Responses of leaf biomechanics and underlying traits to rangeland management differ between graminoids and forbs

QuestionAlthough leaf biomechanical properties have been identified as critical traits for plant-herbivore interactions, their responses to grazing pressure have been poorly investigated. Intensification of rangeland management, associated with fertilization and an increase in grazing pressure, has been shown to favour fast-growing species that can compensate for biomass losses due to grazing. According to the postulated trade-off between resource acquisition and defence, it is often expected that acquisitive traits should be associated with low leaf mechanical resistance. Here we investigated the responses of two leaf biomechanical traits, and their underlying traits, to management intensification.LocationWe used a long-term experiment in a rangeland located in the Mediterranean region of Southern France, in which three treatments corresponding to different fertilizer inputs and sheep grazing pressures were established.MethodsWe sampled 24 abundant graminoid and forb species. The responses of work to shear and force to tear to the treatments were tested together with those of growth-related leaf traits (leaf mass per area, dry matter content). To better understand the observed patterns, we tested whether the difference between species' leaf biomechanics could be explained by morpho-anatomical characteristics such as leaf thickness and density.ResultsConsistent with the acquisition-defence trade-off hypothesis, we found that graminoids from fertilized and intensely grazed areas had lower leaf resistance than those in ungrazed areas. However, no difference in leaf biomechanics was found in forbs despite a significant decrease in leaf mass per area and leaf dry matter content with management intensification. Consistent with this, we found no significant effect of morpho-anatomical traits on either biomechanical trait in forbs.ConclusionsOur results suggest that the observed responses in graminoids result from phenotypic constraints between resource acquisition and biomechanical defence. However, these phenotypic constraints appeared to be released in forbs, questioning the idea of a universal relationship between these two functions. We investigated how leaf biomechanics responds to grazing and fertilization, and its coordination with resource acquisition traits. Management intensification favoured species with lower leaf toughness in graminoids, but not in forbs. In graminoids, low toughness was related to fast acquisition, while this was not the case in forbs. Variations in leaf anatomy are hypothesized to explain these differences.image