Epiphytes as a bioclimatic indicator in Hawaii

Epiphytes occupy tight, climate-defined niches compared with co-occurring life forms such as trees. Because of Hawaii Island's natural climatic diversity, it is an ideal location to understand how these intrinsically climate sensitive plants interact with the atmosphere and evaluate how they may serve as a near-term indicator of climate change. Here we establish a baseline of corticolous epiphyte communities by 1) investigating patterns of epiphyte abundance and species composition across elevation and precipitation gradients on windward Hawaii Island, and 2) using physiological measurements to investigate the relative importance of rain vs. fog in epiphyte-atmosphere interactions. Overall, epiphyte communities showed much finer scale responses to climate variation when compared with structurally dominant vegetation. The precipitation gradient exhibits a clear increase in abundance of all epiphyte groups and an increase in diversity with increasing rainfall. Results across the elevation gradient show a higher abundance of filmy ferns and bryophytes above the lifting condensation level where fog incidence is highest and PET is lowest, as well as a marked difference in species composition. A second focus of this investigation involves using stable isotope ecology to trace differences in rain and fog water sources at two sites of equal precipitation: Volcano and Kohala. Results indicate that the delta18O signature of fog at Volcano is significantly enriched compared to rain, but that sources do not separate at Kohala. Volcano's precipitation patterns include orographic moisture with occasional synoptic scale low-pressure systems that deliver heavy rainfall. Results suggest that the major contributor to precipitation in Volcano is rain whereas horizontally driven moisture is the major contributor at Kohala. Because isotope composition is a marker for vapor source and condensation history, these results indicate very different precipitation patterns at Kohala and Volcano.