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A warmer climate with drier summers will affect the suitability of tree species in their current range in most of Europe. To preserve ecosystem services in the future, many European countries are looking for tree species adapted to the expected future climate and are setting up trials to test them in different environments. In collaboration with forest practitioners and federal and regional authorities, we have established a network of 57 common gardens across large environmental gradients in Switzerland. Over a period of 30 to 50 years, the vitality, growth and survival of 18 tree species from 117 seed sources will be monitored to develop tree species recommendations for forest managers. In this article, we outline the considerations, challenges and trade-offs involved in designing this experiment, as well as the participatory process with a variety of stakeholders, from local foresters to the Federal Office for the Environment. Further, we list experiments testing multiple species on multiple sites in Europe and compare experimental designs, tree species and environmental gradients. The Swiss common garden network complements other European experiments and broadens the potential network by extending the covered environmental gradients to colder and moister conditions. Such targeted assisted migration trials are key to promoting tree species that can cope with the future climate and can help us to foster forest ecosystems that can adapt to rapidly changing climates. [ABSTRACT FROM AUTHOR]

European beech (Fagus sylvatica L.) is an important tree species throughout Europe but shifts in its suitable habitats are expected in the future due to climate change. Finding provenances that are still economically viable and ecologically resilient is an ongoing field of research. We modelled the dominant tree heights of European beech as a trait reflecting growth performance dependent on provenance, climate and soil conditions. We derived dominant tree heights from national forest inventory (NFI) data from six European countries spanning over large ecological gradients. We performed function-on-scalar regression using hierarchical generalized additive models (HGAM) to model both the global effects shared among all provenances and the effects specific to a particular provenance. By comparing predictions for a reference period of 1981–2010 and 2071–2100 in a RCP 8.5 scenario, we showed that changes in growth performance can be expected in the future. Dominant tree heights decreased in Southern and Central Europe but increased in Northern Europe by more than 10 m. Changes in growth performance were always accompanied by a change in beech provenances, assuming assisted migration without dispersal limitations. Our results support the concept of assisted migration for the building of resilient future forests and emphasize the use of genetic data for future growth predictions. [ABSTRACT FROM AUTHOR]

Climate projections indicate large increases in temperature, requiring tree species to adapt or migrate faster than observed historic rates. In Central Europe, discussion in forest management has increasingly focused on how humans can assist these adaptation/migration processes, especially when important forest ecosystem services are at risk. One option is to introduce non-native tree species to help forest ecosystems adapt to climate change. We established a long-term experiment in autumn 2012 on five study sites in climatically different regions of Central Europe. The performance of five non-native tree species from warmer climates (Abies bornmuelleriana Mattf., Cedrus libani A. Rich., Fagus orientalis Lipsky, Tilia tomentosa Moench and Tsuga heterophylla Sarg.) was compared to that of one local species native to each site. We monitored sapling survival during the first 4 years after planting. Small differences in timing of inventory campaigns were overcome by fitting a Weibull function to survival curves and analysing time-harmonized estimates for equal biannual periods. Sapling mortality decreased over time, but early after planting, it differed between species. The native and the two non-native broadleaved species showed higher survival than the non-native coniferous species. Moreover, the site conditions of the open areas seemed to initially hamper the growth of shade-tolerant species. All species performed reasonably well during the relatively short observation period. Any differences between hazard rates almost vanished 4 years after planting, which suggests that all species tested are likely to thrive under current climate conditions. Long-term observations are, however, needed to corroborate these results. [ABSTRACT FROM AUTHOR]

Species distribution models (SDMs), which statistically relate species occurrence to climatic variables, are widely used to identify areas suitable for species growth under future climates and to plan for assisted migration. When SDMs are projected across times or spaces, it is assumed that species climatic requirements remain constant. However, empirical evidence supporting this assumption is rare, and SDM predictions could be biased. Historical human-aided movements of tree species can shed light on the reliability of SDM predictions in planning for assisted migration. We used Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco), a North American conifer introduced into Europe during the mid-19th century, as a case-study to test niche conservatism. We combined transcontinental data sets of Douglas-fir occurrence and climatic predictors to compare the realized niches between native and introduced ranges. We calibrated a SDM in the native range and compared areas predicted to be climatically suitable with observed presences. The realized niches in the native and introduced ranges showed very limited overlap. The SDM calibrated in North America had very high predictive power in the native range, but failed to predict climatic suitability in Europe where Douglas-fir grows in climates that have no analogue in the native range. We review the ecological mechanisms and silvicultural practices that can trigger such shifts in realized niches. Retrospective analysis of tree species introduction revealed that the assumption of niche conservatism is erroneous. As a result, distributions predicted by SDM are importantly biased. There is a high risk that assisted migration programs may be misdirected and target inadequate species or introduction zones. [ABSTRACT FROM AUTHOR]

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Abstract: Common ash (Fraxinus excelsior) is a keystone tree species throughout temperate Europe whose future existence is threatened by an emerging invasive fungal disease. Ash dieback, which first appeared in Poland in the 1990s, has rapidly spread to most eastern, central and northern European countries. The causal agent of the disease, the ascomycete Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea), was recently described as a new species. Given that the disease lethally affects ash trees of all age classes, and that ash tree mortality levels are high, F. excelsior and the many organisms dependent on ash trees are under threat. Based on a literature survey, we provide an overview of the present knowledge on ash dieback, identify practical recommendations and point out research needs. The observation of relatively resistant individual ash trees (although at very low frequency) calls for a rapid germplasm collection effort to establish a breeding program for resistance or tolerance to the disease. Ash trees that appear to be tolerant to the pathogen should not be felled, unless they pose an unacceptable risk to people’s security. Given that the pathogen does not form propagules on wood, and given the importance of deadwood for biodiversity conservation, dead and dying ash trees should be left in the forest. Landscape pathology and genetic tools can be used to reconstruct the dispersal pathways of H. pseudoalbidus and to identify environmental features associated with variation in disease severity, so as to better predict the further development of the epidemic. Observations on differences in susceptibility of various ash species are needed to locate the geographic origin of the pathogen and to identify Fraxinus species which might be used for resistance breeding or even replacement of F. excelsior. Conservation biologists, landscape managers, restoration ecologists, social scientists and tree geneticists need to engage with forest pathologists and the various stakeholders throughout the distributional range of F. excelsior so as to tackle this pressing conservation challenge. [Copyright &y& Elsevier]