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Abstract

Savannah-type ecosystems account for 26–30% of global gross primary productivity
GPP with water being one of the major driving factors. In Europe, savannah-type woodlands
cover an area of about 1.5 million ha. Here, the recent past has shown a significant
5 decrease of precipitation P in winter and spring as well as decrease of total annual
precipitation. Strong effects on local water balance and carbon sink strength have thus
been reported due to changes in precipitation regime.
The objective of this study is to quantify the impact of the extreme drought event
in 2012 on the water balance, gross primary productivity and carbon sink strength of 10 a typical Portuguese cork-oak woodland (montado) compared to the wet year 2011.
Physiological responses of the dominant tree species Quercus suber (L.) are disentangled, employing combined photosynthesis and stomatal conductance modelling.
Precipitation effectiveness ET/P increased from 86% in 2011 to 122% in the dry
year 2012 due to deep soil or ground water access of the Q. suber trees leaving no
15 water for ground water replenishment. Understorey and overstorey GPP were strongly reduced by 53% and 28 %, respectively, in 2012 compared to 2011 due to the late
onset of the autumn rains in 2011 and an additional severe winter/spring drought.
However, the ecosystem was still a carbon sink in both years but with a 38% reduced
sink strength under extreme drought in 2012 compared to 2011. The combined
20 photosynthesis-stomatal conductance model yielded best results if it was allowed to adjust photosynthetic and stomatal parameters simultaneously. If stomatal response was modelled with the Leuning approach, which allows for a different sensitivity to
vapour pressure deficit, the stomatal model parameters were highly coupled. A change
in either of the parameters needed to be compensated by the other to guarantee a sta25 ble sensitivity of stomatal conductance to assimilation, independently from variations in vapour pressure deficit. The Q. suber trees showed a 31% reduced stomatal conductance during the drought period 2012 compared to 2011 due to water supply limitations.
In response to reduced leaf internal CO2 availability, the trees strongly reduced appar-ent maximum carboxylation rate by 39% in 2012 compared to 2011. Unexpectedly, the optimum temperature Topt of maximum electron transport rate decreased during
the drought period, enhancing the susceptibility of the trees to high temperature stress
during the summer. 5 Our results suggest that, if the trend of decreasing annual precipitation and changed precipitation pattern on the Iberian Peninsula continues, sustained effects on local ground water reservoirs, understorey species composition and tree mortality have to be expected in the long term. To successfully model the effect of drought on the montado ecosystem, variable apparent maximum carboxylation rate Vc,max, stomatal con10
ductance parameter m and vapor pressure deficit sensitivity parameter D0 need to be
incorporated in photosynthesis-stomatal conductance modelling.