Drought impact on carbon and water cycling in a Mediterranean Quercus suber L. woodland during the extreme drought event in 2012

[摘要] Savannah-typeecosystems account for 26–30% of global gross primary productivity GPP,with water being one of the major driving factors. In Europe, savannah-typewoodlands cover an area of about 1.5 million ha. Here, the recent past hasshown a significant decrease in precipitation P in winter and spring aswell as a decrease in total annual precipitation. Strong effects on localwater balance and carbon sink strength have thus been reported due to changesin precipitation regime.

The objective of this study is to quantify the impact of the extreme droughtevent in 2012 on the water balance, gross primary productivity and carbonsink strength of a typical Portuguese cork-oak woodland (montado)compared to the wet year of 2011. Physiological responses of the dominanttree species Quercus suber (L.) are disentangled employing combinedphotosynthesis and stomatal conductance modelling.

Precipitation effectiveness ET/P increased from 86% in 2011 to122% in the 2012 dry year due to deep soil or groundwater access of theQ. suber trees leaving no water for groundwater 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 autumnrains in 2011 and an additional severe winter/spring drought. However, theecosystem was still a carbon sink in both years, but with a 38% reducedsink strength under extreme drought in 2012 compared to 2011. The combinedphotosynthesis–stomatal conductance model yielded the best results if it wasallowed to adjust photosynthetic and stomatal parameters simultaneously. Ifstomatal response was modelled with the Leuning approach, which allows fora different sensitivity to vapour pressure deficit, the stomatal modelparameters were highly coupled. A change in either of the parameters neededto be compensated by the other to guarantee a stable sensitivity of stomatalconductance to assimilation, independent of variations in vapour pressuredeficit. The Q. suber trees showed a 37% reduced stomatalconductance during the drought period of 2012 compared to 2011, due to watersupply limitations. In response to reduced leaf-internal CO₂availability, the trees strongly reduced the apparent maximum carboxylationrate by 43% in 2012 compared to 2011. Unexpectedly, the optimumtemperature T_opt of the maximum electron transport rate decreasedduring the drought period, enhancing the susceptibility of the trees to hightemperature stress during the summer.

Our results suggest that, if the trend of decreasing annual precipitation andchanged precipitation patterns on the Iberian Peninsula continues, sustainedeffects on local groundwater reservoirs, understorey species composition andtree mortality have to be expected in the long term. To model the effect ofdrought on the montado ecosystem successfully, variable apparentmaximum carboxylation rate V_c,max, stomatal conductance parameterm and vapour pressure deficitsensitivity parameter D₀ need to be incorporated intophotosynthesis–stomatal conductance modelling.

[学科分类] 地球化学与岩石