Different predictions of traits on elevational distribution of Fagaceae species between ever-wet and seasonally dry regions in Southeast Asia

Plants optimize traits to adapt to an environment, forming the basis of trait–environment relationships. However, it is unclear whether these relationships are evolutionarily and spatially robust, particularly in species-rich tropical forests. In this study, we examined the relationships between species elevational distribution and traits that represent the major axes of resource-use-strategies (leaf traits, maximum height, and wood density), focusing on Fagaceae, which occupies diverse elevational niches in tropical montane forests. We investigated two tropical regions (northern Borneo [NB] and northern Thailand [NT]) with different environmental gradients along elevation. NB has increasing temperatures at lower elevations with high levels of rainfall at all elevations, whereas NT has increasing temperatures and dry soil, particularly during the dry season at lower elevations. Different traits were related to the species distribution in the two regions. In NT, species with high desiccation tolerance in the leaf and stem were distributed at drought-prone low elevations. These species do not occur at stress-moderate high elevations, likely because of strong resource competition. In NB, species with durable leaves were associated with harsh higher elevations and wider elevational ranges. The predictions of elevational ranges by some leaf traits contrasted between the two regions. These results suggest that the influence of traits on growth and survival largely depends on resource gradients along elevation and, presumably, water availability. Our results raise concerns about using a single trait to predict the future distribution of species under climate changes in different environments.