Altitudinal variation in leaf gas exchange, nitrogen and phosphorus contentrations, and leaf mass per area in populations of Frasera speciosa

Foliar nitrogen (N) and phosphorus (P) concentrations, leaf mass per unit area (LMA), and leaf gas exchange were measured in seven populations of Frasera speciosa along a 1700 m gradient, to assess whether altitudinal variation in net photosynthesis rates was related to foliar N and P concentrations, LMA, and stomatal conductance. A secondary goal was to evaluate whether altitudinal trends in foliar chemistry were independent of phenology of the plants. Leaf N and P concentrations per unit mass changed seasonally in all populations, but increased with altitude throughout the growing season. Leaf mass per unit area decreased with increasing altitude, unlike most other studies which have found LMA to increase with altitude. As a result foliar N per unit area of leaf did not change with increasing altitude. Net photosynthesis rates and stomatal conductance did not change with increasing altitude. Transpiration rates increased, while water-use efficiency and the mole fraction of intercellular CO(2) decreased with increasing altitude. The change in LMA was the predominant response to increasing altitude in populations of Frasera, and assuming the decrease in CO(2) partial pressure with increasing altitude imposes a constraint on photosynthesis, increased internal conductance and/or increased photosynthetic capacity may have resulted in maintenance of similar field photosynthesis rates among populations.