Assisted migration across fixed seed zones detects adaptation lags in two major North American tree species

Abstract

Boreal forests are experiencing dramatic climate change, having warmed 1.0°–1.9°C over the last century. Yet forest regeneration practices are often still dictated by a fixed seed zone framework, in which seeds are both harvested from and planted into predefined areas. Our goal was to determine whether seedlings sourced from southern seed zones in Minnesota USA are already better adapted to northerly seed zones because of climate change. Bur oak ( Quercus macrocarpa ) and northern red oak ( Quercus rubra ) seedlings from two seed zones (i.e., tree ecotypes) were planted into 16 sites in two northern seed zones and measured for 3 yr. Our hypotheses were threefold: (1) tree species with more southern geographic distributions would thrive in northern forests where climate has already warmed substantially, (2) southern ecotypes of these species would have higher survival and growth than the northern ecotype in northern environments, and (3) natural selection would favor seedlings that expressed phenotypic and phenological traits characteristic of trees sourced from the more southern seed zone. For both species, survival was high (>93%), and southern ecotypes expressed traits consistent with our climate adaptation hypotheses. Ecotypic differences were especially evident for red oak; the southern ecotype had had higher survival, lower specific leaf area ( SLA ), faster height and diameter growth, and extended leaf phenology relative to the northern ecotype. Bur oak results were weaker, but the southern ecotype also had earlier budburst and lower SLA than the northern ecotype. Models based on the fixed seed zones failed to explain seedling performance as well as those with continuous predictors (e.g., climate and geographical position), suggesting that plant adaptations within current seed zone delineations do align with changing climate conditions. Adding support for this conclusion, natural selection favored traits expressed by the more southern tree ecotypes. Collectively, these results suggest that state seed sourcing guidelines should be reexamined to permit plantings across seed zones, a form of assisted migration. More extensive experiments (i.e., provenance trails) are necessary to make species‐specific seed transfer guidelines that account for climate trends while also considering the precise geographic origin of seed sources.

Ten Years of Provenance Trials and Application of Multivariate Random Forests Predicted the Most Preferable Seed Source for Silviculture of Abies sachalinensis in Hokkaido, Japan

“…Furthermore, the mechanisms of local adaptation of A. sachalinensis remain unclear. To achieve successful silviculture, we should consider not only tree growth but also other factors such as mortality [55].…”

“…Range-wide provenance tests have been used to establish and modify seed zones and seed transfer guidelines to improve the tree growth of many forestry species [55,[58][59][60]. There are three conceptual options for improving the current frameworks: (i) re-set the current seed zones, (ii) facilitate appropriate seed transfer, (iii) facilitate assisted gene flow beyond the seed zones to avoid a reduction in tree growth caused by a mismatch to future climatic conditions [7,61,62].…”

Supporting the adaptive capacity of species through more effective knowledge exchange with conservation practitioners

“…outbreeding depression, disease risk; Cook & Sgrò, 2018 ) and want to understand the contexts in which strategies like genetic rescue (Box 1 ) and assisted migration will yield benefits—questions without easy answers (Bell et al, 2019 ; Ridley & Alexander, 2016 ; Tallmon et al, 2004 ). It is therefore necessary to work with practitioners to provide clear guidance for the circumstances under which current management practices are harmful (Prober et al, 2015 ; Weeks et al, 2016 ), and when new approaches will achieve better conservation outcomes (Bell et al, 2019 ; Etterson et al, 2020 ; Frankham, 2016 ; Weeks et al, 2017 ). Although considerably more difficult than for laboratory‐based studies, building an evidence base that demonstrates benefits outside of model organisms, over multiple generations, in natural systems and under realistic management conditions (e.g.…”

Assisted Migration Field Tests in Canada and Mexico: Lessons, Limitations, and Challenges

“…While assisted migration has received significant attention as a climate change adaptation strategy [88,89], diversification of forestry practices (e.g., use of alternative silviculture systems, planting more tree species, prioritizing harvest in areas of high risk of distur-bance) and genetic diversification in particular have also been proposed to buffer some of the uncertainty in climate change projections [1,31,90]. Crop diversification is commonly practiced by farmers to mitigate uncertainty associated with crop prices and variability in production due to pest outbreaks and extreme weather [91], and portfolio diversification is widely used by investors to stabilize returns from fluctuating markets [92].…”