The role of pollen limitation on the coexistence of two dioecious, wind-pollinated, closely related shrubs in a fluctuating environment

Elucidating the mechanisms of species coexistence is a crucial goal in ecology. Theory suggests that, when resource abundance fluctuates, coexistence can be achieved if each species in a competing pair is better at exploiting resources at opposite extremes of a fluctuating resource spectrum. Nonetheless, the proximal mechanisms allowing coexistence remain largely unexplored. In a previous paper, we showed that the coexistence of two Atriplex species was facilitated by their varying demographic response (in survival, growth and recruitment) to fluctuation in water availability. Here we explore the effect of spatial distribution, and pollen and resource limitation on the reproductive success (production of viable seeds) of the same two species. An analysis of their spatial distribution showed that Atriplex acanthocarpa had a clumped distribution, which is thought to increase the effectiveness of pollination in wind-pollinated plants, while Atriplex canescens had a random distribution, a pattern expected to restrict wind-pollination success. A pollen and resource (water and nutrients) addition experiment implemented through a repeated-measures design demonstrated that seed viability of A. canescens was both pollen and resource limited, but that these effects were negligible in A. acanthocarpa. Under natural conditions, pollen limitation restricted seed number in A. canescens to only one-third of that recorded when manual pollination was performed. By decreasing its fecundity (and consequent potential seedling recruitment), pollen limitation reverses the competitive advantage of A. canescens over A. acanthocarpa when the limiting resource (water) is abundant and seedling recruitment takes place. To our knowledge, our study of this congeneric pair in the Chihuahuan Desert is the first to document a link between pollen limitation and species coexistence.