Climate warming alters photosynthetic responses to elevated CO₂ in prairie plants

The impact of elevated CO₂ concentration ([CO₂ ]) and climate warming on plant productivity in dryland ecosystems is influenced strongly by soil moisture availability. We predicted that the influence of warming on the stimulation of photosynthesis by elevated [CO₂ ] in prairie plants would operate primarily through direct and indirect effects on soil water.

We measured light-saturated photosynthesis (A_net ), stomatal conductance (g_s ), maximum Rubisco carboxylation rate (V_cmax ), maximum electron transport capacity (J_max ) and related variables in four C₃ plant species in the Prairie Heating and CO₂ Enrichment (PHACE) experiment in southeastern Wyoming. Measurements were conducted over two growing seasons that differed in the amount of precipitation and soil moisture content.

A_net in the C₃ subshrub Artemisia frigida and the C₃ forb Sphaeralcea coccinea was stimulated by elevated [CO₂ ] under ambient and warmed temperature treatments. Warming by itself reduced A_net in all species during the dry year, but stimulated photosynthesis in S. coccinea in the wet year. In contrast, A_net in the C₃ grass Pascopyrum smithii was not stimulated by elevated [CO₂ ] or warming under wet or dry conditions. Photosynthetic downregulation under elevated [CO₂ ] in this species countered the potential stimulatory effect under improved water relations. Warming also reduced the magnitude of CO₂ -induced down-regulation in this grass, possibly by sustaining high levels of carbon utilization.

Direct and indirect effects of elevated [CO₂ ] and warming on soil water was an overriding factor influencing patterns of A_net in this semi-arid temperate grassland, emphasizing the important role of water relations in driving grassland responses to global change.