Photosynthetic acclimation and sensitivity to short- and long-term environmental changes

O_LIThe future climate will be characterized by an increase in frequency and duration of drought and warming that exacerbates atmospheric evaporative demand. How trees acclimate to long-term soil moisture changes and whether these long-term changes alter trees sensitivity to short-term (day to months) variations of vapor pressure deficit (VPD) and soil moisture is largely unknown. C_LIO_LILeaf gas exchange measurements were performed within a long-term (17 years) irrigation experiment in a Scots pine-dominated forest in one of Switzerlands driest areas on trees in naturally dry (control), irrigated, and irrigation-stop (after 11 years of irrigation) conditions. C_LIO_LISeventeen years of irrigation increased photosynthesis (A) and stomatal conductance (gs) and reduced the gs sensitivity to increasing VPD but not to soil drying. Following irrigation-stop, gas exchange did not decrease immediately, but after three years, had decreased significantly in irrigation-stop trees. Vcmax and Jmax recovered after five years. C_LIO_LIThese results suggest that long-term release of soil drought reduces the sensitivity to atmospheric evaporative demand and that atmospheric constraints may play an increasingly important role in combination with soil drought. In addition, they suggest that structural adjustments lead to an attenuation of initially strong leaf-level acclimation to strong multiple-year drought. C_LI