Chronology of hydraulic vulnerability in trunk wood of conifer trees with and without symptoms of top dieback

There is evidence that recently occurring top dieback of Norway spruce (Piceaabies(L.) Karst.) treesin southern Norway is associated with drought stress. We comparedfunctional wood traits of20 healthy looking trees and 20 trees with visual signs of top dieback. SilviScan technology was applied to measure cell dimensions (lumen and cell wall thickness) in a selected set of trunk wood specimens where vulnerability to cavitation (P50) datawere available. The conduit wall reinforcement ((t/b)²) was a good proxy for P50. Cell dimensions were measured on wood cores of all 40 trees; theoretical vulnerability of single annual rings could bethus estimated. Declining trees tended to have lower (t/b)² before and during a period of water deficit (difference between precipitation and potential evapotranspiration)that lasted from 2004 to 2006. The results are discussed with respect to genetic predisposition.