The climate envelope of Alaska’s northern treelines: implications for controlling factors and future treeline advance. Primary data and analyses 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 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.

探明调控北方林线的核心机制，对于精准预测生物群区迁移以及陆地生态系统对气候的反馈响应至关重要。长期以来，全球尺度的观测均发现，海拔梯度与纬度梯度上的林线均处于相近的生长季平均气温（growing season air temperature, GSAT）等温线上，由此催生了生长限制假说（growth limitation hypothesis, GLH）：寒冷的生长季平均气温会限制林线树木的地上生长，林线处的生长季平均气温约为6~7摄氏度（℃）。若某林线的生长季平均气温高于6~7℃，则提示存在其他限制因子。尽管全球气候持续变暖，但诸多林线并未出现上侵扩张，且大尺度研究中极少纳入其他气候变量进行分析。本研究旨在验证阿拉斯加北部现存的北方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，并识别当前林线范围以外最有可能发生林线扩张的区域。我们对阿拉斯加北部约12400千米（km）的林线（含2.6万余个采样点）进行了数字化处理，并计算了该区域内的季尺度气候变量。随后构建了预测林线分布的广义加性模型（generalized additive model, GAM），以明确调控林线分布的关键因子。阿拉斯加北部林线的两项生长季平均气温指标均显著高于6~7℃的等温线阈值，均值分别为8.5℃与9.3℃，这表明生长季低温带来的直接生理限制，不太可能解释阿拉斯加北部林线的分布位置。最终模型纳入了累计生长度日、近地表（≤1米（m））多年冻土发生概率以及生长季总降水量，这些变量可共同反映土壤温度的调控作用。研究结果表明，生长季平均气温或许并非阿拉斯加北部林线分布的核心驱动因子，或者其作用受到其他更直接、甚至非气候因子的调控。本模型预测了当前林线范围以外的多个潜在林线分布区域，提示若不受非气候因子限制，林线或可沿这些区域发生扩张。