Crown defoliation & tree rings

 

 

Assessment of crown conditions has been widely used to study impact of various environmental factors on tree health, although little data is available which directly links trees’ crown conditions with their diameter increment. To assess this relationship in southern Swedish oaks (Quercus robur L.) the total of 275 trees from 33 sites were analysed. Dendrochonological methods were employed to recover trees’ growth rate over the 25 years preceding the year of sampling (2000-2004).

 

The study showed good correlation between visually assessed vitality classes and diameter growth. The diameter increment was highest in trees with healthy crowns, intermediate in trees with moderately declined crowns, and the lowest in trees with heavily declining crowns. Oak chronologies suggested a period of growth decline in the beginning of 1990s which we relate to the widespread early-summer drought of 1992. Over the 1990s, the growth in healthy-looking and moderately declining trees recovered, while the growth of heavily declining trees did not.

 

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.

 

Figure. Growth pattern of trees belonging to different classes of crown condition during the 30 years preceding the year of sampling (2002). Error bars shown are 1 SE. Total n = 260; crown class 1, 2, and 3 – 63, 137, and 60 trees, respectively. Three vitality classes were used: 1 – healthy trees with crown defoliation less than 25%, single dead branches with a diameter not exceeding 2-4 cm and fully developed young shoots exceeding 10 cm in length; 2 – moderately declining trees with crown defoliation between 25% and 60%, with a number of dead branches whose diameter was 4-8 cm and reduced length of the young shoots; and 3 – severely declining trees with crown defoliation above 60%, a large number of thick (> 8 cm in diameter) dead branches in the crown, and severely reduced rosette-like appearance of young shoots.