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.

中文翻译：

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在固定种子区的辅助迁移可检测到北美两种主要树种的适应滞后。

北方森林正经历着剧烈的气候变化，在上个世纪中变暖了1.0°–1.9°C。然而，森林更新的实践通常仍然是由固定的种子区框架决定的，在该框架中，既从预定区域收获种子又将其种植到预定区域。我们的目标是确定由于气候变化，源自美国明尼苏达州南部种子区的幼苗是否已更好地适应北方种子区。Bur栎（Quercus macrocarpa）和北部红栎（Quercus rubra））将来自两个种子区（即树木生态型）的幼苗种植到两个北部种子区的16个地点，并进行3年测量。我们的假设有三个方面：（1）南方地理分布更多的树种会在气候已经明显变暖的北部森林中繁衍，（2）这些物种的南部生态型将比北部环境中的北部生态型具有更高的生存和生长，以及（3）自然选择将有利于表现出表型和物候性状特征的幼苗，这些树状性状来自较南部种子区。对于这两个物种，存活率都很高（> 93％），并且南部的生态型表现出与我们的气候适应假设相一致的特征。对于红橡树，生态型差异尤为明显。南部的生态型生存率更高 相对于北部生态型而言，较低的比叶面积（SLA），更快的高度和直径增长以及更长的叶子物候学。伯橡树的结果较弱，但南部生态型比北部生态型还具有更早的芽萌和较低的SLA。基于固定种子区的模型无法解释幼苗性能以及具有连续预测指标（例如，气候和地理位置）的幼苗性能，这表明当前种子区轮廓内的植物适应性确实与气候条件的变化保持一致。为支持这一结论，自然选择有利于南方树木生态型表达的性状。总的来说，这些结果表明，应该重新审查国家种子采购指南，以允许跨种子区播种，这是一种辅助迁移的形式。更广泛的实验（即，