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Piayda, Arndt ; Dubbert, Maren ; Rebmann, Corinna ; Kolle, Olaf ; Costa e Silva, Filipe ; Correia, Alexandra C. ; Pereira, João Santos ; Werner, Christiane ; Cuntz, Matthias:
Drought impact on carbon and water cycling in a Mediterranean Quercus suber L. woodland during the extreme drought event in 2012.
In: Biogeosciences.
Bd. 11
(2014)
Heft 24
.
- S. 7159-7178.
ISSN 1726-4189
DOI: https://doi.org/10.5194/bg-11-7159-2014
Abstract
Savannah-type ecosystems account for 26–30 % of global gross primary productivity GPP, with waterbeing 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 decrease in precipita-tion P in winter and spring as well as a decrease in total an-nual precipitation. Strong effects on local water balance andcarbon sink strength have thus been reported due to changesin precipitation regime.The objective of this study is to quantify the impact ofthe extreme drought event in 2012 on the water balance,gross primary productivity and carbon sink strength of a typ-ical Portuguese cork-oak woodland (montado) compared tothe wet year of 2011. Physiological responses of the dom-inant tree species Quercus suber (L.) are disentangled em-ploying combined photosynthesis and stomatal conductancemodelling.Precipitation effectiveness ET/P increased from 86 % in2011 to 122 % in the 2012 dry year due to deep soil orgroundwater access of the Q. suber trees leaving no waterfor groundwater replenishment. Understorey and overstoreyGPP were strongly reduced, by 53 and 28 %, respectively,in 2012 compared to 2011, due to the late onset of the au-tumn rains in 2011 and an additional severe winter/springdrought. However, the ecosystem was still a carbon sink inboth years, but with a 38 % reduced sink strength under ex-treme drought in 2012 compared to 2011. The combinedphotosynthesis–stomatal conductance model yielded the bestresults if it was allowed to adjust photosynthetic and stomatalparameters simultaneously. If stomatal response was mod-elled with the Leuning approach, which allows for a differ-ent sensitivity to vapour pressure deficit, the stomatal modelparameters were highly coupled. A change in either of theparameters needed to be compensated by the other to guar-antee a stable sensitivity of stomatal conductance to assim-ilation, independent of variations in vapour pressure deficit.The Q. suber trees showed a 37 % reduced stomatal conduc-tance during the drought period of 2012 compared to 2011,due to water supply limitations. In response to reduced leaf-internal CO2 availability, the trees strongly reduced the ap-parent maximum carboxylation rate by 43 % in 2012 com-pared to 2011. Unexpectedly, the optimum temperature Toptof the maximum electron transport rate decreased during thedrought period, enhancing the susceptibility of the trees tohigh temperature stress during the summer.Our results suggest that, if the trend of decreasing an-nual precipitation and changed precipitation patterns onthe Iberian Peninsula continues, sustained effects on localgroundwater reservoirs, understorey species composition andtree mortality have to be expected in the long term. To model the effect of drought on the montado ecosystem success-fully, variable apparent maximum carboxylation rate Vc,max,stomatal conductance parameter m and vapour pressuredeficit sensitivity parameter D0 need to be incorporated intophotosynthesis–stomatal conductance modelling.