Department of Biology, University of Minnesota Duluth, Duluth, Minnesota, 55812, USA.
The Nature Conservancy in Minnesota-North Dakota-South Dakota, Duluth, Minnesota, 55802, USA.
Ecol Appl. 2020 Jul;30(5):e02092. doi: 10.1002/eap.2092. Epub 2020 Mar 19.
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.
北方森林正在经历剧烈的气候变化，在上个世纪已经升温 1.0°C-1.9°C。然而，森林更新实践通常仍然受到固定种子区框架的支配，在这种框架下，种子既从预定义的区域中收获，也被种植到该区域中。我们的目标是确定美国明尼苏达州南部种子区的幼苗是否由于气候变化已经更好地适应了北方种子区。从两个种子区（即树种生态型）收获的白栎（Quercus macrocarpa）和北方红栎（Quercus rubra）幼苗被种植在两个北方种子区的 16 个地点，并进行了 3 年的测量。我们的假设是三重的：（1）地理分布更南的树种将在气候已经大幅变暖的北方森林中茁壮成长；（2）这些物种的南部生态型在北方环境中的存活率和生长率将高于北部生态型；（3）自然选择将有利于表现出源自更南部种子区的树木特有的表型和物候特征的幼苗。对于这两个物种，存活率都很高（>93%），南部生态型表现出与我们的气候适应假设一致的特征。红栎的生态型差异尤为明显；南部生态型的存活率更高，比北部生态型的比叶面积（SLA）更低，生长速度和直径增长更快，叶片物候期更长。白栎的结果较弱，但南部生态型的芽萌发时间也比北部生态型更早，比叶面积（SLA）也更低。基于固定种子区的模型无法像基于连续预测因子（如气候和地理位置）的模型那样解释幼苗表现，这表明当前种子区划分内的植物适应确实与不断变化的气候条件一致。这一结论得到了支持，自然选择有利于更南部树种生态型所表达的特征。总的来说，这些结果表明，应该重新审查国家种子采购指南，以允许在种子区之间进行种植，这是一种辅助迁移的形式。需要进行更广泛的实验（即起源试验），以制定考虑到气候趋势的特定物种的种子转移指南，同时还要考虑种子来源的确切地理位置。
