Agricultural and Forest Meteorology 204 (2015) 48–57 Contents lists available at ScienceDirect Agricultural and Forest Meteorology j o ur na l ho me pag e: www.elsevier.com/locate/agrformet Effects of an extremely dry winter on net ecosystem carbon exchange and tree phenology at a cork oak woodland Filipe Costa-e-Silva a,∗ , Alexandra C. Correia a , Arndt Piayda b , Maren Dubbert c , Corinna Rebmann b , Matthias Cuntz b , Christiane Werner c , Jorge Soares David a , João Santos Pereira a a CEF, Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal b Deptartment Computational Hydrosystems, UFZ Helmholtz Centre for Environmental Research, Permoserstraße 15, 04318 Leipzig, Germany c Agroecosystem Research, BayCeer, University of Bayreuth Universitätsstraße 30, 95447 Bayreuth, Germany a r t i c l e i n f o Article history: Received 2 June 2014 Received in revised form 12 January 2015 Accepted 29 January 2015 Keywords: CO2 fluxes Evergreen oak Leaf area index Mediterranean woodland Quercus suber L. Tree diameter increment a b s t r a c t In seasonally dry climates, such as the Mediterranean, lack of rainfall in the usually wet winter may originate severe droughts which are a main cause of inter-annual variation in carbon sequestration. Leaf phenology variability may alter the seasonal pattern of photosynthetic uptake, which in turn is deter- mined by leaf gas exchange limitations. The current study is based on the monitoring of an extremely dry winter in an evergreen cork oak woodland under the Mediterranean climate of central Portugal. Results are focused on net ecosystem CO 2 exchange (NEE), phenology and tree growth measurements during two contrasting years: 2011, a wet year with a typical summer drought pattern and 2012, with an extremely unusual dry winter (only 10 mm of total rainfall) that exacerbated the following summer drought effects. Main aims of this study were to assess the effects of an extreme dry winter in (1) annual and seasonal net ecosystem CO 2 exchange, and in (2) cork oak phenology. The dry year 2012 was marked by a 45% lower carbon sequestration (−214 vs. −388 g C m −2 year −1 ) and a 63% lower annual tree diameter growth but only a 9% lower leaf area index compared to the wet year 2011. A significant reduction of 15% in yearly carbon sequestration was associated with leaf phenological events of canopy renewal in the early spring. In contrast to male flower production, fruit setting was severely depressed by water stress with a 54% decrease during the dry year. Our results suggest that leaf growth and leaf area maintenance are resilient ecophysiological processes under winter drought and are a priority carbon sink for photoassimilates in contrast to tree diameter growth. Thus, carbon sequestration reductions under low water availabilities in cork oak woodland should be ascribed to stomatal regulation or photosynthetic limitations and to a lesser extent to leaf area reductions. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Cork oak (Quercus suber L.) open woodlands cover an area of about 2–2.5 million ha in the western Mediterranean (Aronson et al., 2009). These are man-made ecosystems exploited with Abbreviations: (C/N), carbon-to-nitrogen ratio; (DBH), diameter at breast height; (DOY), day of year; (ET), ecosystem evapotranspiration; (GPP), gross primary pro- ductivity; (LAI), leaf area index; (LAImax), maximum leaf area index; (LAI min ), minimum leaf area index; (LUE), light use efficiency; (NEE), net ecosystem exchange; (PAR), photosynthetically active radiation; (PLC), percentage loss in hydraulic con- ductivity; ( md ), midday leaf water potential; ( pd ), predawn leaf water potential; (Reco), ecosystem respiration; (se), standard error of the mean; (SLA), specific leaf area; (SWC), soil water content; (u*), friction velocity; (VPDmax), maximum vapor pressure deficit. ∗ Corresponding author. Tel.: +351 21365 3515; fax: +351 21365 3338. E-mail address: filipecs@isa.utl.pt (F. Costa-e-Silva). low-impact agro-forestry, with high biodiversity and conservation value (Bugalho et al., 2011). In Portugal these woodlands cover 0.74 million ha and represent 23% of all forested area. Cork oak has a significant economic value. It provides 0.7% of Portugal gross domestic product and supplies 54% of the worldwide cork pro- duction (Evangelista, 2010). Cork is a natural product consisting of continuous annual layers of suberized tissue produced by phel- logen, a secondary meristem wrapping the inner bark. Cork wine bottle stoppers is the main product and to obtain commercial grade cork stripping is done traditionally every 9 years. Cork removal can only be safely done when the phellogen cells are actively dividing, in late-spring and early-summer, to prevent injuries to the tree (Costa et al., 2003). Cork oak is well adapted to the adverse semiarid Mediter- ranean climate and its ecophysiology has been well studied in the last decades (e.g., Otieno et al., 2007; Pereira et al., 2009; Vaz http://dx.doi.org/10.1016/j.agrformet.2015.01.017 0168-1923/© 2015 Elsevier B.V. All rights reserved.