Limited influence of air temperature and precipitation on six-year survival and growth of non-native tree species in a Central European multi-site field trial

The biggest challenge for forest management in the near future will be the silvicultural adaptation of forests on sites where climate change will increase the risk for drought induced tree mortality and for a decrease of the growth performance. One management option is the diversification by promoting climate change adapted nonnative tree species. Currently, we have in depth experience with only a few non-native tree species in Central Europe. Other tree species that may complement the species pool in Central Europe need to be carefully selected by a range of criteria. Before establishing larger silvicultural experiments at stand-level, the suitability of species to grow well with a low risk of failure under expected future climatic conditions as well as a good performance already today should be tested, amongst other aspects. We present results of a multi-site common garden experiment where growth and survival of five non-native tree species (Abies bornmuelleriana, Cedrus libani, Fagus orientalis, Tilia tomentosa, and Tsuga heterophylla) and one climate adapted local native species per site are tested in five field trials in Austria, Germany and Switzerland along a temperature (7.9-10.4 degrees C mean annual Temperature) and precipitation (490-1147 mm annual precipitation) gradient. Each species was replicated on three plots per site using a block-wise design. Lower survival and growth of the non-native species as the native reference on most sites indicate that the studied non-native species are today not as well adapted to current climatic conditions as native species. However, the performance of the non-native species was sufficiently high to justify further trials. Survival and growth rates were mostly equally high on the two sites at the opposing ends of the temperature and humidity gradient and lower on all other sites. Height growth and survival were only marginally affected by the strong summer drought in 2018 in all except one site, presumably due to the low water storage capacity of the sandy soils on this site. This indicates that the tested non-native tree species grow well in the wide range of climatically changing weather conditions from our transnational study sites across Central Europe. So far, the tested tree species did not reach their climatic growth limitation. However, edaphic site conditions may have influenced the performance of the tested non-native species as well and must be considered when discussing the suitability of these tree species.