A functional comparative approach to facilitation and its context dependence. (Special Feature: Mechanisms of plant competition.)

摘要

Our ability to generalize about broad patterns and outcomes of competitive interactions among plants has improved from a better understanding of functional traits. Facilitative interactions among plants also exhibit strong trait-based patterns; however, very little empirical research has addressed the trait basis of different facilitative mechanisms or the applicability of traditionally defined functional strategies to facilitation. We present a series of predictions regarding the trait basis of a variety of facilitative mechanisms based on empirical patterns of trait responses to environmental filters and knowledge of plant-environment feedbacks. Using a leaf-height-seed framework to predict plant responses to facilitative effects of neighbours, we identify two categories of facilitative mechanisms based on similar predicted responses of specific leaf area (SLA), height and seed size: (i) facilitation of low-SLA, large-seeded and tall plants in environments typified by periodically unfavourable conditions; and (ii) facilitation of high-SLA, small-seeded and tall plants in persistently severe environments. We suggest that facilitation related to herbivory will be idiosyncratic with respect to these functional traits. Competition for limiting resources is predicted to reinforce trait-based responses to facilitation in periodically severe environments, particularly under light competition, but to offset facilitative effects to some degree in persistently severe environments. Thus, biotic interactions may have stronger consequences for shifts in trait distributions in environments typified by disturbance, pulsed soil moisture and short growing seasons limited by low temperatures. Facilitative effects, in addition to facilitative responses, may also be regulated by functional traits, but there is much less empirical evidence for links between functional traits and ameliorative effects on the environment. However, we predict that leaf area index or other integrative traits related to canopy density are likely to be important effect traits for most facilitative mechanisms and should be integrated into more trait-screening programs. Based on the exceptional contribution that trait-based approaches have made for understanding other interactions, we suggest that taking a functional comparative approach to facilitation is an opportunity to improve our ability to identify general patterns and consequences of positive interactions.