STOMATAL REGULATION IN A CHANGING CLIMATE - A FIELD-STUDY USING FREE-AIR TEMPERATURE INCREASE (FATI) AND FREE-AIR CO2 ENRICHMENT (FACE)

This study investigates effects of climate warming (+2.5 degrees C abo ve ambient) and elevated CO2 concentration (600 mu mol mol(-1)) on the stomatal functioning and the water relations of Lolium perenne, using Free Air Temperature Increase (FATI) and Free Air CO2 Enrichment (FAC E), Compared to growth at ambient temperature, whole-season temperatur e increase reduced leaf stomatal conductance, but only at the top of t he canopy (-14.6 and -8.8% at ambient and elevated CO2, respectively), However, because higher canopy temperature raised the leaf-to-air vap our pressure difference, leaf transpiration rate increased (+28% at am bient and +48% at elevated CO2) and instantaneous leaf water use effic iency, derived from short-term measurements of assimilation and transp iration rate, declined (-11% at ambient and -13% at elevated CO2). Nev ertheless, at the stand level, growth at +2.5 degrees C reduced transp iration due to fewer tillers per plant and a smaller leaf area per til ler, This sparser vegetation was also more closely coupled to the atmo sphere 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 app lied (Cowan 1977; Ball, Woodrow & Berry 1987; Leuning 1995), The latte r model accounted for the highest proportion of variability in the dat a (58%) and was insensitive to CO2 and temperature regime, which sugge sts that the principles of stomatal regulation are not affected by cha nges in CO2 or climate.