Forster, B.; Knizek, M.; Grodzki, W. (eds.) 1999: Methodology ofForest Insect and Disease Survey in Central Europe.
Proceedings of the Second Workshop of the IUFRO WP 7.03.10, Apri120-23, 1999, Sion-Chateauneuf, Switzerland.
Birmensdorf, Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) 60-66.
Forest decline in Scots pine stands in Visp (Valais, Switzerland)
A dendroecological study
Andreas Rigling, Paolo Cherubini and Antti Pouttu
Swiss Federal Institute for Forest, Snow and Landscape Research, CH-8903 Birmensdorf, Switzerland
1 Introduction
Chronic anthropogenic disturbances have long affected forest landscapes in Valais, one of the main valleys of the Swiss Inner Alps. Here, the first human settlements date back to pre- Roman times (Welten 1982, Bieri-Steck 1990, Burga and Perret 1998). Over the last 7500 years, the extent of downy oak (Quercus pubescens Willd.), silver fir (Abies alba Miller) and Norway spruce (Picea abies (L.) Karst.) forests have been reduced becauseof human activities such as felling for agriculture, selective timber cutting, dry wood and litter utilization, and forest grazing (Haas and Rasmussen 1993, Bumand 1976, Lingg 1986, Kempf 1985). As a result, the frequency of Scots pine (Pinus silvestris L.) increased and, today, pine forests extend well beyond their natural range (Plumettaz Clot 1988).
Since the beginning of this century high mortality rates of Scots pine (Pinus silvestris L.) have been observed in parts of the valley. During the last 30 years, a number of research projects have been undertaken to try and determine the cause of.the mortality. During the 1970s, most explanations involved injury by fluorides from nearby aluminium smelters (Fliihler et al.
1981). Following the installation of emission-reduction technologies in the aluminium smelters, symptoms of fluoride injury disappeared, but mortality rates remained roughly the same. In the early 1990s, mortality rates in the vicinity of Visp, in "Telwald", increased.
However, the spatial distribution patterns of dead trees in the forest has changed over time: in the past the dead trees were distributed diffusely in the forests as single trees but now they appear increasingly clustered into groups of some trees up to whole stands. These high mortality rates are causing considerable concern about the long-term sustainability of the protection forests on the valley sides.
This paper aims to apply dendroecological methods to discuss the main causes of the recent die back.
2 Material and Methods
The "Telwald" lies between the villages Visp and Visperterminen, in canton Valais (Switzerland). The altitude ranges from 700 to 1400 m asl., exposition is west to west/northwest and the slope is between 20° and 40°. The geological substrate is a"Biinderschiefer" (schists), the soils are Rendzic leptosols (Rendzinas) rich on fine earth with high water holding capacity. The Humus forms are mull to moder. With a precipitation of 600 mm per year, the climate in the vicinity of Visp should be considered dry.
Three stands have been investigated:
• Stand 1 is well growing, crown transparency varies between 0 and 30%, and the trees reach heights up to 20 m. The stand has two strata of trees: the upper stratum consists mostly of Scots pine (80%) and Norway spruce (20%), whereas in the lower stratum Norway spruce dominates (70%) over Scots pine (30%). The dead pines are diffusely distributed. Insects currently do not represent a relevant problem.
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• Stand 2 has two strata: Scots pine dominates over Norway spruce in both strata. Crown transparency varies between 25 to 80%. The tree height is up to 15m. The distribution of the dead pines is both diffuse and clustered. Insects do not currently represent a relevant problem.
• Stand 3 is part of the mortality-front. The trees are dying, their crown transparency is high ( 40-95% ). The stand consists 100% of Scots pine growing in only one stratum. Tree height is up to 15m. The dead pines are diffusely distributed. Insects, mainly the pine shoot beetles (Tomicus piniperda and T. minor) do represent a serious problem.
Fieldwork was done in February 1997. In each stand, the diameters at breast height (1.3m) (DBH) of all trees higher than 1.3 m within an area of 30 by 30 m were measured and the basal area was calculated. For each tree, total height was measured and crown transparency was estimated.
For the dendroecological investigations, only dominant and co-dominat trees were sampled (Tab. 1).
From each tree we took two cores with an incremental borer at DBH. The preparation of the samples was done after Pilcher (1990). If we did not reach the pith, the missing years were estimated according to Braker (1981).
Table 1: Number of sampled trees.
Species Number of Number of
living trees dead trees
Stand 1 Scots pine 12 9
N. spruce 13 -
Stand2 Scots pine 14 21
N. spruce 11 -
Stand3 Scots pine 28 10
N. spruce - -
Ring width measurement was done using the linear table Lintab (Frank Rinn, Heidelberg, Germany) and the Time Series Analysis and Presentation Program TSAP (Rinn 1996). To build chronologies all the cores were synchronised using the crossdating technique (GleichHi.ufigkeit values and Student's t-test) and brought together into tree and site chronologies. The year of death was determined by using the crossdating technique. The dated year of death is affected by a certain error, because the last visible ring on the core does not always represent the year of death. To analyse more carefully the last tree rings on each cores, microsections were prepared.
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 area is low. Mostly suppressed Scots pines with small DBH died.
Figure 1: Age structure of the dominant trees. Figure 2: Distribution ofDBH, basal area and stem density.
Scots pine dead • Scots pine alive o Spruce alive A
- .l ..
s! 0fl)
i
N
.L..l.
"" fb
• ....
"" f!j
"' 000
1800 1820 18'{0 1860 1880 1900 1920
Figure 3: Stand chronologies
500 450 1400
8 350
-
::::. 300 .s :9 250
bO
200 150
lOO
50 0
Stand 1 - Stand 1 ···
&
1940 Basal area 56m2/ha
No. of stems I 011 Stklha
10
s --·-- -···-·-·-·-·-··---· -· -- ··:···- ('
0 • rJJJ,.
nl-.
ill .tl I "' ..s m w " ro M
3o
l
Stand31
_ ... _ . . Jllo.: .. 1789 Stklha Basal area ... ... 53m2/ha .
'c;
20 .
. - ··-···iLL. . . .
IIJ. . . i
I.
Rs m w " ro M
DBH-Ciasses, upper limits
ftScots pine pine
I
Spruce nFir, Larch, Birch IL)dead alive UDowny oak I
Stand2- Stand 2 ···
Stand3 -
Cll G) bJ)
]
0I
"'0
en
B
1800 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000
62
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 - - - . 1990 91 92 93 94 9S (96)
missing rings during the last decade. Figure 5 shows the number of Scots pines where tree rings were missing, for each stand and for each year since 1987. No rings were missing in stand 1.
In stand 2, and more clearly in stand 3, an increasing number of rings were missing. For example, tree rings for 1996 were missing. on 70% of the Scots pines at stand 3. We believe that the main cause for missing rings is the shoot or maturation feeding of the pine shoot beetles.
..,
"' 0 · z u ·
·a =
:1 0 t )
"'
"'
..
1990
..
..
91
..
,. ...
....
92 93 94 (95X96)
..
.. .
-
.. ..
PhlOCIII
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
=
70 I\ : :
= ::: ... , ...
i60 ... - ... .
·8 so
""
.s
40 30
j 20
··-···--
Figure 6: Date of death
00
0 .,00
B . . •··• ·-·00
•1950 1955 1960 1965 1970
00 .... 8 0 0
s ... ; ..
aJa
0-?· ...• ••
...
1975 1980 1985 1990 1995 2000
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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