Variation in mean density of earlywood vessels distribution at breast height of European ash trees, representing different health conditions at time of sampling, over the calendar years 1978–2007. Density of vessels was defined as the number of earlywood vessels per mm² of annual ring area and calculated as a mean for three trees representing the group of healthy (solid line), weakened (dashed line) or dead trees (dotted-dashed line), respectively, and for 5-y time intervals. Vertical bars denote standard errors.