Provenance trials in the service of forestry assisted migration: A review of North American field trials and experiments.

• Assisted migration (AM) is being considered to adapt forests to climate change. • We review 104 North American common-garden studies and experiments focused on AM. • Forty-seven species were investigated, but studies focused on commercial conifers. • Recent studies include local environmental variables and effects of extreme weather. • Strengths and weaknesses of studies and pathways for future research are summarized. As greenhouse gas emissions rise unabated and temperatures continue to rise, forest managers have begun to consider assisted migration (AM) of trees as a strategy to adapt forests to climate change. To investigate the risks and benefits of AM, old provenance trials have been repurposed and new common-garden experiments initiated. In this paper, we review three decades of common-garden based research into the adaptation of species and provenances (geographically specific varieties) to current and future climates in North America. Using a filtered data base of 106 papers and reports, we investigate multiple aspects of this work, including subject species, tree response variables and their climatic predictors, statistical techniques, and the patterns of adaptation revealed in results. Although the reviewed work contained valuable insights into provenance-climate relationships, much of this work focused on growth in commercial conifer species and the correlation of that growth with annual or monthly climatic averages. A minority of studies investigated the effects of climate variables with direct mechanistic effects on growth or phenology, including temperature extremes, frost and drought indices, or growing-season duration. Furthermore, recent research has opened new avenues of investigation into the influence of local soil and topographic variables, and even ectomycorrhizal communities over provenance performance. We make the case that future common-garden research into provenance-climate relationships should focus on climatic variables with demonstrated mechanistic connections to growth, survival, and phenology. The effects of extreme weather merit particular attention because extreme weather is likely to be a key determinant of long-term tree health and persistence. [ABSTRACT FROM AUTHOR]