Vegetation dynamics in arid areas are complicated by strong temporal variation in species abundances, as revealed by long-term studies. This variation in large part is driven by environmental variation. In the Sonoran desert, for example, prolonged drought usually leads to large declines of dominant perennial plants. Episodic recruitment associated with unusually heavy precipitation leads to population recovery. An important question is the role of such temporal variation as a mechanism of species coexistence. Due to the long life spans of perennial plants and lack of theoretical models for perennial plant dynamics in arid environments, we developed a size-structured model of community dynamics in a temporally varying environment. This model has the advantage of being able to make good use of demographic data for the goal of quantifying species coexistence mechanisms. Our study starts by modifying the standard lottery model by allowing all individuals in the guild, juvenile or adult, to compete for space to establish and grow. We make the lottery model more realistic by further including size-dependent competitive ability in addition to size-dependent schedules of fecundity and mortality.
The competitive ability of a single individual scales with its body size, and determines its probability of establishment as a seedling or growth for larger size classes. Different scaling relationships capture differences in species' life-history strategies: shorter-lived smaller-sized fast growing vs longer-lived larger-sized slow growing species. Through quantification of storage effect, we can see how species differences in scaling relationships affect the strength of species coexistence. To begin with, we assume that the environment just affects early seedling recruitment. We compare the stability of the community under current patterns of environment variation and future scenarios. The model focuses on species such as the saguaro, which has a narrow window of recruitment, and its associated species. One strength of our model is the ability to make good use of the long-term size-structured data for the saguaro. More importantly, it allows the storage effect to be estimated for the cases where competition for resources for growth occurs over a range of size classes, something not possible with the original lottery model without compromising the data. Further developments will include the effects of environmental variation on competitive ability.