Abstract
The functional trait composition of plant communities reflects the influence of ecological constraints on their assembly. In high-elevation peatlands of the Andes, multiple abiotic and biotic stresses are exerted on herbaceous plant communities. We hypothesized that dominant cushion-plant species found in these ecosystems strongly influence the dynamics of their subordinate plants by providing protection against external stress. To test the hypothesis, we addressed the variation of functional traits reflecting how subordinate plants invest in durable tissues and develop below and above the canopy of two cushion-plant species dominating the ecosystem, with varying intensity of herbivore pressure. We expected different functional composition of plants living below (understory stratum) and above (emergent stratum) the canopy, related to cushion-plant protection. In addition, we considered the functional composition of basic morphological guilds, forbs and graminoids, as they invest differently in plant development and respond differently to herbivore pressure. We found contrasted functional compositions among morphological guilds and according to the position relatively to cushion-plant canopy. These variations support a protective influence of cushion-plants on subordinate plants for both strata, with investment into less costly leaves and a larger stature when protection is greater, depending on cushion-plant species and cushion-plant height. External stress mainly affected emergent plants and led to different responses according to the guild. Graminoids showed avoidant strategy, while forbs shifted from avoidant to tolerant strategy with increasing herbivore pressure. Both between-species and intraspecific trait variations contributed to explain functional changes of guilds, but the relative contributions depended on morphological guild and stratum. A basic trade-off in physiological and morphological variability determined different relative contribution in forbs and graminoids. While functional ecology classically focuses on the composition of spatially delimited communities, we here underline the need to address separately the functional composition and assembly dynamics of distinct guilds making a community. In addition, our survey is the first to investigate the functional ecology of high-elevation tropical peatlands and provides novel insights into vegetation ecology in this stressful ecosystem.