The main trigger of the Scots pine decline in the Wallis (Switzerland) is the changing climate. At elevations up to 1200 m, the climatic conditions become too warm and too dry for the Scots pine growing in the Wallis. They are about to be replaced by broadleaved trees, in particular by the sessile oak.
Fig. 1 - Dead pine tree with mistletoe in the central
Photo: Andreas Rigling (WSL)
the last decades, numerous Scots pine trees have died at lower elevations of
the Wallis – partly on extended areas. At the same time, Scots pine is reproducing
insufficiently. The Swiss Federal Research Institute WSL has intensively investigated
the decline and insufficient reproduction rate of the Scots pine and developed
conclusions about the causes of these changes.
In the Wallis, the Rhone Valley and its side valleys contain characteristic Scots pine forests stretching from the base of the valley at approximately 450 m up to an elevation of approximately 1500 m above sea level. In some areas, you can find Scots pine stands as far up as the forest line. The Scots pine forests protect the population from avalanches, rock fall and erosion as well as being recreation areas for people and a natural habitat for rare plant and animal species.
Occasionally, since the beginning of the twentieth century, dying Scots pines have been reported. In the 1970’s and 1980’s, pronounced needle necroses have been found on Scots pine and were correlated to the fluorine emissions from the nearby aluminum industry. Emissions were drastically reduced by installing filters at the beginning of the 1980’s, resulting in the disappearance of the needle injury symptoms.
At the beginning of the 1990’s, when strong attacks of various harmful insects and pathogens were frequently observed, a new Scots pine decline started. In some places the dying Scots pine was replaced by different tree species, such as the sessile oak or the white beam. In other places, no tree species were present as substitutes in order to take over functions such as protection from rock fall.
This dramatic development led the Swiss Federal Research Institute WSL, together with the office for Forest and Landscape of the canton Wallis to start a comprehensive, interdisciplinary research project to understand the causes of the extended Scots pine decline. Furthermore, the objective was to formulate the needs of action and appropriate management methods, respectively. In addition to the canton Wallis, this project was supported with generous funding by the Federal Office for the Environment (FOEN) and the Velux foundation.
Today the Scots pine is not only declining in the Wallis but also in other inner-alpine, dry valleys such as in the regions of Innsbruck, southern Styria, Carinthia, Vinschgau and in the Aosta valley. This alpine-wide phenomenon excluded local emissions of air pollutants as a possible cause; nevertheless, one needs to look for common factors, as within each region of investigation complex interactions may be the cause for the decline phenomena.
At first, it was recorded where and how many Scots pine trees were dying in the Wallis. To accomplish this, we investigated on the 1x1 km grid of the Swiss National Inventory LFI how many trees have died or been harvested since the inventories of 1983-1985 (LFI I) and 1993-1995 (LFI II) and how many seedlings have come up. The analyses have clearly demonstrated that during the past 20 years the Scots pine trees that have died were mainly growing at the lower and drier elevations below 1200 m a.s.l., in particular in the central Wallis. At higher and cooler elevations not more Scots pine trees were dying than on average across Switzerland, whereas at lower elevations twice as many were dying (Fig. 2). We grouped the plots according to mean precipitation and temperature into very dry, dry, and moderate humid sites. We also found that at very dry sites, twice as many trees have died compared to the humid sites.
|Fig. 2 - Scots pine mortality in the Wallis compared to the Swiss average (1984–2003).|
With 500 to 1000 mm annual precipitation, the central Wallis is the driest region of Switzerland. However, there are great local differences: specifically, from west to east in the Rhone Valley, precipitation is significantly decreasing due to the mountain shielding. The precipitation maximum is shifting from summer to winter. As a result, the stress on vegetation due to drought during the summer increases towards the east and reaches its maximum near the village of Visp, while the annual sum of precipitation may vary considerably.
After a period with sufficient precipitation, the years since 1990 were clearly drier than average in the Wallis. For example, at lower elevations of the central and eastern part of the main Rhone Valley as well as in the side valleys of the Vispa River, there were only 390 to 490 mm of precipitation in 1998 (on average 78% of the annual precipitation), 300 to 430 mm (62%) in 2003 and 400 to 570 mm (85%) in 2004. Usually, there is 500 to 600 mm of precipitation.
The development over the last 100 years however, does not show any decrease in precipitation or a significant increase in dry periods. On the other hand, there has been a strong increase of temperature in the Wallis as well as across the rest of Switzerland during this same time period.
It is notable that there is a strong increase in summer temperatures during the last 20 years. The number of hot days with an average temperature of higher than 20°C has significantly increased at lower elevations during recent years: Visp is recording an increase from a former 18 days to about 40 days to date, with a record of 77 days during the hot summer of 2003 (Fig. 3). The higher temperatures, in particular during summer time, leads to an increased stress due to greater transpiration. This results in the trees shutting their stomata earlier in order to reduce water use on dry and hot days.
|Fig. 3 - Number of hot days (daily mean temperature higher than 20°C) in Visp since 1980.|
The drought stress of a tree mainly depends on the soil water which is available for uptake. On the LWF-plot at Visp, we have been monitoring the soil water content since July 2001 on an hourly basis. Each summer, the water reserves are exhausted also at greater depths. Small amounts of precipitation (< 10 mm per day; line B in Fig. 4) evaporate very quickly and hardly drain into the upper soil layer. In greater amounts the water seeps into the surface but the lower soil layers stay dry (see line A in Fig. 4). Thus, the lower soil layers are only a limited water provider. As long as there are no significant rainfalls, the water level remains on a constant low level which is not available for the plants until fall/winter. Only strong rainfalls, such as those experienced in the summer of 2002, are able to increase the amount of available soil water at lower soil layers.
In fall and winter on the other hand, the water reserves are recharged depending on the weather. When the water reserves are not recharged completely during winter, plant dehydration will set in earlier during summer.
In conclusion, it could be demonstrated that during the vegetation period, trees grown on the respective study plot are suffering under drought stress on a regular basis. This reduces resistance against pests. When the drought lasts for an extended period of time, such as in 2003, Scots pine trees are the first to die.
|Fig. 4 - Soil water content in Visp during 2002 at different soil depths and varying precipitation. (Precipitation data: G. Schneiter, WSL).|
The increase of temperature during summer months increases the transpiration rate and leads to an increased drought stress for trees during drought years with limited soil water availability. At the study site nearby Visp, where already more then 60% of the Scots pine trees have died since 1996, most of those trees have died during the years after hot and dry summers (e.g, 1998 and 2003) (Fig. 5). But during this same time period only 15% of broadleaf trees died, including mainly birch, cherry, oaks and few white beam trees as well.
If there are several drought years in a row, the negative impact on Scots pine is even more severe which can be shown with tree ring analyses. In general, the Scots pine growing in the Wallis responds to dry years with increased needle dropping and a decreasing needle and shoot length. It appears that Scots pine trees with reduced needle biomass are more susceptible to stresses such as co-occurring infestation by mistletoe, nematodes and blue-stain fungi.
The sub-Mediterranean sessile oak which replaces the Scots pine at several locations is generally better adapted to drought than the Scots pine. Growth studies however show that sessile oak can also reach its limits during extreme drought years.
|Fig. 5 – Mortality rate of Scots pine during years after hot and dry summers.|
The occurrence of pests and diseases on Scots pine was also investigated within this project. In general, it has been shown that a temperature increase accelerates the infestation by pests and fungi, either directly by a faster development of the pests or indirectly by a weakening of the trees.
An important stress factor for the Scots pine in the Wallis is the mistletoe. As a warm-loving half parasite the mistletoe withdraws water and nutrients from the tree but conducts photosynthesis itself. Due to the temperature increase during the last 100 years, today the temperature sensitive mistletoe can be found at 200 m higher elevations than around 1900. After the drought years of 1998 and 2003, on the LWF-plot near Visp, two to three times more Scots pine trees died with an intermediate to strong infestation by mistletoe compared to Scots pine trees with no infestation by mistletoe.
The Scots pine knows a great number of fungal diseases on roots, stem and needles which can weaken the trees, if not kill them. Some of them were found on Scots pine trees in the Wallis. It is not known yet whether climate warming will lead to a more severe infestation by fungi in the long run.
Nematodes of the Bursaphelenchus (up to approximately 1 mm long nematode) were found in the sap wood of dying Scots pine trees in the Wallis. These nematodes are also supposed to weaken or even kill Scots pine trees. The nematodes are well known to have a higher reproduction rate at higher summer temperatures.
The pests found in dead Scots pine trees are mostly secondary, which means that they primarily attack already weakened trees. In particular, after dry years; therefore, the attack increases. But it is also known that increased temperatures accelerate the development of the larvae.
The possible climate change scenarios show a large variation. Mainly a slight increase in winter precipitation and a decrease in summer precipitations are expected. The predicted temperature increase within this century is more than twice as high as recorded during the last century. In particular, the increase of summer temperatures will further weaken the Scots pine growing at lower elevations. The potential pests may benefit from the warming. This may affect not only the mortality rate but also the reproduction rate of the Scots pine. The Scots pine belt may then move towards higher elevations as other tree species such as silver fir, larch and spruce may also be weakened by climate change. In lower valleys, the sessile oak will progress and eventually dominate. If the climatic conditions turn even drier, the sessile oak will also suffer. This may lead to a possible scenario of progressing desertification of current forest stands at lower elevations of the Wallis. According to our judgment, this may significantly restrict the management options for silviculture.