| Tropical dry forests are among the most endangered ecosystems worldwide and the Hawaiian Islands are located within a biodiversity hotspot. This dissertation examines the roles of biological, climatic, and anthropogenic factors in determining the levels of endangerment, patterns of species richness, structure, and composition in Hawaiian dry forest trees. Chapter 1 examines the current endangerment of dry forest trees. Forty-five percent of Hawaiian dry forest trees are at risk of endangerment. Dry forest trees at risk have a significantly larger range size compared to trees at risk in other Hawaiian forest types. Among dry forest trees, those with hermaphroditic breeding systems, autochorous dispersal mechanisms, conspicuous flowers, and dry fruit were all more likely to be at risk of endangerment. In Chapter 2 field measurements, GIS, and remote sensing are used to examine patterns of diversity in remaining dry forest fragments. Big Island contains the largest area of remaining native dry forests (1.4% of island area) and is among the highest in species richness. Anthropogenic factors such as fragmentation, or biological interactions related to resource competition, appear to be the dominant determinants of species richness as opposed to climate-controlled processes. In Chapter 3 remote sensing is used to examine the leaf phenology of Hawaiian forests over the past 9 years. Dry forest NDVI (Normalized Difference Vegetation Index) is more tightly coupled with precipitation seasonality compared to rainforest NDVI. While dry forests experienced reduced photosynthesis during the 2002-2003 El Nino drought, rainforest photosynthesis increased. In Chapter 4 a paleo-ecological analysis of the sediments from Kealia Pond, Maui is used to examine climate change and human impact on lowland forests over the past 2,600 years. Native forests appear to have been relatively resilient to fire throughout much of the Polynesian period. The greatest reduction in overall diversity occurred during the European period, and primary accounts by Europeans during the 18th and 19th centuries provide a historical narrative that supports this interpretation. Furthermore, there is strong evidence of increased precipitation during the Medieval Climate Anomaly (977-1212 AD) and this hydrological shift may have supported rapid human population growth and expansion into dry marginal environments. |
