Increment allocation along the stem axis of dominant and suppressed trees in reaction to drought-results from 123 stem analyses of Norway spruce, Scots pine and European beech

Stem analyses were carried out on three common tree species in Central Europe to evaluate whether the strong drought of 2003 affected stem increment patterns in dominant and suppressed trees differently. Following Sterba (1981) we expected that dominant trees would react with an increased stem increment in the upper stem section (= acrotone) while suppressed trees would not change their stem increment pattern. Our data set included 41 Norway spruce (Picea abies [L] Karst), 46 Scots pine (Pinus sylvestris L.) and 36 European beech (Fagus silvatica L.) trees of social classes 1-4 (acc. to Kraft 1884) from stands distributed over Eastern Bavaria. To compare stem increment patterns for trees of different heights the basal area increment BAI was calculated in relative heights (10 % intervals). Drought (2003) and post-drought BAI (2004-2007) were compared with the pre-drought BAI (1999-2002). The BAI at 50 % height (BAI50) and 10 % height (BAI10) and the ratio BAI50/BAI10 were used to test increment differences between the classes. The drought in 2003 led to a loss in BAI10 of 35 % in spruce and 20 % in pine and beech. The BAI50 loss was generally weaker with 25 % in spruce, 5 % in pine and 15 % in beech. However, in terms of differences between the classes, only spruce exhibited a growth behaviour as expected: allocation pattern along the stem axis in the dominant classes 1-3 turned stronger acrotone but remained unchanged in suppressed class 4. In pine and beech, the loss in BAI10 and BAI50 during drought was higher for dominant trees than for suppressed trees, but changes in the BAI50/BAI10 ratio were independent of the social status. For all species, post-drought recovery in 2004-2007 was stronger for dominant than for suppressed trees. In spruce-which reacted strongest to drought-the more acrotone stem increment allocation in class 1-3 in 2003 is interpreted as drought stress-a reaction that has been shown also for other stress sources. Class 4 trees in spruce reacted equally strong as class 1-3 but the stem increment did not become more acrotone which indicates a high pre-drought stress level due to suppression. Pine and beech reacted less strong to drought. In contrast to spruce, increment loss increased from dominant to suppressed trees. In pine, the stem increment pattern in class 1 did not change while in class 2-4 a trend towards more acrotone increment is in correspondence with the higher drought stress in these classes. Beech, finally, did not significantly change its increment pattern in all of the classes.