Phenotypic effects of the nurse Thylacospermum caespitosum on dependent plant species along regional climate stress gradients

Abstract

Contrasting phenotypes of alpine cushion species have been recurrently described in several mountain ranges along small‐scale topography gradients, with tight competitive phenotypes in stressful convex topography and loose facilitative phenotypes in sheltered concave topography. The consistency of phenotypic effects along large‐scale climate stress gradients have been proposed as a test of the likely genetic bases of the differences observed at small‐scale. Inversely, plastic phenotypic effects are more likely to vanish at some points along climate stress gradients. We tested this hypothesis for two phenotypes of the alpine cushion species Thylacospermum caespitosum at four points along regional gradients of cold and drought stress in northwest China. We measured the traits of the two cushion phenotypes and quantified their associated plant communities and environmental variables along the regional temperature and aridity gradients. Cushion height, convexity and stem density overall showed significant effect of phenotypes. Difference in tightness of cushions between phenotypes was consistent across climate conditions, whereas differences in cushion convexity and height between phenotypes increased with increasing cold stress. Phenotypic effects on species richness and abundance were consistent along both climate gradients but not effects on species composition, while there were no phenotypic effects on environmental variables. Additionally, RII (relative interaction index) curves were linear along the drought gradient but unimodal along the temperature gradient, likely due to the occurrence of contrasting species pools at the different sites. We conclude that the consistency of phenotypic effects of T. caespitosum was high for species richness and abundance and mainly explained by differences in interference mediated by likely heritable differences in cushion tightness. Additionally, our study shows that the shapes of the relationship between plant responses to neighbours and environmental stresses are not necessarily driven by niche‐based deterministic factors.

Phenotypic structure of plant facilitation networks

“…To properly capture not only the continuous nature of the measured variable but also its variation with time and space, intraspecific trait variability should be included in the study of plant facilitation networks (Poisot et al ., 2015). For example, some studies have shown that a single nurse species may have different phenotypes that have very contrasting effects on their facilitated plants (Michalet et al ., 2011; Jiang et al ., 2018; Pistón et al ., 2018). To capture the intraspecific temporal variation of traits is especially problematic for seedlings, which have fast growing rates and it is hard to determine when to measure them (Butterfield and Briggs, 2011).…”

The balance of canopy and soil effects determines intraspecific differences in foundation species’ effects on associated plants

“…(), and Jiang et al. () stressed the importance of above‐ground traits—in particular canopy compactness—to explain the negative effects of tight phenotypes as compared to loose ones on associated species, although Schöb et al. () showed opposite effects.…”

“…Tighter phenotypes have been shown to reduce temperature and light but increase soil moisture and litter compared to loose phenotypes (Classen, Chapman, Whitham, Hart, & Koch, 2007;Crutsinger, Strauss, & Rudgers, 2010). Michalet et al (2011), Al Hayek et al (2015b), Bonanomi et al (2016), and Jiang et al (2018) stressed the importance of above-ground traits-in particular canopy compactness-to explain the negative effects of tight phenotypes as compared to loose ones on associated species, although Schöb et al (2013) showed opposite effects.…”

Benefactor facilitation and beneficiary feedback effects drive shrub‐dominated community succession in a semi‐arid dune ecosystem

“…They explained this interesting result by species pool effects, especially the lack of stress‐intolerant species under the most extreme conditions of stress. Similar species pool effects have also been shown to potentially affect the shape of the relationship between plant–plant interactions and environmental severity in salt marsh communities (Pennings, Selig, Houser, & Bertness, ), desert systems (Noumi, Chaieb, Michalet, & Touzard, ) and alpine communities (Jiang et al., ).…”