Background/Question/Methods Island floras are characterized by disharmonic and phenotypically distinct species. Whether island plant syndromes contribute to their vulnerability to global threats such as invasive plant displacement is generally assumed, but rarely studied directly. One prominent idea is that island plants have more conservative life history strategies, making them weak competitors. While this idea has been examined with respect to leaf physiological traits, an anatomical perspective is largely missing. As one of the most heavily invaded and endangered floras in the world, the Hawaiian Islands offer a compelling case study to investigate anatomical trait differences between native and invasive plant species. We examined leaf epidermal anatomical traits linked to gas exchange, such as stomatal density, distribution, and size, in a phylogenetically diverse sample pool of 74 woody species (45 native, 29 invasive). Epidermal peels of both abaxial and adaxial leaf surfaces were taken from field-collected plants and scanned for digital analysis using ImageJ. Stomatal density was quantified by analyzing three sub-samples for each leaf surface, and stomatal size was calculated for 6-12 stomata guard cell lengths and width. We predicted greater densities of smaller stomata in invasive compared to native plants, consistent with more rapid gas exchange rates in invasive species. Results/Conclusions Across all species, the distribution of stomata were consistently hypostomatous, as is typical for woody tropical plants. Other traits were highly variable across species. For example, stomatal density varied by an order of magnitude, and stomatal size varied by 4x. Nonetheless, some trends emerged, including greater stomatal density in invasive compared to native species, as predicted. Stomatal size was not consistently different between invasive and native species and demonstrated considerable variation within and among species. How these patterns in stomatal anatomy relate to physiological performance underlying growth rate and vigor remains to be examined. However, this study provides evidence that invasive-native trait differences exist in leaf anatomy, highlighting this as a fruitful area for future research, particularly in tropical islands where few species have been anatomically characterized.
