Increased sensitivity to drought across successional stages in natural Norway spruce (Picea abies (L.) Karst.) forests of the Calimani Mountains, Romania

Abstract

Winter drought becomes a limiting factor of forest stand growth by the end of the twentieth century. Disturbances strongly influence the structure of natural forests. The frequency and severity of natural disturbances, as well as drought events, are expected to increase with climate change. Our study investigated if forests with differing forest structures related to disturbance histories also differed in sensitivity to drought. In a natural forest landscape in the Calimani Mountains of the Eastern Carpathians, Romania, we used six forest patches to represent different successional stages, from early- to late-successional stages. We used two temporal resolutions of the Standardized Precipitation-Evapotranspiration Index to describe short and long water resource dynamics within or between hydrological years, respectively. We detected an increase in the importance of winter drought across all successional stages; it was first identified in the oldest patch in the 1970s, and it consecutively affected younger patches. We observed that different forest structures do not lead to substantial differences in trends in drought–growth relationships. A shift in sensitivity to water availability from early spring to winter occurred over the twentieth century. These findings suggest that the impact of climate change on Norway spruce forest ecosystems of the Eastern Carpathians will likely be difficult to mitigate at a local scale using traditional forestry practices.