Risk of extreme climate impacts on European Norway spruce forest: drought and frost in the climate emergency

Shifts in weather patterns causing more severe and frequent climatic extreme events, par- ticularly periods of hot drought and late-spring frost, cause stress and damage to many tree species. Here we use 12 members of the UKCP Regional (12km) ensemble to assess the changing risk of tree stress posed by hot droughts and late-spring frosts. As an exemplar tree species, we use Norway spruce (Picea abies) to demonstrate a method that links the European distribution of the species to extremes in the projected climate data ensemble at the continental scale. Monthly climate projections were used to calculate the standardised precipitation and evapotranspiration index (SPEI) and bias corrected daily temperature projections were used to calculate a frost buffer of the number of days between last date of spring frost and the date of leafing of Norway spruce. The UKCP Regional climate model ensemble shows substantial changes in the distribution of SPEI likelihood and severity between the pseudo-global warming threshold temperatures of 10C and 20C indicating concern in relation to drought severity and frequency in Alpine, Atlantic, and Mediter- ranean biogeographic regions. In addition, late spring frosts particularly in the Atlantic and Continental biogeographic regions will continue to affect sensitive young trees such as Norway spruce. The approach removes the time series estimation of climate extremes providing a single method of estimating effects of the climate emergency across shared so- cio-economic pathways. Finally, the approach could be useful to direct the deployment of suitable selected forest reproductive material from tree-breeding programmes in Europe.