My postdoctoral work focuses on understanding the ecological and evolutionary factors that have helped shape the distribution of native C3 and C4 grasses in Hawaii. Previous work has demonstrated sharp altitudinal crossovers between C3 and C4 dominance on tropical mountains across the world. The dominance of C4 grasses at lower elevations has been attributed to the higher mean annual tempertature of those areas. This is justified by the quantum yield hypothesis, which holds that under light-limited conditions, C4 plants will outperfom C3 plants at higher temperatures. If quantum yield were a primary determinant of C4 distribution, then C4 grasses should dominate in warm tropical understory environments-- but C4 grasses are mostly found in warm, high-light environments. We hypothesize that a combination of factors, including light, moisture, and nutrient availability, are as important as temperature in determining C3 or C4 ecological dominance. We'll test these ideas in Hawaii with a combination of GIS mapping and fieldwork along altitudinal transects.

At a more general level, we hope to also re-evaluate the usefulness of C3/C4 functional type categories. C4 photosynthesis has evolved independently in a variety of grass lineages, and much of the pioneering work on comparative C3/C4 grass physiology only included a handful of species that were not particularly well sampled from the grass phylogeny. It is quite possible that certain ecological and physiological traits that we commonly associate with being a C4 grass may instead be traits associated with belonging to a particular grass lineage. We will begin to explore these issues by analyzing Hawaiian and North American C3/C4 grass distributions within a phylogenetic context.

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