| In most plant communities, neighbours are likely to interact in at least two ways---negatively through competition and positively by facilitation. However, typically only the end product of these interactions is measured thereby detecting only the outcome, not the interactions themselves. This thesis focuses on understanding net interactions within an annual plant community in the Negev Desert, Israel by measuring the effects of spatial pattern, seed density, plant density, and specific species effects at different life-stages (by successive measurements) and levels of abiotic stress. I first tested the assumption that seeds in seed banks are generally clumped. Using geostatistics, a consistent clump size of 85cm2 was detected across time, stress level, and seed size. There was however variation in the amount of seed present on the dune over time. Subsequently, I experimentally manipulated both the fine scale spatial pattern of patches of seed and the local density of seeds in small patches. Increasing local seed density generally had negative effects on measures of plant growth, whereas patches of seed with adjacent planted patches had increased performance. These results suggest that there is interference between seeds and plants within patches but positive interactions between the patches. The more general effects of density dependence were also tested at the seed and emergent plant levels. Emergence of seedlings was negatively affected by increases in seed density which also supports the interpretation that seed-seed interference may be occurring in this plant community. Mean plant size was negatively affected by increases in plant density, but survival was unaffected. Finally, I tested the prediction that a larger annual, Erodium laciniatum, acts as a benefactor species to nearby smaller annuals. Erodium was either added to or removed from patches of seed or vegetation. In both experiments, Erodium acted as a benefactor by increasing performance of neighbours (i.e., aboveground biomass or survival). Hence, competition and facilitation both play important roles in this plant community and their relative importance is influenced by life stage but not by level of abiotic stress. These studies are the first to demonstrate facilitation similar to shrub-understorey systems but at a much finer spatial scale. |
