内部过程未表现出密度依赖性；实验补水（experimental addition of water）处理亦未对所有检测的性能指标产生影响。密度依赖过程存在年际变异，但土壤湿度的效应则无此特征。
8 本研究结论为：幼苗出苗过程与定植植物群落内的部分过程存在密度依赖性，但定植植物群落同时还受资源限制（resource limitation）等其他过程的调控。