Positive interactions in plant communities

Interactions among species are central to the study of plant communities and ecological theory.Until recently,ecologists have focused on competition as the most important interaction among plant species.During the last two decades,however,evidence for the importance of positive interactions(facilitation)among plants in many plant communities has accumulated.Positive interactions among plants occur when the presence of one plant enhances the growth,survival,or reproduction of a neighbor.Such interactions can be considered mutualistic when both species derive benefit from the interaction.Some cases of facilitation may be better described as commensalism in which one species benefits from another,but the other is not affected.Often,positive interactions reduce the negative effects of competition,so the net effect of neighbors is still negative.Positive interactions occur when one organism makes the local environment more favorable for another either directly(such as by ameliorating harsh environmental characteristics,altering substrate characteristics,or increasing the availability of a resource),or indirectly(such as by removing potential competitors,introducing other beneficial organisms or protecting their neighbors from herbivores or parasites).The net effect of plant interactions is frequently measured as the ratio of some measure of performance,usually biomass,of individuals with neighbors removed to that of control plants with neighbors left intact.It is difficult to distinguish positive interactions from negative interactions because both effects operate simultaneously,and we can only observe the net effect of plant interactions.Several indices have been used over the years to explore the net effect of plant interactions.The relative interaction intensity(RII)in plants has been proposed because it has useful mathematical and statistical properties,thus avoiding many of the problems associated with other frequently used indices.In addition,RII can be used to measure multispecies interactions at the community level.Certain ecological models(e.g.individual-based modelling)incorporating plant facilitation provide useful tools for exploring some of the fundamental processes within plant communities(e.g.species distributions along environmental gradients,biomass-density relationships and size inequality in plant populations).The stress-gradient hypothesis(SGH)predicts that the frequency of facilitative and competitive interactions will vary inversely across abiotic stress gradients,with competitive interactions being most important when abiotic stress does not strongly limit the ability of plants to acquire and exploit resources,and facilitative interactions being more important when abiotic stress is high or when consumer pressure is intense.The SGH has been supported by numerous studies in many ecosystems,and has provided a foundation for studying the balance between facilitation and competition in plant communities.A growing number of empirical studies do not support the SGH,however,It may be possible to refine specific predictions relevant to the SGH by explicitly considering the life history of the interacting species(relative tolerance to stress versus competitive ability)and the characteristics of the stress factor(resource versus non-resource)over wider stress gradients.In addition,inclusion of consumer-incurred biotic stress can alter the predicted outcome of interactions along resource-and non-resource-based stress gradients for both benefactors and beneficiaries.Positive interactions may influence community spatial patterns,permit coexistence,enhance diversity and productivity,and drive community dynamics.Inclusion of facilitation into the theory,models and paradigms of population and community ecology may alter many basic predictions.We also consider future directions for research on facilitation,emphasizing the need to increase our understanding of(1)the balance between positive and negative interactions and their effects on community organization,(2)positive interactions in the context of global warming,(3)integrating positive interaction with evolution,and(4)the potential role of positive interactions in ecosystem restoration.