Evidence of climate-induced stress of Norway spruce along elevation gradient preceding the current dieback in Central Europe

Abstract

Effect of drought during 2017 and 2018 resulted in radial stem increment reduction to 78% and 61%, respectively, of the levels occurring in normal year 2016 in Central Europe. Norway spruce (Picea abies (L.) Karst.) is currently the most threatened commercial tree species in Central Europe. This is due to increased drought stress from advancing climate change as well as the species’ distribution outside its natural range. Tree water status and water movement through a tree are key parameters influencing tree growth and vitality. This study is focused on the growth and stress reaction of spruce to climatic conditions, analysing stem diameter variation along an elevation gradient (381–995 m a.s.l.) in the Czech Republic. Tree water deficit based on the zero-growth concept (TWD), calculated from high-frequency dendrometer records and the temporal dynamics of radial growth, was studied for 3 years (2016–2018). Two of these 3 years were affected by severe drought during the growing season. Contrary to our expectations, the observed TWD showed no clear linear decline with rising elevation. The most severe tree desiccation was observed in experimental sites at middle elevations of about 600 m a.s.l. Here, we show that both the timing and level of tree water deficit had an impact on annual stem radial increment (SRIannual). Severe drought had a substantial negative impact on SRIannual of Norway spruce in both 2017 and 2018. Drought conditions in 2017 and 2018 resulted in reduction of SRIannual relative to measurements for the wetter year in 2016 to 78% and 61%, respectively. We report the evidence that the current climatic conditions in the Central European region are not suitable for growing Norway spruce at lower and middle elevations and that forest management needs to react immediately to this situation.