The Hawaiian C4 euphorbia adaptive radiation: an ecophysiological approach to understanding leaf trait diversification

Abstract

Foliar traits, such as properties of venation, stomata, papillae, composition, and gross anatomy can provide important information about plant adaptation to the environment as these traits greatly influence plant physiological processes. Examining leaf traits in relationship to the natural physical environments in which they occur can provide a detailed understanding of plant function and adaptation to a set of given environmental conditions. My research focuses on the native Euphorbia subgenus Chamaesyce of Hawaii, a group of C4 eudicots that have diversified across dramatic habitat gradients from one putative herbaceous colonizing species into 29 endemic woody taxa, within the last five million years. This lineage includes a variety of life forms, ranging from subshrubs a few centimeters in height, to trees over six meters tall. Members of the radiation are adapted to diverse habitats, including wet, mesic, and dry forests, bogs, and coastal zones. In this dissertation work, leaf anatomy and physiology were explored in an ecophysiological context. I measured a total of 104 leaf traits from 27 Hawaiian Euphorbia taxa across five Hawaiian Islands to test the hypotheses that leaf traits are aligned with environmental factors including rainfall, precipitation, humidity, vapor pressure deficit, elevation, and with habitat irradiance, and that leaf traits are coordinated in plant function. In most cases, I found that leaf traits correlated with environmental factors similarly to what has been reported in previous studies of distantly related species sampled within or across communities. I confirmed that the C4 Hawaiian Euphorbia lineage has diversified across habitat types in their overall growth form and that there is exceptional variation in foliar characteristics for these taxa indicating strong adaptation to the diverse environments and habitats. Thus, I found very large variation across taxa in leaf morphology and nutrient composition; in stomatal distribution, size and densities; the presence of papillae; and venation characteristics. This work captures, in detail, some of the greatest variation for leaf traits across taxa within a genus ever reported and demonstrates the rapid evolutionary diversification of many aspects of leaf structure and function.