Forest ecosystem functioning in response to wetter winters along a large climatic gradient (WetWin)

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Climate models consistently project moderate to large increases in air temperatures for most temperate ecosystems over the next century, with warming in winter exceeding warming in all other seasons for central and northern Europe. Furthermore, winter precipitation is expected to increase and to shift from snow towards rain. In short, European winters will become warmer and wetter. The net effect of these warmer and wetter winters for ecosystem functioning, however, is unclear.Warmer soils are generally expected to enhance soil biotic activity and nutrient availability for plants. In regions with little natural snow pack and warm winters, this increase in available nutrients will either result in increased plant growth or, in particular if winter precipitation increases, in increased nutrient leaching. Towards colder regions with deeper and longer snow cover, loss of snow may result in colder soil temperatures due to missing insulation by the snow pack, reduced mineralization and decreased plant growth. Here, additional rainfall may exacerbate N limitation due to increased N leaching. The broad objective for this project is the investigation of the importance of winter precipitation for the ecology and biogeochemistry of temperate broad-leaved forests. More specifically, we will test for the effects of increased winter precipitation and its interactions with reduced snow cover. It is hypothesized that effects of nitrogen leaching are stronger at warmer sites, while snow cover change causes stronger responses at colder sites. Therefore, the manipulation experiments will be carried out along a strong climatic gradient which exemplifies climate warming effects from formerly colder and snowier conditions in the east of the gradient to future warmer and less snowy conditions in the west of the gradient. Net ecosystem effects to changes in winter precipitation in tree growth and nitrogen leaching will be quantified and potential effects in these proxies will be explained by the following suite of parameters: (1) soil biotic activity, decomposition, and mineralization; (2) nutrient availability and leaching (3) above and below-ground growth and resource allocation of adult and juvenile individuals of the naturally dominating forest tree of central Europe, i.e. Fagus sylvatica, and understory vegetation. We will identify the relative sensitivities of these ecological and biogeochemical response parameters to winter precipitation change along a large gradient of winter temperature from across Northeastern Germany and northern Poland in order to develop a better understanding of the potential effects on ecosystem functioning and sound up-scaling to future climatic conditions.