Leaf trait trade-offs and their implications for ecosystem dynamics in Hawaiian and continental floras

The leaf traits of dominant species are known to be important to local ecosystem dynamics. We compared the leaves of non-monocotyledoneous Angiosperm species in the mountain forests of Hawai¨i with those in a pooled database from continental floras. We found that the leaves of Hawaiian species had significantly higher mass per area (LMA) and tensile strength than those from continental floras. In general, they were not tougher at a certain LMA, although there was a weak indication that Hawaiian low-LMA plants are tougher at a given LMA than continental plants. This pattern was maintained even when only understorey plants from nutrient-poor habitats were considered. The leaves of Hawaiian species contained significantly lower nitrogen (% dry weight) than those of continental species. They also had more N at a given LMA. We suggest that abiotic factors alone cannot account for the patterns observed, that they could be at least partly explained by the absence of large vertebrate herbivores over evolutionary time in Hawai´i.  Empirical evidence and model simulations support the idea that the absence of large generalist herbivores through evolutionary time drives plant-soil-microbial feedbacks towards a tighter mode of nutrient cycling, and results in more conservative and unpalatable leaves in Hawai´i.