Crown defoliation & site properties

We studied crown defoliation in oak (Quercus robur and Q. petraea), analysed in 845 trees in the three southern-most counties of Sweden (Scania, Blekinge, and Halland).

Crown defoliation showed significant increase between 1988 and 1999. Defoliation increased on average for more than 20%, suggesting a considerable deterioration in the status of oak crowns.

Changes in crown defoliation were related to the pH values of the upper 20-30 cm of the mineral soils. Trees on the soils with higher pH (>4.0, in BaCl2 filtrate) had significantly lower increase in crown defoliation than trees on more acid soils (with pH < 4.0), indicating that less favourable soil conditions may further enhance oak decline (Figure A).

At the time of the first inventory (1988) the mean defoliation was higher in younger trees (60-100 years old) as compared to older trees (> 100 years old). This difference, however, disappeared in 1999, both age classes showed significantly higher but not significantly different levels of defoliation (Figure B).

Younger oaks might be the first to react to the decline-causing factors at the beginning of the period studied.

Management of oak stands, modifying age structure of oak population and soil properties of the stands, will likely affect the future conditions of oak crowns.

Figure.  Dynamics of crown conditions in 845 trees inventoried in the southern Sweden during 1988 and 1999. Error bars are 95% distribution limits of respectful variables. Results of repeated measures ANOVA with percent of crown defoliation in 1988 and 1999 as repeated-measure dependent variable. Independent variables were pH (A), tree age (B).

pH values refer to the upper 20-30 cm of the mineral soil. On figure A, n values for each combination of factor levels are given as point labels.

A.

B.