Plant litter trait variation between native and nonnative species across steep climate gradient in Hawaiian Islands

Oceanic islands have high biodiversity due to high rates of endemicity, which is now severely threatened by global change, including biological invasions. Invasive plants are predicted to displace native plants via vigorous resource use associated with fast growth rates and population expansion. The corresponding dynamics associated with invasive plant litter offer important insights to bridge live foliage traits associated with competition with invasive plant effects on ecosystem function via litter decomposition. Evidence has accumulated to support the prediction that invasive species produce higher quality litter than native species, which decomposes more rapidly, in turn providing positive feedback that facilitates their expansion. However, litter quality can vary among and within species across climate gradients, which is likely to contribute to spatial variation in native-invasive plant interactions. In this study, we synthesize a large body of litter trait data using systematic review methods and quantitative analyses, to investigate litter trait differences between native island plants and non-native plants established in natural habitats across steep elevation (7.5 – 2660 m) and mean annual rainfall (272 – 6362 mm) gradients of the Hawaiian Islands. We found that litter traits are highly variable in both native and invasive species, with considerable overlap in multivariate trait space. Intraspecific and interspecific differences were the main sources of litter trait variation, which explained 40% and 41% of the total variance, respectively. Nonetheless, as predicted, invasive plants had litter that tended to be of higher nutritional quality and lower toughness than native plants, although this difference explained only 8% of the total variance across all traits. Interestingly, litter traits varied significantly with respect to temperature and rainfall, and the patterns differed between native and invasive plants. These results corroborate previous studies on live foliage traits that climate mediates invasive-native plant interactions across the heterogeneous environment of Hawaii. These patterns emphasize the importance of considering litter as part of the functional syndrome of plants and for a better understanding of how invasive plants may alter their novel ecosystems.