Shade tolerance plays a major role in plant distribution across the light gradient. However, plants not only have to deal with low light but also with herbivores. In a temperate rainforest we have shown that herbivores: i) impact plant fitness, particularly in the shade, ii) limit plant functional responses to low light, and iii) modify patterns of selection on functional traits in the shade. However, these studies were conducted with single plant species. Here, we delved further into the interplay between herbivory and shade tolerance in the tree species community of this temperate rainforest. First, we evaluated the relationship between shade tolerance and herbivory. Second, we asked whether the ontogenetic change in shade tolerance of tree species is related to herbivory pressure during the seedling stage. Third, given that relative growth rate (RGR) is a key descriptor of plant species strategies and that it has been related to both herbivory rate and shade tolerance, we addressed the differential linkage of shade tolerance and herbivory rate with RGR via functional traits. Fourth, we asked whether RGR predicted tolerance to herbivory. We used field measurements, field experiments, and some species features were obtained from the literature.
Results/Conclusions
First, we found a negative association between tree species shade tolerance and a field herbivory index standardized by leaf lifespan (N = 15 species). Second, a logistic regression analysis showed a significant relationship between the herbivory rate sustained at the seedling stage and the probability for a tree species to change its shade tolerance at later ontogenetic stages. Specifically, tree species sustaining higher herbivory levels early in their ontogeny became less shade tolerant as juveniles (N = 14 species). Third, we found that RGR was associated negatively with shade tolerance and positively with herbivory rate (N = 11 species), but shade tolerance and herbivore resistance did not covary with the same functional traits. Whereas shade-tolerance was strongly related to photosynthetic rate and to a lesser extent to leaf mass ratio and dark respiration, herbivory rate was closely related to specific leaf area and leaf mass ratio, and slightly associated with protein content. Fourth, seedlings from tree species with higher RGR showed greater tolerance to experimental herbivory in the study forest (N = 9 species), and RGR showed a significant association with leaf lifespan. In sum, we conclude that herbivores may play a significant role in plant adaptation to low light.