Competition is often thought to be stronger than facilitation. In alpine plant communities this relationship may reverse due to abiotic stress. Mat-forming plants may mediate these processes by modifying the vital rates of plants growing inside them through amelioration of abiotic conditions. These demographic effects may underlie the strong positive associations between individuals in many of these systems. Interannual weather variation and climate change may affect the strength and direction of these plant-plant interactions, though the effect of these changes on demography is unknown. We hypothesized that plant vital rates within versus outside mat-forming plants would be improved due to facilitation, and that the magnitude of these differences would increase with climatic stress, such as drought.
We set up fifty 2 × 2 m permanent plots in an alpine community in the Rocky Mountains, Colorado (~3540 m.a.s.l.). Over five years we tracked all core vital rates (growth, survival, fecundity, and recruitment) for over 2300 individuals of 17 species. We tested our hypothesis by comparing vital rates across species, age classes, growing environments (in mat or not in mat), and along gradients in abiotic environments (temperature and soil moisture).
Results/Conclusions
We found species-level differences in vital rates across growing environments, with strongest effects for seedling recruitment. The effects of growing environment on adult survival, growth and fecundity were weak for most species. These patterns were associated with greater water availability inside mats than outside. This strong environmental filtering on early age classes led to clustering of individuals within mats. Interestingly, species that tolerated well abiotic conditions outside mats performed worse inside mats, possibly indicating high interspecific competition. These results were consistent across years despite high interannual variation in snow pack and water availability. Examining vital rates thus identifies microclimate-dependent mechanisms by which facilitation and competition occur. Demographic and dynamic approaches to community ecology will produce further insights than are possible from analysis of species co-occurrence patterns.