Relationship between crown condition and tree diameter growth in southern Swedish oaks

We studied correlation between crown conditions and tree-ring widths in 260 trees of pedunculate oak (Quercus robur L.) growing on 33 sites in southern Sweden. The tree-ring increment over 1998-2002 was highest in trees with healthy crowns, intermediate in trees with moderately declined crowns, and lowest in trees with heavily declining crowns. The time period with significant correlation between crown status and tree-ring increment varied between 10 years (given autocorrelation in tree-ring chronologies preserved) and 4 years (autocorrelation removed). In pairwise comparisons of three crown classes, differences in tree-ring increment between trees with healthy crowns and trees with heavily declining crowns were the most pronounced, Fisher LSD P value staying below 0.05 over 13 years (autocorrelation preserved ) or 4 years (autocorrelation removed). Over two 5-year periods (1993-1997 vs. 1998-2002) the cumulative increment increased significantly for trees with healthy crowns, did not change in trees with moderately declining crowns, and significantly decreased in trees with heavily declining crowns. For trees with healthy crowns, this dynamics may represent growth recovery after 1992 drought. Instead, oaks with defoliation above 60% appear to reach a threshold in their ability to recover growth. At sites on nutrient-poor soils cumulative increments over 1998-2002 differed significantly among trees with different crown condition and no differences were observed at sites on nutrient-rich soils. Analyses and interpretation of the oak growth trends as recovered from tree-ring chronologies may be improved by controlling for the crown status of the trees sampled, e.g., by using sampling strategy that would represent the average crown and growth conditions of the sites.