DICE-risk: Potenzielle Invasivität der Douglasie in der Schweiz: eine standortsbezogene Evaluation. Schlussbericht des Projekts im Forschungsprogramm Wald und Klimawandel

Since its introduction from North America to Europe in the middle of the19th century,\nDouglas fir (<I>Pseudotsuga menziesii</I> (Mirb.) Franco) has become the most important non-native tree\nspecies planted in Central European forests for its fast growth and good wood quality. Also in\nSwitzerland, Douglas-fir is the number one non-native tree species, even though its abundance is\nmuch lower than in Germany or France. Thirty years ago, more than 1000 stands with plantations\nof Douglas fir were documented nationwide (Bürgi and Diez 1986). Meanwhile, the nature conservancy\nhas raised the question of its invasion potential under climate change. With this study\nfinanced by the FOEN, we document where Douglas fir can establish in Switzerland. In particular,\nwe tried to answer which forest sites are suitable for Douglas fir, whether established populations\ncan negatively affect forest ecosystems, and to which degree the age of Douglas fir stands affects\nthe spread into neighbouring stands via recurrent seed dispersal.<br/>\nThe inventory of non-native tree species in Swiss forests (Bürgi and Diez 1986) served as a\nnationwide stratified systematic sample consisting of 58 forest stands with three or more Douglas\nfirs individuals aged 55 years or more. In those stands, young Douglas fir individuals were located\nand measured within a circle with a radius of 100 m around the mature Douglas fir trees. Also, the\nfollowing parameters were assessed: pH of the topsoil measured by <I>Hellige PEHAMETER</I> with an\naccuracy of 0.5 pH units, and understory light conditions using a LAI gear (leaf area index). The\npotential production area of Douglas fir in Switzerland was calculated by calibrating models with\nclimate variables of Douglas fir presence in Switzerland and at the origin of Douglas fir in the\ncoastal zone of NW USA and Canada.<br/>\nReferring to our sample, Douglas fir has been most frequently planted in beech forests at low\nelevations, and in particular in woodruff-beech communities (Galio odorati-Fagetum). We found\nsmall regeneration (< 130 cm) at elevations up to 1300 m asl, with no clear trend of abundance.\nEstablished regeneration (> 130 cm) decreases towards higher elevations. The presence of both\nsmall and established Douglas fir regeneration depends on understory light conditions and the\nnumber of mature Douglas fir trees. Improved light conditions corresponded to stands with recent\nfellings, along forest roads and in disturbed areas (e.g. windthrow). Established Douglas fir often\nshowed signs of tending measures. The number of established Douglas fir was neither correlated\nwith climatic variables nor with the pH of the topsoil. These results can be explained by the fact\nthat Douglas fir has usually been planted at sites with suitable climate and acidic soils.<br/>\nDistribution models of Douglas fir in Switzerland indicate additional suitable sites at low elevations\nwhere the tree species may be planted. A model using annual climate data from the origin\nof the coastal Douglas fir in North America resulted in a markedly larger range of suitable growing\nconditions. This indicates a considerably larger niche for this tree species in Switzerland under the\ncondition that climate seasonality is neglected.<br/>\nIn the frame of this field study, we found no stands with strong Douglas fir spread. Accordingly,\nno real or potential negative influences could be detected. A neighbourhood analysis showed\nonly few Douglas fir stands in close proximity to nature reserves of national importance. Considering\na buffer of 100 m around Douglas fir stands as sufficient, this applies to 0.1 to 1% of the sites\nlisted in the inventory of Bürgi and Diez (1986).

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