A dendroecological study
3 Results and Discussion
The age structure of the dominant trees (Fig. 1) shows that the oldest Scots pines in all three stands are about the same age, ea. 190 years. They germinated at the beginning of the 19th century. At that time severe forest destructions (i.e. logging and clearing by burning) took place in the Alps. The distribution patterns of the living Scots pines in stand 1 and 2 are similar. Most of the trees germinated within 50 years, until 1860. However stand 3 shows a different distribution pattern: the older trees are the same age as in the other stands, but most of the trees germinated after 1890, within 10 years. The dead Scots pine are the same age as the living ones. In stand 1 the Norway spruces are 30 years younger than the Scots pines- in stand 2 they are the same age.
The distribution of the DBH (Fig. 2) shows that the dead pines in stand 1 have smaller DBH than the living ones. This indicates that the dead pines were suppressed. For the future, it seems likely that competition will increase, because the Norway spruces are advancing and will compete with the Scots pines more and more. In stand 2 the dead and the living Scots pines show similar distribution patterns. Stand 3 shows a very high stem density, while basal
Figure 1: Age structure of the dominant trees. Figure 2: Distribution ofDBH, basal area
The mean chronologies of the different species at the different stands have a similar trend and synchronous pointer years, i.e. very narrow and very wide tree rings (Fig. 3). After 1991 the ring-width growth of the Scots pines in stand 3 shows an abrupt decrease, whereas all other chronologies show an increase (black circle). Growth fluctuations with frequencies from 5 to 30 years can be explained by climate (Rigling and Cherubini 1999). This applies also to the increase of growth after 1991. This means that the growth decrease of stand 3 is not due to climate.
Microsections (Fig. 4) taken from Scots pines heavily attacked by pine shoot beetles in stand 3 show a growth reduction starting at the beginning of the '90s (after 1990 or 1993).
Discontinous (see tree no. 26, year 1994) and missing rings (years 1995 and 1996) were detected.
The cores show an f'i&ure 4: Microsections
increasing frequency of r - - - .
The last-formed tree ring on each core provides information on the year of death of the trees.
The distribution of last-formed tree rings is shown in Figure 6. Stand 1 and 3 show regular distributions. Stand 2 shows an increasing mortality rate in the late 1980s. According to Rigling and Cherubini (1999) the climatic condition can not explain the mortality at that time.
Spatia-temporal distribution of dead trees at the site (i.e. the higher frequency of death in only a few years and the spatial cluster-distribution of dead trees) suggests that insect attacks could be an explanation of the phenomenon.
Figure 5: Missing rings (1987 -1996) lJ
=
Figure 7 shows the tree growth of Scots pines prior to death in comparison to the mean chronology of the living pines. The growth of the pines prior to death of stand 1 and 3 decreased continously until their death. After 1940 (stand 1) and 1955 (stand 3), they showed slower growth rates than the living pines. The continous decrease of growth could be related to the increasing competition within the stands during that time. In stand 2 the growth of Scots pines prior to death until a few years before death was similar to that of living pines.
The growth prior to death comes to an abrupt end. Some trees showed slower growth rates, others faster rates in comparison with the living pines. This indicates that not only suppressed trees with reduced growth died. Together with the fact that rings are missing in the living pines of stand 2 exactly at the time of beginning of the growth reduction (Fig. 6) confirms the hypothesis of insect attack.
Figure 7: Growth trends of Scots pine prior to death 500
450 400
§ 350
;::;
i
300i
250150 lOO 50
Stand 1
1800
8 450
s
; 300
i
250500 . - - ---.
450 m 400 350
t
250150
100 ! ---· --·-···· 1---- F':-·vtf'/1··\;-···-·-'-1:+ --VJIW IIVII
.50
1800 1820 1840 1860 1880
Mean chronology of living trees - Single curves of dead trees
4 Conclusions
The complexity of forest ecosystems and the special situation in V alais, where the Scots pine forests have been strongly influenced by human activities for centuries explains the presence of several causes of the mortality of the pines in the 'Telwald'. Based on our dendroecological studies we can exclude overaging as explanatory factor, because the trees are only about 200 years old. Rigling and Schweingruber ( 1997) showed that on comparable sites Scots pine can reach ages up to 450 years.
Competition as explanatory process for mortality:
The dead pines diffusely distributed in stand 1 and 3 have the same age and have smaller DBH compared to the living pines. This can be explained by increasing competition from Norway spruce (stand 1), together with high stand densities (stand 3), which are consistent with an advanced stage in the natural succession of the forests. This leads to a shortage of light and water and finally to death. The dead pines show a continuous decrease in radial growth prior to death. The year of death varies from one individual to another, and in most cases is determined by an additional stress factor
Insect attack as explanatory parameter for mortality:
The diffuse and clustered dead pines in stand 2 have the same age and have a similar DBH distribution as the living pines. Competition seems not to be high, as shown by the low stand density and the little percentage of Norway spruces in the undercover. Initially dead pines have normal growth rates, followed by an abrupt growth reduction which led to death. In the living pines rings are missing exactly at the time of beginning of the growth reduction.
Pine shoot beetle as explanatory parameter for mortality:
The whole stand 3 is infested by pine shoot beetles. Signs of their shoot feeding are visible in every crown. The clustered dying Scots pines show an abrupt growth reduction, starting between 1990 and 1993, and very often trees fail in forming tree rings All the pines, despite their individual situation within the stand are declining.
Acknowledgements
This project was funded by the Swiss long-term forest ecosystem research programme (Langfristige Waldokosystemforschung- LWF). We acknowledge the cooperation with the forest service of Valais. We thank our colleagues 0. Bosch, O.U. Braker, A. Clark, M.
Dobbertin, R. Engesser, B. Forster, J.L. Innes, and F.H. Schweingruber for their critical ad vices.
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