Competitive ability, stress tolerance and plant interactions along stress gradients

Abstract

Exceptions to the generality of the stress-gradient hypothesis (SGH) may be reconciled by considering species-specific traits and stress tolerance strategies. Studies have tested stress tolerance and competitive ability in mediating interaction outcomes, but few have incorporated this to predict how species interactions shift between competition and facilitation along stress gradients. We used field surveys, salt tolerance and competition experiments to develop a predictive model interspecific interaction shifts across salinity stress gradients. Field survey and greenhouse tolerance tests revealed tradeoffs between stress tolerance and competitive ability. Modeling showed that along salinity gradients, (1) plant interactions shifted from competition to facilitation at high salinities within the physiological limits of salt-intolerant plants, (2) facilitation collapsed when salinity stress exceeded the physiological tolerance of salt-intolerant plants, and (3) neighbor removal experiments overestimate interspecific facilitation by including intraspecific effects. A community-level field experiment, suggested that (1) species interactions are competitive in benign and, facilitative in harsh condition, but fuzzy under medium environmental stress due to niche differences of species and weak stress amelioration, and (2) the SGH works on strong but not weak stress gradients, so SGH confusion arises when it is applied across questionable stress gradients. Our study clarifies how species interactions vary along stress gradients. Moving forward, focusing on SGH applications rather than exceptions on weak or nonexistent gradients would be most productive.

Aridity preferences alter the relative importance of abiotic and biotic drivers on plant species abundance in global drylands

“…For example, it has often been questioned whether the changes in the sign and frequency of plant–splant interactions across environmental gradients are driven by the environment itself, or by the changes in species composition along the studied gradients (e.g. Gross et al., ; Hacker & Gaines, ; Liancourt, Le Bagousse‐Pinguet, Rixen, & Dolezal, ; Qi et al., ; Soliveres & Maestre, ). Additionally, compositional shifts towards communities dominated by species specialized to thrive under arid conditions could make plant communities less sensitive to further increases in aridity (Ulrich et al., ).…”

Facilitation from an intraspecific perspective – stress tolerance determines facilitative effect and response in plants

“…() found that, under drought conditions, two competitive grass species showed stronger facilitative response to neighbours than was the case for coexisting drought‐tolerant species. A similar result was found in more comprehensive experimental studies for plants grown along a salinity or pH gradient (He et al ., ; Qi et al ., ). In general, our findings are compatible with most previous studies and theoretical predictions about the trait dependence of facilitative response, although there is conflicting evidence as well (Donovan & Richards, ).…”

“…This has been confirmed by many empirical studies conducted in salt marshes and along other salinity gradients (Bertness, ; Bertness & Shumway, ). If shading was the main mechanism in our study, too, then facilitative effects should be size‐dependent, with large plants exhibiting a stronger facilitative effect than small plants (Qi et al ., ). This rationale had inspired our hypothesis about salt‐tolerant plants having a stronger facilitative effect than sensitive plants, because they are bigger under saline conditions.…”

“…However, experimental studies mostly focused either on the beneficiaries (Liancourt et al ., ; He et al ., ) or on benefactors alone (Schöb et al ., ); that is, the trait dependence of both aspects of facilitation has not been explored simultaneously. Furthermore, although a few experiments about reciprocal facilitative interactions have been carried out (Pugnaire et al ., ; Holzapfel & Mahall, ; Tirado et al ., ; Qi et al ., ), the explicit differentiation between traits affecting facilitative effect and response has not been made. Here, we define facilitative effect as the ability of a benefactor to ameliorate stressful environmental conditions for its neighbours, and facilitative response as the potential of a plant to benefit from the presence of neighbours.…”

Wave stress and biotic facilitation drive community composition in a marginal hard‐bottom ecosystem

“…Many studies suggest a decline in facilitation toward the most severe conditions, as facilitators are less successful in ameliorating stress in very harsh environments (Kawai and Tokeshi ; but see He and Bertness , Qi et al. ). Therefore, while the facilitation of mussels at 8 m depth increases with exposure throughout the wave exposure gradient (Fig.…”