The drivers and mechanisms of C4 and C3 grass distributions in Hawaiʻi under current and future climates

Ph.D. University of Hawaii at Manoa 2012.Includes bibliographical references.The two most common photosynthetic pathways in flowering plants are C4 and C3 photosynthesis. Forty percent of species in the Poaceae (the grass family) have C4 photosynthesis while the other 60% use the C3 photosynthetic pathway. C4 plants generally have a high optimum temperature range; while C3 plants have a lower optimum temperature range. Over the last 200 years, over 400 grass species having both C4 and C3 photosynthetic pathways have been introduced to Hawaiʻi, with 100 of these species becoming naturalized. In Hawaiʻi, most nonnative grasses that have altered the grass fire cycle and threaten native plant communities are C4 grasses. Therefore, it is important to understand the drivers and limitations of C4 and C3 grass distributions in Hawaiʻi. In addition, to understanding current distributions of C4 and C3 grasses, it is also important to determine how these distributions will be impacted by aspects of global change such as climate warming, altered precipitation patterns, and elevated CO2. This work used field surveys across broad elevation transects, historical data, manipulative experiments, and climate-niche modeling to determine what climatic factors drive C4 and C3 grass distributions, along with determining how these distributions might change in response to global change. Relative C4/C3 grass abundance and richness patterns were found to be broadly driven by temperature. This suggests that the C4-C3 transition point (the point of dominance between C4 and C3 grasses) will shift upward in elevation with climate warming. Distributional patterns for individual C4 species were found to be complex and require species-specific evaluation. C3 species consistently responded as predicted with climate warming and elevated CO2 revealing that these species will likely have smaller distributions in the future at higher elevations in Hawaiʻi and may become more competitive in drought conditions with increased CO2. C4 plants were found to have positive growth and physiological responses to elevated CO2, however, some species may have physiological limitations under drought stress that cannot be overcome with increased CO2. Similar patterns are expected for ecosystems elsewhere in the tropics, where both C4 and C3 grass persist