The climate envelope of Alaska’s northern treelines: implications for controlling factors and future treeline advance. Mapping seasonal climate variables 2019 - 2021.

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 degrees celsius (°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 to identify areas beyond the current treeline where advance is most likely. We digitized ~12,400 kilometers (km) of treelines (greater than 26K (26,000) 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 meters (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.

厘清调控北方林线（treeline）的关键机制，对于精准预测生物群系迁移及陆地生态系统对气候的反馈响应至关重要。长期以来，全球尺度的观测研究均发现，海拔林线与纬度林线均分布于相近的生长季气温（Growing Season Air Temperature，GSAT）等温线区间，由此催生了生长限制假说（Growth Limitation Hypothesis，GLH）：寒冷的生长季气温会限制林线树木的地上生长，林线处的生长季气温均值约为6~7摄氏度（℃）。若某林线的生长季气温均值高于6~7℃，则提示存在其他限制因子。全球范围内诸多林线虽经历气候变暖却并未出现扩张，而大尺度研究中极少考虑其他气候变量的影响。本研究的目标在于：验证阿拉斯加北部现存寒带林线是否符合生长限制假说的等温线阈值，明确对林线分布最具预测能力的环境因子，并识别当前林线以外最有可能发生林线扩张的区域。我们数字化了阿拉斯加北部总长约12400千米的林线（涵盖超2.6万个采样点），并计算了该区域的季节气候变量；随后构建广义可加模型（Generalized Additive Model）以预测林线存在概率，进而明确决定林线分布的关键驱动因子。阿拉斯加北部林线的两项生长季气温均值指标均显著高于6~7℃的等温线阈值（分别为8.5℃与9.3℃），这表明生长季低温带来的直接生理限制，难以解释阿拉斯加北部林线的分布位置。本研究最终入选的模型变量包括累积生长度日、近地表（≤1米）多年冻土（permafrost）发生概率以及生长季总降水量，这些变量共同体现了土壤温度的重要调控作用。研究结果表明，生长季气温均值可能并非阿拉斯加北部林线分布的核心驱动因子，抑或其影响受到其他更直接、甚至非气候的调控过程所介导。我们的模型预测了当前林线以外的多个区域具备林线扩张潜力，这意味着若不受非气候因子制约，林线或将沿这些区域实现进一步扩张。