In plant ecology, much attention has been paid to the question of whether and under what circumstances neighbors ought to confer fitness benefits via facilitation or costs via competition. Classic theory predicts that the strength of abiotic stress should inform whether neighbor interactions result in net benefits or costs; under more abiotically stressful conditions, it is expected that facilitation is likely to be stronger than competition, as neighbors may ameliorate unfavorable microclimate conditions and/or confer associational defenses against herbivores. Conversely, in more abiotically permissive environments, it is expected that that plants are more likely to compete with neighbors for resources, as the benefits of these facilitative properties decline. Although often invoked, these predictions are broad and fail to consider how individual trait variation and its underlying genetic variation influence the outcomes of these ecological interactions. We performed common garden studies and manipulative field experiments using the ecological model species Boechera stricta (Brassicaceae) to investigate the role of genetic variation, trait variation, and environmental stress in determining whether the density of heterospecific neighbors conferred facilitative or competitive effects on focal plants.
Results/Conclusions
By growing >300 B. stricta genotypes in a common garden environment, we found significant genetic variation in the relationship between heterospecific neighbor density and reproductive output. Furthermore, these interactions varied among years, suggesting that environmental variation modifies the impact of genotype on the level of competition or facilitation experienced. To tease apart effects of microclimate and naturally occurring neighbor density, which may be correlated, we took two approaches. First, we manipulated abiotic stress using rainout shelters over field plots containing two focal genotypes varying in herbivore defense strategies. These two genotypes responded differently to increased drought stress; drought intensified competition for one genotype, while attenuating the magnitude of competition experienced by the other genotype. Finally, we manipulated the density of a heterospecific neighbor in potted B. stricta transplants of the same genotypes. Here, one genotype experienced greater competition from neighbors, while the other benefited from neighbor density, possibly due to associational defenses. Together, these results provide mixed evidence for the general applicability of the stress gradient hypothesis in predicting the magnitude and direction of species interactions, and emphasize the contributions of genetic and trait variation, in addition to abiotic conditions, to ecological outcomes among close neighbors.