Leaf physiological responses to extreme droughts in Mediterranean Quercus ilex forest. Plant Cell Environ

Global climate change is expected to result in more fre-quent and intense droughts in theMediterranean region.To understand forest response to severe drought, we used a mobile rainfall shelter to examine the impact of spring and autumn rainfall exclusion on stomatal (SL) and non-stomatal (NSL) limitations of photosynthesis in a Quercus ilex ecosystem. Spring rainfall exclusion, carried out during increasing atmospheric demand and leaf development, had a larger impact on photosynthesis than autumn exclusion, conducted at a time of mature foliage and decreasing vapour pressure deficit. The relative importance of NSL increased with drought intensity. SL and NSL were equal once total limitation (TL) reached 60%, but NSL greatly exceeded SL during severe drought, with 76 % NSL parti-tioned equally between mesophyll conductance (MCL) and biochemical (BL) limitations when TL reached 100%. Rain-fall exclusion altered the relationship between leaf water potential and photosynthesis. In response to severe mid-summer drought stress, An and Vcmax were 75 % and 72% lower in the spring exclusion plot than in the control plot at the same pre-dawn leaf water potential. Our results revealed changes in the relationship between photosyn-thetic parameters and water stress that are not currently included in drought parameterizations for modelling applications. Key-words: climate change; maximum carboxylation rate; mesophyll conductance; photosynthesis; rainfall exclusion; stomatal conductance