The mechanisms that determine how a non-native species becomes a dominant, invasive member of a plant community remain poorly understood. Previous studies examining the causes of invasion have focused on invader traits and community attributes (e.g. diversity) that promote resistance to invasion. However, more recent studies have attempted to predict invasiveness using phylogenetic relatedness and/or novelty of non-natives relative to natives, with contradictory results. A successful non-native species should be similar enough to the native residents to persist in the habitat but different enough to compete well for resources. Previously, the relationship between phylogenetic novelty and invasiveness has been examined at relatively large spatial scales. We expand these studies to encompass multiple spatial scales (from individuals to islands) and a precipitation gradient. We collected functional trait data on native and non-native plants across the Hawaiian Islands. Studies on phylogenetic novelty of invasives are well suited to the Hawaiian Islands because native species exhibit strong phylogenetic relatedness due to geographic isolation. The data include leaf thickness, specific leaf area, chlorophyll content, maximum photosynthetic capacity, and stomatal conductance, among others.
The functional trait responses were highly variable among native and non-native species, and we found no uniform patterns in response to precipitation within either group, contrary to our expectations. For both native and non-native species, physiological responses were correlated with some but not all of the morphological traits, and these relationships were significantly influenced by precipitation levels. We also found that plant habit (trees versus shrubs versus vines) also significantly influenced functional trait responses. Lastly, changes in spatial scale (within a site to across islands) significantly influenced the functional traits of our study species, highlighting the need for integration of spatial variation in studies of plant community dynamics and assembly processes. Our results suggest that within Hawaii, native and invasive species overlap in their functional traits, and that the degree of overlap changes with spatial scale and resource availability. Increasing drought associated with global climate change may therefore have mixed effects on the community composition of Hawaiian forests.