Wood density as a screening trait for drought sensitivity in Norway spruce

We linked hydraulic vulnerability in Norway spruce (Picea abies (L.) Karst.) trunkwood with different cambial age to wood density and applied the findings on annual density variations in healthy and declining trees from southern Norway during 1990 to 2010. We hypothesized that drought stress due to the 2003 or 2006 European heat waves were the triggers for tree decline and focused analyses on the structure of wood that was produced after, as well as before, signs of stress, i.e., when decreases in height or diameter growth were visible. In the data set comprising previously published and new measurements, P-50, i.e., the pressure potential necessary to induce a 50% loss in hydraulic conductivity, was negatively related to wood density. Declining trees produced wider annual rings with lower density than vigorous trees before their radial and height increment started to decline in 2003 or 2006. Trees that produced low-density wood under favorable water availability were more stressed by a sudden drought event because of higher P-50 values in their sapwood. Due to the strong genotypic relationship between wood density and growth, we suggest that spruce trees selected for fast growth might experience limited hydraulic performance under the impact of extreme heat waves.