Effects of Added Water on Photosynthesis of Bistorta vivipara: The Importance of Water Relations and Leaf Nitrogen in Two Alpine Communities, Pikes Peak, Colorado, U.S.A.

Abstract

Photosynthetic, water relations, and leaf nitrogen responses of Bistorta vivipara, a wide-ranging alpine species, to added water were measured in order to determine the extent to which water limits photosynthesis. One centimeter of water per week (equivalent to about 1 standard deviation of natural precipitation) was added to plots in fellfield and moist meadow communities. There were generally no early- or mid-season treatment effects for any of the measured responses. At the end of the growing season photosynthetic rates were higher in watered plots than in control plots in both communities even though the summer was much wetter than average. This suggests that photosynthetic rates might be further limited by lack of water during growing seasons with average or below-average precipitation. Treatment differences in leaf water potential, stomatal conductance, and leaf nitrogen concentration were not present at the end of the season. However, each of these three parameters was found to limit photosynthesis, and each was greater in the watered plots than in control plots at the end of the season. The stronger correlations of leaf water potential and stomatal conductance with photosynthesis compared to the weak correlation of leaf nitrogen to photosynthesis suggests watering primarily leads to more open stomates, which increases photosynthesis. Potential indirect effects of watering on photosynthesis through increased nutrient availability caused by increased mineralization or nutrient movement are apparently unimportant.