Behavior of the ´European´ Douglas-fir

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) with its increasing proportion in Europe holds the promise to be the most economical and ecological important non-native European coniferous forest tree. It outcompetes numerous native European species by better growth properties, equal or better quality of timber and the ability to cope effectively with drought. Douglas-fir has a large native distribution range in western North America. In the USA and Canada, this species is represented by two varieties (`coastal` and `interior`), which are geographically largely distinct, but hybridize when in contact. The `coastal` variety possesses better growth properties than the `interior` variety, which in turn is more cold-hardy and more drought resistant however more sensitive to fungal pathogen under humid conditions. In Europe, Douglas-fir was introduced almost two centuries ago. For European forestry, according to 40-years-old- European provenance research, the `coastal` variety from specific American areas is recommended, whereas the `interior` variety is principally a `no go` variety in all but a few European countries. Interestingly, individuals of both varieties as well as their admixed (inter-varietal hybrid and introgressed) individuals were found in imported American seeds from such recommended `coastal` variety areas. In addition, the distribution of Douglas-fir in Europe was shaped for more than 140 years by import of seed of unknown origin as well as mixing of seed sources. This resulted also in variety-mixed stands, which regenerate naturally and where the inter-varietal hybridisation is expected to take place. The presence of both varieties and/or hybrids in imported seed sources and within existing European stands raises pivotal questions: How frequent are inter-varietal hybridisation events in European stands? What impacts do hybridisation and hybrids/introgressed individuals have on the persistence and development of an introduced species such as the `European` Douglas-fir in its new environment? Being of high significance in evolutionary and adaptation ecology these questions are the focal point of the proposed project. The primary aim of the proposed project is to study the occurrence and distribution of inter-varietal hybrids and introgressed individuals within variety-mixed European Douglas-fir stands and their natural regeneration. Moreover, the drought tolerance of inter-varietal hybrids and the two varieties shall be compared. This will allow identifying trends in the development of `European` Douglas-fir within the established stands also with respect to climate change. The secondary aim is to analyse the genetic structure and composition of American Douglas-fir seed areas relevant for seed transfer to Central Europe, and to compare adaptive phenological and growth traits between selected American and European progenies. This not only permits us to recognize how `variety-pure`, and genetically divergent the imported American provenances are, but also to detect and understand similarities/differences in the adaptive traits when progeny is produced outside the distribution range.

Across Europe, there is hardly a tree species so controversial and caught between nature conservation and forestry as the North American Douglas-r (Pseudotsuga menziesii (Mirb.), Franco). It is the most frequent introduced conifer in European forests when we take into account the number of European countries where it grows in forests (>35 countries), and the area it occupies in Europe (>823,534 hectares, 0.40% of the European forest area). The year of first introduction of Douglas-fir differs largely between European countries (1827 in the UK, 1950 in Macedonia). In its native range, in the western North America, Douglas-r is divided into two geographically distinct varieties: P. var. menziesii (coastal variety), and P. menziesii var. glauca (interior variety), which also varies in growth rates and susceptibility to biotic and abiotic factors. Both varieties hybridize in contact zones resulting in variety-hybrids. In Europe, the presence of variety-hybrids is limited. This is due to the uneven occurrence of both varieties in European forests, in which the coastal variety dominates among old trees but also in the natural regeneration. Consequently, variety-hybrids with contributing coastal, but not the interior parent were identified. The dominance of coastal variety is also matching interests of the European forestry, which cultivates this variety for its superior growth and health performance. With help of genetic analyses, we could show that the coastal variety also prevails in the majority of old stands (96%), which have been planted in the past using seeds of unknown variety and geographic origin, and was introduced from a restricted area in the US (from the western Cascade range). On the contrary, the interior variety was introduced from a much larger geographic area in Amerika (2.000 km). Both introduced varieties host identical levels of genetic diversity when compared to native areas. When comparing genetic diversity between adult trees and natural regeneration within Europe, no difference was found when performing extensive analyses (using larger numbers of individuals, larger area coverage, but less populations), while a generalising analysis showed significant reduction in the total genetic diversity. Also seedlings grown from seeds collected in Europe revealed a lower genetic diversity when compared to the American seedlings, in addition to signicant differences in the adaptive traits (morphological characteristics, timing of bud burst). The climate of the European introduced range is most similar to the climate of the coastal variety from the western Cascade range, from which the majority of the coastal European Douglas-r, but not from the European interior variety originates. Analyses of climatic distances suggest that old European stands did not undergo strong climatic selection, but also that climatic similarity is only of minor importance for successful growth and reproduction when this conifer reaches a new range.