Components of ‘competitive ability’ in the LHS model: Implication on coexistence for twelve co-occurring Mediterranean grasses

Abstract

The leaf–height–seed (LHS) model has been proposed as a simple trait-based functional classification. We investigated whether the two components of competitive ability, i.e. competitive response (CR) and competitive effect (CE), are captured by the LHS model and whether these two components are independent for 12 coexisting Mediterranean grasses. Two greenhouse experiments were conducted to estimate competitive effect and response of 12 coexisting grass species from Mediterranean habitats in Jordan. We applied a phytometer design and calculated CR and CE using the relative interaction index (RII). Mature plant height, seed mass and leaf dry matter content (LDMC, used as the leaf trait) were measured for each species. Correlations and trade-offs between the three traits and the components of competitive ability, CR and CE, were analyzed with principal components analysis (PCA). The LHS model was a good predictor of competitive ability but CR and CE were independent and related to different traits. CR was positively correlated with seed mass and CE with plant height. LDMC was neither correlated to CR nor to CE. Based on these findings, we suggest that there are three primary strategies allowing coexistence in Mediterranean communities, which are related to competition: (1) large CE, i.e. large negative impact on other species associated with large stature, (2) large CR, i.e. resistance to competition associated with large seeds, and (3) competition avoidance associated with small seeds.