Global climate change is expected to result in more frequent and intense droughts in the Mediterranean 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 (S(L) ) and non-stomatal (NS(L) ) 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 NS(L) increased with drought intensity. S(L) and NS(L) were equal once total limitation (T(L) ) reached 60%, but NS(L) greatly exceeded S(L) during severe drought, with 76% NS(L) partitioned equally between mesophyll conductance (MC(L) ) and biochemical (B(L) ) limitations when T(L) reached 100%. Rainfall exclusion altered the relationship between leaf water potential and photosynthesis. In response to severe mid-summer drought stress, A(n) and V(cmax) 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 photosynthetic parameters and water stress that are not currently included in drought parameterizations for modelling applications. © 2010 Blackwell Publishing Ltd.
