Links between Transpiration and Plant Nitrogen: Variation with Atmospheric CO2 Concentration and Nitrogen Availability

Abstract

Transpiration is closely linked to plant nitrogen (N) content, indicating that global or other changes that alter plant N accumulation or the relative requirements of plants for water and N will affect transpiration. We studied effects of N availability and atmospheric CO2 concentration, two components of global biogeochemistry that are changing, on relationships between whole-plant transpiration and N in two perennial C3 species, Pseudoroegneria spicata (a tussock grass) and Gutierrezia microcephala (a half-shrub). Two indices of plant N requirement were used: N accretion (N in live and dead tissues) and N loss in litter (N in dead tissues). Transpiration was analyzed as the product of N accretion or loss by plants and the ratio of transpiration to N accretion or loss. The two indices of plant N requirement led to different conclusions as to the effects of N availability on plant use of water relative to N. Transpiration scaled proportionally with N accretion, but transpiration per unit of N loss declined at high N. Carbon dioxide enrichment had little effect on the ratio of transpiration to N accretion and no effect on transpiration per unit of N loss. The two species accumulated similar amounts of N, but the half-shrub used more than twice as much water as the grass. Nitrogen availability and CO2 concentration influenced whole-plant transpiration more by changing plant N accumulation than by altering the stoichiometry between transpiration and plant N. Species differences in total water use, by contrast, reflected differences in the scaling of transpiration to plant N. A better understanding of species differences in water and N dynamics may thus be required to predict transpiration reliably.