# Assisted Migration Field Tests in Canada and Mexico: Lessons, Limitations, and Challenges
## 8 Conclusions  
Evidence from these field tests suggests that in general, tree populations grow well when moved to planting locations 2 to 3 \({}^{\circ}\)C mean annual temperature colder than their origin. Tree species can also establish and grow well when moved outside of their natural distributions to sites with climates within their modelled historic climatic niches.  
In the case of a shade-tolerant species such as _Abies religigosa_, the use of nurse plants may be necessary to protect the target species by ameliorating extreme temperatures, either day/night fluctuations, the coldest temperatures of winter, or the warmest temperatures during the warm, dry season (March-May in Mexico). With the use of nurse plants, an upward shift of populations by up to 400 m appears to have no negative impact on growth or survival.  
However, as heat waves and unusual drought periods become more frequent, prolonged, and intense, it might be advisable to use nurse plants to increase the survival of the targeted seedlings even for shade-intolerant pine species. For example, British Columbia experienced two extreme droughs during the summer of 2017 and 2018, which caused significant mortality in young plantations [110]. Whereas shade from adjacent vegetation has long been considered undesirable for shade-intolerant tree species in British Columbia, frequent, extreme droughs may require reconsideration of that view in drought-prone regions or coarse, shallow soils.  
Long-term, multi-site field tests located in disparate climates are needed to develop accurate climate transfer and response functions for priority species. Collaboration among neighbouring jurisdictions and careful field test design can help reduce costs.  
For some species such as _Pinus albicanulis_, developing disease-resistant trees through selective breeding may be a higher priority than assisted migration, and have a greater impact on tree health and population size. However, once resistant material is available, it can be integrated into a restoration program that includes assisted migration.  
Planning of field provenance trials requires a long-term budget and human resource commitment. Collecting and extracting seed, producing seedlings, and locating and preparing test sites can take 5 years; an additional 10-15 years of field testing, with frequent test site maintenance, may be needed before extreme climate events and pest disturbances reveal patterns of adaptation.  
Assessment of invasion risk is warranted prior to, and regular monitoring after, implementation of an assisted migration test or program, particularly for tree species having weedy traits, if naturalization of the migrated species is not desired. Careful screening for hitch-hiking pests on seedlings prior to migration out of range is also recommended.  
C.S.-R. and S.N.A. conceived the paper. C.S.-R., G.O. and S.N.A. provided experimental data and analysis per species. [PERSON] developed the mitigating risk section. [PERSON], G.O. and S.N.A. led the writing. All authors have read and agreed to the published version of the manuscript. Funding was provided by the Coordination de la Investigacion Cientifica of the Universidad Michoacana de San Nicolas de Hidalgo to [PERSON].  
This paper is an undertaking of the Forest Genetic Resources Working Group / North American Forest Commission/Food and Agricultural Organization of the United Nations. Thanks to [PERSON] for phenotyping the _Pinus albicanulis_ experiment in 2018, and tolain [PERSON] for preliminary analyses of the resulting data. The _Pica glauc \(\times\) engelmannii_ provenance trial map was created by [PERSON]. Thanks to [PERSON], co-investigator of the _Pica glauc \(\times\) engelmanniti_ provenance trial.  
The authors declare no conflict of interest.