Dispersal potential of Douglas fir in Swiss forests
WHFF Project 2016.06
Project Management: Thomas Wohlgemuth
The most important facts in brief
- Due to the increased cultivation of the high-yield and drought-resistant Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco), the invasiveness risk is analyzed
- Silvicultural measures to control Douglas-fir when undesirable establishment occurs, dimension sizes for buffer zones, and rejuvenating forest sites were considered in more detail
- The effects in pure stands of Douglas-fir in Central Europe showed a significant reduction of species diversity in fungi, arthropods and birds
- Negative effects are smaller when Douglas-fir is admixed. There is a considerable need for research on the degree of mixing Douglas fir that leads to negative effects
The alien Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) enjoys an excellent reputation in Central European forestry and is therefore heavily used for forestry in the neighboring countries Germany and France. In Switzerland, it is regarded as a tree of the future with a broad site spectrum under present climatic conditions because of its resistance to summer drought. The opportunities for forestry use are offset by the risks.
The present study aimed to clarify this risk in more detail. In particular, the following questions were asked:
- On which forest sites and in which adjacent openland ecosystems does Douglas-fir rejuvenate best?
- What dimensions of buffer zones should be selected around open lands such as bogs or around special forest sites to minimize seed entry from adjacent Douglas-fir?
- What silvicultural measures exist to control Douglas-fir when undesirable establishment occurs?
Under present climatic conditions, Douglas-fir can be used at many forest sites, but its climatically suitable acreage at lower elevations in Central Europe will be reduced as temperatures continue to rise, and its optimum growth will shift to elevations higher than 1000 m above sea level. In general, coastal Douglas-fir is proposed for forestry use, especially with provenances from southern Oregon and California. Inland Douglas fir, which is susceptible to needle blight, should generally be avoided.
The opportunities presented for Douglas-fir cultivation are offset by the risks associated with the introduction of this alien tree species. The Douglas fir cannot spread into Central European forest communities on its own and thus displace plant species relevant to nature conservation. However, if nutrient-poor and also dry site conditions prevail, the Douglas fir can move with the native tree species or even overgrow them. The findings confirm the field observations from Germany and Austria, which urged caution against the carefree use of the Douglas fir in silviculture in these countries.
While Douglas-fir does not appear to substantially degrade topsoil, it is likely to cause additional nutrient depletion on lean soils. However, on productive sites, where Douglas-fir is predominantly grown in Switzerland, one cannot speak of a strong negative effect of Douglas-fir on the soil.
In contrast, biodiversity is strongly negatively affected by the cultivation of Douglas fir, especially at very high percentages or in pure stands, in most of the studies considered. This is reflected in lower numbers of fungi in the soil under Douglas fir, fewer arthropod individuals and species in tree bark (summer and winter) and crown (winter only) of Douglas fir, resulting in less food especially for wintering forest birds.
In addition, Douglas-fir shows lower diversity of organisms involved in deadwood decomposition. Several studies indicate that negative impacts on diversity and functions are reduced in mixed stands. In addition, more and more potential native pest organisms are likely to adapt to Douglas-fir such as native bark beetle species or newly introduced pest organisms.
Currently, the situation in Switzerland is neither dramatic nor risky compared to Germany and France, where the national proportion of Douglas fir in forests is about ten times higher. However, their proportion will increase in Swiss forests in the coming decades as a result of planting on the Central Plateau. In terms of risk distribution, the forestry use of Douglas fir should be based on mixed stands.
The project was supported by the Swiss Forest and Wood Research Promotion WHFF-CH of the Federal Office for the Environment BAFU.