Topographic Controls on Stomatal and Mesophyll Limitations to Photosynthesis in Two Subalpine Conifers.

Premise of research. Leaf stomatal and mesophyll conductances limit photosynthesis and influence water use efficiency. Few studies have quantified the relative limitations imposed by these CO2 diffusion pathways on photosynthesis in mature conifer trees under natural conditions. Here, we report observations of stomatal and mesophyll conductance changes during seasonal drying across contrasting topographic positions in two Rocky Mountain conifers. We predicted that topographic controls on soil water availability and energy balance would determine limitations to photosynthesis by mesophyll conductance across conifer species with contrasting patterns of stomatal and hydraulic traits. Methodology. Concurrent measurements of leaf gas exchange and carbon isotope discrimination were used to estimate stomatal (g s) and mesophyll (g m) conductance in branches of an isohydric species, lodgepole pine (Pinus contorta), and an anisohydric species, Engelmann spruce (Picea engelmannii), in the central Rocky Mountains. Quantitative limitation analysis of photosynthesis (A) was then performed using data from CO2 response curves. Pivotal results. Stomatal conductance imposed greater limitations on photosynthesis (42%–67%) than mesophyll conductance (5%–17%), but no significant differences in g m were observed between the two conifer species. At the mesic lower hillslope position, A , g s, and g m increased during the growing season despite declines in soil moisture. In contrast, at the drier upper hillslope position, declines in soil moisture and increases in air temperature during the growing season are correlated with reductions in g s but not with A or g m. Conclusions. Adjustments in g m played a potentially important role in sustaining photosynthesis and improving plant water use efficiency when stomatal conductance decreased with water limitation during the growing season at the research site. Sustained g m with seasonal drought may be an important mechanism allowing conifers to survive and maintain competitive dominance in low-resource habitats. [ABSTRACT FROM AUTHOR]