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 a rea (LMA), and leaf gas exchange were measured in seven populations of Fras era speciosa along a 1700 m gradient, to assess whether altitudinal variati on in net photosynthesis rates was related to foliar N and P concentrations , LMA, and stomatal conductance. A secondary goal was to evaluate whether a ltitudinal 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. Le af 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 photo synthesis rates and stomatal conductance did not change with increasing alt itude. Transpiration rates increased, while water-use efficiency and the mo le fraction of intercellular CO2 decreased with increasing altitude. The ch ange in LMA was the predominant response to increasing altitude in populati ons of Frasera, and assuming the decrease in CO2 partial pressure with incr easing altitude imposes a constraint on photosynthesis, increased internal conductance and/or increased photosynthetic capacity may have resulted in m aintenance of similar field photosynthesis rates among populations.