Small-scale, spatial patterns within plant communities have received much attention in recent years, but directionality of such patterns has rarely been considered. We studied spatial patterns of different life stages of shrubs and winter annuals in a Mojave Desert plant community on stabilized, aeolian sand deposits. Directional associations of adult shrubs were observed between the species Ambrosia dumosa and Acamptopappus sphaerocephalus and within Acamptopappus. Seedlings of both shrub species emerged preferably on the northern side of shrubs. Winter annuals showed strongly directional patterns with respect to shrubs, and these patterns varied from year to year. The directional patterns were largely caused by microclimatic conditions during germination and seedling establishment. Because of the low solar altitude during seedling emergence, soil surface temperatures were greatly affected by shading from shrub canopies and the microtopography of shrub hummocks. In addition, preferential wind directions affected spatial patterns of seeds. Existing directional patterns of the annual vegetation cause a positive feedback loop for the patterns of following years, because the litter contributes to local nutrient enrichment, reduces soil evaporation, and functions as an effective seed trap for both shrubs and annual species.

Key words: spatial patterns; directionality; microtopography; desert shrubs; annuals; community ecology