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.
Bibliographical noteFunding Information:
Experimental plantings were funded by a grant from the Wildlife Conservation Society Climate Adaptation Fund to M. W. Cornett, M. A. White, and J. R. Etterson, originating from the Doris Duke Charitable Foundation. Additional financial support for this work was generously provided by The Nature Conservancy through the Cox Family Fund for Science and Research, and the Carolyn M. Crosby Foundation. S. Handler, C. Swanston and K. Hall provided invaluable advice in conceptualizing and designing the project. We thank the Integrated Biosciences Program at the University of Minnesota‐Duluth for support to L. Kavajecz. We thank C. Dunham for planting coordination and field operations. J. Mead, A. Tse, D. Myhre, D. Thiel, B. Cogger, R. Sullivan, and K. Campbell provided field assistance. R. Sagar assisted with spatial data preparation and field measurements. B. Gross and B. Palik reviewed earlier drafts of this manuscript and provided invaluable insights for refinements. We thank the Superior National Forest, the Minnesota Department of Natural Resources Division of Forestry, Saint Louis County, and Lake County for the permission to locate experimental plantings on their lands.
© 2020 by the Ecological Society of America
- Quercus macrocarpa
- Quercus rubra
- adaptation lag
- assisted migration
- boreal forest
- climate adaptation
- population differentiation
- seed zone