Scale-dependent variation in leaf functional traits clarifies mechanisms of invasion

Comparing the functional traits of co-occurring native and invasive plant species can offer insight regarding mechanisms of invasion. Previous studies have failed to reach a consensus, indicating that the extent of trait differences between native and invasives might depend on the environmental context and on the spatial grain of the analysis. Here we evaluate the scale-dependency of native-invader trait comparisons within the Hawaiian Archipelago, a globally important region of high endemicity and high invasibility. We evaluated trait (e.g. gas exchange, leaf nutrient concentration, specific leaf area) differences locally and regionally (i.e. within and across islands) and found that while invasives are more resource acquisitive at the regional scale, native-invader differences are highly idiosyncratic at localized scales, varying both in direction and magnitude within islands. Our findings clarify how region-wide species comparisons may fail to characterize interactions happening at local scales, thereby misleading or obscuring the mechanisms underlying invasion.

Methods
Morphological and physiological traits were measured on individual plants of native Hawaiian species and non-native invasive species across a range of sites in Hawaii. The dataset includes 21 sites across four of the Hawaiian Islands, with a total of 87 species. We removed obvious outliers or unreasonable values, including negative photosynthesis, transpiratoin, or stomatal conductance, and extreme values of specific leaf area (SLA). Leaf chlorophyll content was measured on three leaves per individual and averaged together to produce one value per individual. The remaining traits were measured on one leaf per individual. Individual replicates were averaged to site level and island level for statistical analyses in our manuscript. All variables were then log10 transformed, with the exception of soil pH, because it is already on a logarithmic scale. The final dataset includes the logscaled data rather than the raw data.