# Assisted Migration Field Tests in Canada and Mexico: Lessons, Limitations, and Challenges
## 4 Lessons
### Picea glauc \(\times\) engelmannii Hybrid Complex  
Negative impacts of climate change on plantation productivity may be mitigated by planting seed sources from locations that are 3 \({}^{\circ}\)C MCMT warmer than the plantation.  
The potential benefit of assisted migration is illustrated in the interior spruce genecology function for the test site at High Level, Alberta, where the most productive populations are those that originate from locations having a MCMT approximately 3 \({}^{\circ}\)C warmer than the plantation (i.e., populations that have been transferred coldward 3 \({}^{\circ}\)C MCMT) (Figure 3). The strong relationship between productivity and seed source climate at this site and others provides convincing evidence of the degree to which evolution has molded populations to be locally adapted.  
Figure 3: Genecology function for a provenance test site near the town of High Level, Alberta, showing that the most productive populations at the High Level site are those that originate from locations having a mean coldest month temperature (MCMT) approximately 3 \({}^{\circ}\)C warmer than the plantation. The vertical bar identifies the MCMT of High Level, Alberta (\(-23\) \({}^{\circ}\)C).  
Comparisons of the modelled trajectories of population productivity over time show that productivity is slightly greater for the local than the migrated population at establishment (i.e., when plantation MCMT = \(-18\)\({}^{\circ}\)C); however, shortly after establishment (i.e., when plantation MCMT \(\approx-15\)\({}^{\circ}\)C), the relative productivity of the migrated population surpasses that of the local population, suggesting that assisted migration can help mitigate climate change impacts to productivity (Figure 4). Where forest productivity is low or long-lived plantations are desired (as in many conservation situations), the increasing advantage of migrated populations over time can be particularly valuable.  
These analyses corroborate a growing body of literature suggesting that assisted migration can help forestall some of the negative impacts of climate change on forest productivity [47; 59]. In lodgepole pine (_Pinus contorta_), where the results of long-term provenance trials have been widely examined, assisted migration is also expected to have a positive impact on productivity [46].  
Mexico. Abies religiosa. The Species Performs Well When Moved 400 m Upward and Local Shrubs Are Used as Nurse Plants  
Upward transfer of seed sources by up to 400 m (equivalent to planting at a site 2 \({}^{\circ}\)C mean annual temperature colder than the seed source, given a lapse rate of 0.5 \({}^{\circ}\)C/100 m elevation [66]) does not appear to have any negative impacts on the planted seedlings if the seedlings are planted under the shade of local shrubs serving as nurse plants. When planted under nurse plants, survival was 83% averaged across sites and provenances; when planted outside the nurse plant crown, average survival was only 14%. Thus, at least for the case of this shade-tolerant _Abies religiosa_ species, assisted migration must be protected assisted migration, by using nurse plants as a companion for the target migrated species [65]. This finding is consistent with that of St. Clair, [PERSON], and [PERSON] [59] who found that Douglas-fir seed sources moved to sites 2 \({}^{\circ}\)C cooler than their origin still retained acceptable long-term survival and productivity.  
Figure 4: Modelled response function showing predicted relative productivity trajectory of “local” (solid line) and migrated (broken line) populations of _Picea glauc \(\times\) engelmannii_ hybrid complex (MCMT_P = \(-18\) and \(-15\)\({}^{\circ}\)C, respectively) when planted at the climate origin of the local population, represented by the vertical bar.