Stomatal, mesophyll and biochemical limitations to photosynthesis and their relationship with leaf structure over an elevation gradient in two conifers.

Photosynthetic responses across complex elevational gradients provides insight into fundamental processes driving responses of plant growth and net primary production to environmental change. Gas exchange of needles and twig water potential were measured in two widespread coniferous tree species, Pinus contorta and Picea engelmannii, over an 800-m elevation gradient in southeastern Wyoming, USA. We hypothesized that limitations to photosynthesis imposed by mesophyll conductance (g) would be greatest at the highest elevation sites due to higher leaf mass per area (LMA) and that estimations of maximum rate of carboxylation (V) without including g would obscure elevational patterns of photosynthetic capacity. We found that g decreased with elevation for P. contorta and remained constant for P. engelmannii, but in general, limitation to photosynthesis by g was small. Indeed, estimations of V when including g were equivalent to those estimated without including g and no correlation was found between g and LMA nor between g and leaf N. Stomatal conductance (g) and biochemical demand for CO were by far the most limiting processes to photosynthesis at all sites along the elevation gradient. Photosynthetic capacity (A) and g were influenced strongly by differences in soil water availability across the elevation transect, while g was less responsive to water availability. Based on our analysis, variation in g plays only a minor role in driving patterns of photosynthesis in P. contorta and P. engelmannii across complex elevational gradients in dry, continental environments of the Rocky Mountains and accurate modeling of photosynthesis, growth and net primary production in these forests may not require detailed estimation of this trait value.