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Extreme drought event impacts carbon and water cycling in a Mediterranean woodland

Christiane Werner1, Arndt Piayda2, Maren Dubbert1, Filipe Costa e Silva3, Alexandra Correia3, Joao Perreira3, Matthias Cuntz2
1 AgroEcosystem Research, BAYCEER, University Bayreuth
2 UFZ Leipzig
3 ISA, Lisbon, Portugal

P 3.24 in Fluxes between the atmosphere and ecosystems

Poster Session 2 on Tuesday, 16:30-18:00

Savannah-type ecosystems account for 26-30% of the global gross primary production with water being one of the major driving factors. The recent past has shown significant decline in precipitation which is predicted to continue under climate change. Particularly, extreme drought spells may endanger ecosystem functioning as they strongly effect local water balance and carbon sink strength. Here, we quantify changes in the water balance, gross primary productivity, carbon sink strength and tree phenology of a typical Portuguese savannah-type woodland (montado) under extreme drought. 2012 was the second driest year since 1950 with a strong precipitation anomaly in winter (less than 20% of average rainfall) followed by a dry summer. We evaluate the physiological response of the dominant tree species Quercus suber (L.), employing combined photosynthesis and stomatal conductance modelling.

Precipitation effectiveness ratio increased up to 122% in 2012 compared to the moist previous year, indicating that evapotranspiration exceeded precipitation input due to ground water access of the Q. suber trees leaving no water for ground water replenishment. By the lack of water in the upper soil and deep ground water reservoirs, the understorey and overstorey gross primary productions were reduced by 53% and 28% in 2012 compared to 2011. Nevertheless, on an annual basis, the ecosystem remained a carbon sink in both years, but with a 61% reduced sink strength during the dry spell. A significant reduction annual NEE was associated with leaf phenological events of canopy renewal. Annual tree diameter growth was reduced by 63% whereas leaf growth was less affected with leaf area index decreasing only 9% in 2012 in comparison to 2011.

Applying a combined photosynthesis and stomatal conductance model, best model fit to gross primary productivity and transpiration of Q. suber trees could be achieved keeping apparent maximum carboxylation rate Vcmax as well as stomatal conductivity parameter m and vapor pressure deficit sensitivity parameter D0 of the stomatal conductance formulation variable. The Q. suber trees showed 20% reduced stomatal conductance gs and even much higher reduction of apparent maximum carboxylation rate Vcmax of 39% during drought. However, stomatal response was strongest at the onset of drought while strongest reduction of Vcmax occurred at the end of the drought period.

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 productivity may be expected in the long-term.

Letzte Änderung 19.06.2014