16. Warm Winter, Wet Spring, and an Extreme Response in Ecosystem Functioning on the Iberian Peninsula

Introduction. The Iberian Peninsula (IP) experienced unusual meteorological conditions in winter and spring 2015/16 (WS15/16) with a warm winter followed by wet conditions in late winter and spring (Figs. 16.1a–c). The unusual succession of these events coincided with an extremely positive anomaly in vegetation productivity on local and regional scales over the IP with unusually high regional vegetation greenness (Figs. 16.1d–f; a proxy for ecosystem productivity) and high crop yields (JRC MARS Bulletins 2016, https://ec.europa.eu/jrc/en/research -topic/crop-yield-forecasting). Climatic changes can affect the intensity and frequency of extreme events (Seneviratne et al. 2012), and these changes are widely recognized to impose substantial impacts on terrestrial ecosystems (Reichstein et al. 2013). However, interpreting and quantifying climate-induced ecosystem impacts such as the vegetation productivity on the IP in WS15/16 remains challenging as continuous site-level measurements that span over a decade are generally rare, and even the longest site measurements are only available for the last 25 years (http://fluxnet.fluxdata.org/data /fluxnet2015-dataset/). While long-term climatic changes impose fundamental impacts on terrestrial ecosystems (Parmesan and Yohe 2003; Walther et al. 2002), cause–effect chains under climatic extremes are often highly nonlinear (Frank et al. 2015) and typically include instantaneous and lagged effects (Arnone et al. 2008). Ecosystem responses to climate extremes are specific to the ecosystem type affected (Teuling et al. 2010), depend on nutrient status, ecosystem health, and pre-exposure; and extreme climatic events can lead to little ecosystem responses while moderate events can trigger large responses. Similarly, ecosystem responses can be mitigated or amplified across seasons (Wolf et al. 2016). For example, higher spring carbon uptake due to higher spring temperatures could compensate for carbon losses under drought conditions over the contiguous United States in summer 2012 (Wolf et al. 2016). To improve our understanding of extreme responses of ecosystem productivity, the concept of compound events is particularly useful. A compound event is a combination, or in our case succession, of events in which the single drivers are not necessarily extreme themselves but lead to an extreme impact (Field et al. 2012; Leonard et al. 2014). A warm winter followed by wet spring in a Mediterranean ecosystem is one example of a compound event in which single drivers (winter temperature and spring precipitation) are not record-breaking extremes themselves, but this favorable combination of meteorological variables can lead to highly positive impacts on ecosystem productivity if other stressors are absent. In particular, for the ecosystem studied, other stressors could include, but are not limited to, short but intense cold spells in AFFILIATIONS: Sippel—Max Planck Institute for Biogeochemistry, Jena, Germany, and now at Norwegian Institute of Bioeconomy Research (NIBIO), Ås, Norway; el-Madany, Migliavacca, Mahecha, Flach, and reichStein—Max Planck Institute for Biogeochemistry, Jena, Germany; carrara— CEAM, Fundación de la Comunidad Valenciana Centro de Estudios Ambientales del Mediterraneo, Paterna, Spain; kaMinSki and voSSbeck—The Inversion Lab, Hamburg, Germany; otto— Environmental Change Institute, University of Oxford, Oxford, United Kingdom; thonicke—Potsdam Institute for Climate Impact Research, Potsdam, Germany