Résumé
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 (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. S-L and NSL were equal once total limitation (T-L) reached 60%, but NSL greatly exceeded S-L during severe drought, with 76% NSL partitioned equally between mesophyll conductance (MCL) 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.