# Assisted Migration Field Tests in Canada and Mexico: Lessons, Limitations, and Challenges
## 1 Introduction  
Climate change is expected to decouple forest tree populations from the climates to which they have become locally adapted over many generations [1; 2; 3; 4]. Local adaptation can involve critical trade-offs; frequently, populations from colder sites are more frost resistant, although they have less growth potential than populations from warmer locations that have genetically extended growing seasons [5; 6; 7], and populations from drierlocations may be more drought resistant or tolerant [8; 9]. Suitable climatic habitat for given biomes, tree species, and their populations is predicted to occur at more polar latitudes and higher elevations as climate change progresses [10; 11; 12; 13; 14]. Indeed, natural migration of some tree populations via seed dispersal to more polar latitudes and higher elevations has already been observed [15; 16; 17], although this is not happening at the speed needed to keep populations coupled with the climatic habitat to which they have evolved. Such ongoing decoupling is creating increasing stress for tree populations. Perhaps most evident is drought stress, which makes trees weaker and less able to defend against insect and pest attacks [18]. This cascade of abiotic and biotic stresses is resulting in massive mortality of tree populations around the world [19; 20; 21]. The largest negative impacts will likely happen at the rear-edge (lower latitudinal, elevational, or seric limit) of the natural distribution of each tree species [22; 23; 24; 25]. The decoupling of forest tree populations from their suitable climatic habitat presents an evolutionary dilemma: populations must adapt or migrate; otherwise, the likely outcome is extirpation [3]. In the very short term, individual trees may tolerate climate change through phenotypic plasticity [26], for example, by reducing leaf area and growth rate to tolerate increasing temperatures and decreasing precipitation. In the long run, adaptation could involve natural selection of individuals more tolerant of new stresses and evolutionary rescue, when genetic adaptation allows a population to recover from demographic effects initiated by environmental change that would otherwise cause extirpation [27]. Natural populations of forest trees usually have high levels of within-population genetic variation, due to their generally large effective population sizes and outcrossing mating systems [28]. Consequently, in most tree species, there is a wide range of different genotypes where natural selection can act to confer more resistance or tolerance to drought stress, pest outbreaks, or high temperatures in subsequent generations. However, if adaptation does not occur quickly enough and decoupling continues for several generations, then the average fitness of individuals within the population or the size of the population may drop too low for it to recover through adaptation and evolutionary rescue [27]. Due to the high likelihood that climate change will negatively impact forest health and productivity [13; 29; 30], assisted migration--the human assisted relocation of genotypes through reforestation and restoration intended to mitigate future impacts of climate change on forest health and productivity--is a climate change adaptation strategy that warrants critical examination. Even the most critical views against assisted migration concede that more field experimental evidence of assisted migration is needed [31]. We agree that the risks and benefits of assisted migration need to be informed with more experimental evidence, particularly with well-designed provenance trials. Here, we present three case studies of field experiments established to evaluate assisted migration in forest trees. Two of these studies pertain to tree species of conservation concern (_Pinus albicans_ (whitebar pine) in Canada and _Abies religigosa_ (Sacred fir) in Mexico) and the other deal with economically important tree species (_Picea glauc \(\times\) engelmannii_ hybrid complex known as interior spruce, in Canada).