Water loss physiology and the evolution within the Tasmanian conifer genus Athrotaxis (Cupressaceae)

The Tasmanian montane and rainforest conifer genus, Athrotaxis, provides a system for investigating the relationship between leaf form and function and its adaptive significance. The two species differ markedly in leaf size, degree of imbricacy and stomatal distribution. Hybrid swarms in the field and glasshouse grown hybrid progeny are highly variable for these traits. In glasshouse grown plants of the true species and a diverse hybrid progeny, stomatal conductivity and density varied by about 200% among individuals with a strong correlation between these traits. Hybrids were found to produce lower stomatal densities and less discrimination of 13C than either species, leading to a negative relationship between stomatal density and delta 13C. In contrast to the highly variable stomatal densities and delta 13C in glasshouse plants, field plants were very conservative in both characters. Relatively low stomatal density and high water use efficiency in field plants suggests strong natural selection to optimize the trade-off between assimilation and water loss. Foliar conductance in the light for the hybrids and A. selaginoides was only 4-6 times as great as, and was strongly correlated with, conductance in the dark. This suggests either incomplete stomatal closure or greater cuticular conductance. In contrast, A. cupressoides appeared to be less "leaky", which may reflect adaptation to its more exposed habitat.