Structure and dynamics of Sonoran Desert perennial plant communities

Arid ecosystems are dominated by perennial plants that vary substantially in growth form, phenology and life-history strategies. Species exhibit common and specific responses to variation in soil moisture, which is a function of precipitation pulses and modulation of inputs by spatially patchy vegetation through root uptake and reduced evaporation rates beneath canopies. This spatial and temporal variation in soil moisture strongly regulates both the structure and dynamics of desert perennial plant communities, which was investigated across Sonoran Desert upland ecosystems.;To understand species-specific population dynamics and functional strategies a suite of functional traits and short-term drought survival were measured for seedlings of 20 species of woody desert perennials. Species were differentiated by two functional axes associated with survival and relative growth rate, producing distinctly different strategies between species that rely on facilitation for establishment and those that do not. Furthermore, facilitative response was also found to influence mature plant functional strategies, which were studied through the analysis of leaf functional traits, water use efficiency and growth form across mature plants of 54 woody species. Leaves of desert perennials were proximately carbon limited due to whole plant water limitation, generating a broad range of leaf carbon deployment strategies that were strongly related to growth form, phenology and life-history strategies.;In order to determine the direct, indirect and modulatory effects of precipitation and existing vegetation on recruitment and mortality structural equation modeling was used to analyze 65 years of vegetation change. Facilitation (positive plant-plant interaction) played a particularly important role in influencing recruitment, as did precipitation in both direct and indirect ways, while competition drove mortality. Based on these empirical results, a simulation model was developed that demonstrated both buffering and amplifying effects of facilitation on community dynamics. To put this work in a broader context, the modeling effort was expanded to assess effects of facilitation and competition on community stability across gradients of environmental severity. The models demonstrated that complex community dynamics can be generated by the interplay between facilitation and competition in moderately severe environments, whereas qualitatively similar behavior was exhibited across communities in more severe environments (including most deserts).