Understanding the key mechanisms that control northern treelines is important to accurately predict biome shifts and terrestrial feedbacks to climate. At a global scale, it has long been observed that elevational and latitudinal treelines occur at similar mean growing season air temperature (GSAT) isotherms, inspiring the growth limitation hypothesis (GLH) that cold GSAT limits aboveground growth of treeline trees, with mean treeline GSAT ~6–7°C. Treelines with mean GSAT warmer than 6–7°C may indicate other limiting factors. Many treelines globally are not advancing despite warming, and other climate variables are rarely considered at broad scales. Our goals were to test whether current boreal treelines in northern Alaska correspond with the GLH isotherm, determine which environmental factors are most predictive of treeline presence, and identify areas beyond the current treeline where advance is most likely. We digitized ~12 400 km of treelines (>26 K points) and computed seasonal climate variables across northern Alaska. We then built a generalized additive model predicting treeline presence to identify key factors determining treeline. Two metrics of mean GSAT at Alaska's northern treelines were consistently warmer than the 6–7°C isotherm (means of 8.5°C and 9.3°C), indicating that direct physiological limitation from low GSAT is unlikely to explain the position of treelines in northern Alaska. Our final model included cumulative growing degree-days, near-surface (≤1 m) permafrost probability and growing season total precipitation, which together may represent the importance of soil temperature. Our results indicate that mean GSAT may not be the primary driver of treeline in northern Alaska or that its effect is mediated by other more proximate, and possibly non-climatic, controls. Our model predicts treeline potential in several areas beyond current treelines, pointing to possible routes of treeline advance if unconstrained by non-climatic factors.

中文翻译：

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阿拉斯加北部林线的气候包络线：对控制因素和未来林线推进的影响

了解控制北方林线的关键机制对于准确预测生物群落变化和陆地对气候的反馈非常重要。在全球范围内，长期以来一直观察到高程和纬度树线出现在相似的平均生长季节气温 (GSAT) 等温线，激发了生长限制假说 (GLH)，即冷 GSAT 限制了树线树的地上生长，平均树线 GSAT ~6–7°C。平均 GSAT 温度高于 6–7°C 的树线可能表明其他限制因素。尽管变暖，全球许多林线并没有向前推进，并且很少在广泛的范围内考虑其他气候变量。我们的目标是测试阿拉斯加北部当前的北方林线是否符合 GLH 等温线，确定哪些环境因素最能预测林线的存在，并确定当前林木线以外最有可能前进的区域。我们数字化了约 12 400 公里的林线（> 26 K 点）并计算了阿拉斯加北部的季节性气候变量。然后，我们建立了一个预测林线存在的广义加性模型，以确定决定林线的关键因素。阿拉斯加北部林线的两个平均 GSAT 指标始终高于 6-7°C 等温线（8.5°C 和 9.3°C 的平均值），表明低 GSAT 的直接生理限制不太可能解释北部林线的位置阿拉斯加州。我们的最终模型包括累积生长期天数、近地表（≤1 m）永久冻土概率和生长季总降水量，这些都可能代表土壤温度的重要性。我们的结果表明，平均 GSAT 可能不是阿拉斯加北部林木线的主要驱动因素，或者它的影响是由其他更接近且可能与气候无关的控件介导的。我们的模型预测了当前林木线以外几个区域的林木线潜力，指出了如果不受非气候因素的约束，可能的林木线前进路线。