Assisted migration vs. close-to-nature forestry: what are the prospects for tree populations under climate change?

Climate change is currently perceived as the most important challenge faced globally by ecosystems and human society. The predicted changes of temperature and precipitation patterns are expected to alter the environmental conditions to which forest trees in Europe are adapted, and expose them to new pests and pathogens. This would unavoidably lead to a huge loss of ecosystem services provided to society, and at the local scale may potentially endanger the very existence of forests. In this study, we reviewed biological background and limits of mechanisms by which tree populations may cope with climate change: adaptation by natural selection, gene flow, epigenetic phenomena and phenotypic plasticity, as well as forest management strategies, which rely on these mechanisms. We argue that maintaining genetic diversity is important in the long-term view but natural selection cannot ensure sufficiently rapid response to environmental change. On the other hand, epigenetic memory effects may change adaptively relevant traits within a single generation, while close-to-nature forestry practices are the basic requirement to make use of epigenetics. Assisted migration, as a frequently suggested mitigation option, relies primarily on the knowledge gained from provenance research; the review analyses potential pitfalls of this strategy. We suggest that all approaches, i.e., leaving a part of forests without management, close-to-nature forestry, and transfer of forest reproductive materials from sources presumably adapted to future climates are combined across the landscape in an integrative manner.