Title: Ecophysiological controls of conifer biogeographic distributions
Description: The absence of the majority of conifers from tropical forests remains an unresolved evolutionary and biogeographic enigma. Correlations between a flattened conifer leaf morphology and tropical persistence have led to the hypothesis that shade tolerance is critical for conifer survival in these environments. Yet, little is known of the physiological mechanisms constraining conifer distributions. This dissertation, using the diversity of leaf morphologies and biogeographic distributions found among conifer families, examines this hypothesis from a physiological perspective. The first chapter examines the photosynthetic process in shade-house grown saplings of conifers coexisting in the Central Highlands of Vietnam, an area of unusual tropical conifer diversity. Pinus krempfii, a flat-leaved pine endemic to the densely-canopied tropical forests of Vietnam, demonstrates an intermediate photosynthesis between needle-leaved Pinus and flat-leaved Podocarpaceae, thus providing support for shade tolerance as an important driver of conifer success in tropical forests.

In chapter two, respiratory traits are examined in adult conifers growing naturally in the Central Highlands of Vietnam. Flat-leaved conifer species are found to have lower respiration than needle-leaved species. Finally, in chapter 3, the respiratory temperature responses of members from the two largest conifer families, the Pinaceae and Podocarpaceae, are compared. Consistently lower respiration in Podocarpaceae compared to Pinaceae suggests that there are phylogenetic underpinnings to the respiratory differences between the two families. These findings correlate to the biogeographic differences between the two families, with Podocarpaceae being notably more successful in tropical forests than Pinaceae. Together, all three chapters demonstrate that lower foliar carbon fluxes and flattened leaf morphologies are characteristics of a shade tolerance strategy that is necessary for conifer persistence in tropical forests. Ultimately, this dissertation makes a unique link between conifer physiologies and their current biogeographic distributions that will pave the way for a mechanistic tackling of their future survival in a changing climate.