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要旨:
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 (SL) and nonstomatal
(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 partitioned
equally between mesophyll conductance (MCL) and
biochemical (BL) limitations when TL reached 100%. Rainfall
exclusion altered the relationship between leaf water
potential and photosynthesis. In response to severe midsummer
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 photosynthetic
parameters and water stress that are not currently included in drought parameterizations for modelling applications.
