Stomatal regulation in a changing climate: a field study using Free Air Temperature Increase (FATI) and Free Air CO2 Enrichment (FACE)

Abstract

ABSTRACTThis study investigates effects of climate warming (+ 2.5°C ubove ambient) and elevated CO2 concentration (600 μmol mol−1) on the stomatal functioning and the water relations of Lolium perenne, using Free Air Temperature Increase (FATI) and Free Air CO2 Enrichment (FACE). Compared to growth at ambient temperature, whole‐season temperature increase reduced leaf stomatal conductance, but only at the top of the canopy (‐14.6 and ‐8.8% at ambient and elevated CO2, respectively). However, because higher canopy temperature raised the leaf‐to‐air vapour pressure difference, leaf transpiration rate increased (+28% at ambient and +48% at elevated CO2) and instantaneous leaf water use efficiency, derived from short‐term measurements of assimilation and transpiration rate, declined (‐11% at ambient and ‐13% at elevated CO2). Nevertheless, at the stand level, growth at + 2.5°C reduced transpiration due to fewer tillers per plant and a smaller leaf area per tiller. This sparser vegetation was also more closely coupled to the atmosphere and maintained a drier internal microclimate. To assess whether the stomatal behaviour observed in this experiment could be explained by prevailing concepts of stomatal functioning, three models were applied (Cowan 1977; Ball, Woodrow & Berry 1987; Leuning 1995). The latter model accounted for the highest proportion of variability in the data (58%) and was insensitive to CO2 and temperature regime, which suggests that the principles of stomatal regulation are not affected by changes in CO2 or climate.