The climate envelope of Alaska's northern treelines: implications for controlling factors and future treeline advance
The climate envelope of Alaska's northern treelines: implications for controlling factors and future treeline advance
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DOI：
10.1111/ecog.05597
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发表时间：
2021-10
期刊：
影响因子：
5.9
通讯作者：
C. Maher;R. Dial;N. Pastick;R. Hewitt;M. Jorgenson;P. Sullivan
中科院分区：
文献类型：
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作者：
C. Maher;R. Dial;N. Pastick;R. Hewitt;M. Jorgenson;P. Sullivan
了解控制北部树状线的关键机制对于准确预测生物群的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，人们观察到海拔和纬度的树线出现在类似的平均生长季气温(GSAT)等温线上，这引发了生长限制假说(GLH)，即寒冷的GSAT限制了树线的地上生长，平均树线GSAT~6-7°C。平均GSAT温度高于6-7℃的树线可能表明其他限制因素。尽管全球变暖，但许多树线并没有向前推进，其他气候变量很少在大范围内被考虑。我们的目标是测试目前阿拉斯加北部的北部树线是否符合GLH等温线，确定哪些环境因素最能预测树线的存在，并确定当前树线之外最有可能前进的区域。我们数字化了大约12400公里的树线(26K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树线存在的广义加法模型，以确定决定树线的关键因素。阿拉斯加北部树线的两个平均GSAT指标始终高于6-7℃等温线(平均8.5℃和9.3℃)，这表明低GSAT的直接生理限制不太可能解释阿拉斯加北部树线的位置。我们的最终模型包括累积生长度日数和近地表(≤1m)永久冻土概率，这两个因素一起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树线的主要驱动因素，或者它的影响是通过其他更接近的、可能是非气候的控制来调节的。我们的模型预测了当前树线以外的几个地区的树线潜力，指出了如果不受非气候因素限制，树线可能前进的路线。
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 and near‐surface (≤1 m) permafrost probability, 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.