# Assisted Migration Field Tests in Canada and Mexico: Lessons, Limitations, and Challenges
## 5 Limitations
### Canada. Picea glauca \(\times\) engelmannii Hybrid Complex  
The _Picea glauca \(\times\) engelmannii_ provenance trial contains one of the widest ranges of population and test site climates in North America, so one may expect a high likelihood of observing local adaptation at these sites. Nonetheless, transfer functions were nearly flat at almost half of the test sites at age six [56], suggesting (at those sites) that the species complex lacks local adaptation. Further, where local adaptation was observed, it varied widely, and was only weakly related to site climate. Sites that did not display strong local adaptation may have escaped extreme climate events or biotic agents or may have lacked local soil edaphic conditions needed to reveal local adaptation (i.e., needed to translate genetic differences among populations into phenotypic differences) [70]. Older tests or different test site locations may have resulted in stronger evidence of local adaptation or more test sites showing local adaptation. Therefore, when developing BC's climate-based seed transfer system, its designers chose to err on the side of caution by including data from only those sites having the steepest transfer functions where local adaptation was strongest [48].  
How Far Is Too Far? Determining a Suitable Migration Distance  
The climate distance populations are migrated in assisted migration initiatives (i.e., the migration distance) that can significantly impact the success of those efforts; migration distances that are too short may be ineffective, while those that are too long may result in early mortality due to cold injury. Comparisons of modelled response functions for populations from a range of climates can help identify optimum migration distances by identifying seed source climates expected to maximize productivity over the expected lifetime of the plantation ([47]; see also _Picea glauca \(\times\) engelmannii_ example, Figure 4).  
Nonetheless, establishing plantations with seed sources whose 1940s climate matches a plantation's current climate would address any adaptation lag that has arisen since the onset of rapid anthropogenic climate warming in the 1950s. However, accounting solely for anthropogenic climate change by attempting to optimize adaptation at establishment could compromise adaptation in later decades, given continued climate change, especially for long-lived species. Therefore, the optimum migration distance should also consider climate change during the lifespan of a plantation by weighing the risk of maladaptation during seedling establishment with the risk of maladaptation towards the end of a plantation's lifespan [51]. A recently implemented assisted migration system in BC adopted the projected climate at one quarter of the plantation's expected lifespan as the adaptation target date [48].  
The logic employed in this proposal is not a radical or risky departure from previous seed sourcing practices, as it attempts to re-establish pre-existing genotype-climate relationships and does not depend on projections of the late 21 st century climate. Arguably, the migration distance calculation proposed here, being dependent on only the 1931-1960 climate mean of the seed source and the projected plantation climate of approximately 2040, is less prone to error than migration distances interpreted from adaptation lags shown in transfer functions (Figure 3) or by comparison of modelled response functions for populations from a range of climates (Figure 4), which can vary considerably with transfer range, model function, tree age, and different dependent and independent variables.