Links between transpiration and plant nitrogen: Variation with atmosphericCO2 concentration and nitrogen availability

Transpiration is closely linked to plans nitrogen (N) content, indicating t hat global or other changes that alter plant N accumulation or the relative requirements of plants for water and N will affect transpiration. We studi ed effects of N availability and atmospheric CO2 concentration, two compone nts of global biogeochemistry that are changing, on relationships between w hole-plant transpiration and N in two perennial C-3 species, Pseudoroegneri a spicata (a tussock grass) and Gutierrezia microcephala ia half-shrub). Tw o 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 analy zed as the product of N accretion or loss by plants and the ratio of transp iration 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 o f water relative to N. Transpiration scaled proportionally with N accretion , but transpiration per unit of N loss declined at high N. Carbon dioxide e nrichment had little effect on the ratio of transpiration to N accretion an d no effect on transpiration per unit of N lass. The two species accumulate d similar amounts of N, but the half-shrub used more than twice as much wat er as the grass. Nitrogen availability and CO2 concentration influenced who le-plant transpiration more by changing plant N accumulation than by alteri ng the stoichiometry between transpiration and plant N. Species differences in total water use, by contrast, reflected differences in the scaling of t ranspiration to plant N. A better under standing of species differences in water and N dynamics may thus be required to predict transpiration reliably .