Alaskan carbon-climate feedbacks will be weaker than inferred from short-term manipulations: Alaskan Benchmark Data and Model runs

This submission aimed to assess differences in short-term step warming manipulations and long-term chronic response to climate change in Alaskan ecosystems. Briefly, climate warming is occurring fastest at high latitudes. Based on short-term field experiments, this warming is projected to stimulate soil organic matter decomposition, and promote a positive feedback to climate change. We show here that the tightly coupled, nonlinear nature of high-latitude ecosystems implies that short-term (< 10 year) warming experiments produce emergent ecosystem carbon stock temperature sensitivities inconsistent with emergent multi-decadal responses. We first demonstrate that a well-tested mechanistic ecosystem model accurately represents observed carbon cycle and active layer depth responses to short-term summer warming in four diverse Alaskan sites. We then show that short-term warming manipulations do not capture the non-linear, long-term dynamics of vegetation, and thereby soil organic matter, that occur in response to thermal, hydrological, and nutrient transformations belowground. Our results demonstrate significant spatial heterogeneity in multi-decadal Arctic carbon cycle trajectories and argue for more mechanistic models to improve predictive capabilities. The model used in the current study is available publicly (https://github.com/jinyun1tang/ECOSYS), and the current submission contains the python/ matlab codes for analyzing output from the model (includng a readme file to explain the codes). The benchmark data, also enclosed, was collected from a range of published and publicly available sources (extracted using GRABIT: https://www.mathworks.com/matlabcentral/fileexchange/7173-grabit). These sources describe warming induced changes in tundra/ boreal ecosystems.

本次投稿的研究旨在评估阿拉斯加生态系统内短期阶梯式增温操控手段与长期气候变化慢性响应之间的差异。简言之，高纬度地区的气候变暖速率位居全球之首。基于短期野外实验，当前学界预测此类增温将促进土壤有机质（soil organic matter）分解，并形成对气候变化的正反馈（positive feedback）效应。本研究显示，高纬度生态系统具有紧密耦合的非线性本质，这意味着短期（<10年）增温实验所得到的生态系统碳库温度敏感性的涌现特征，与多十年尺度的实际响应并不匹配。我们首先证实，一款经过充分验证的机理生态模型可以准确表征阿拉斯加四个不同样地中，观测得到的碳循环与活动层深度（active layer depth）对夏季短期增温的响应。随后我们发现，短期增温操控实验无法捕捉植被以及由此关联的土壤有机质所呈现的非线性长期动态——这类动态由地下的热、水文与养分转化过程共同驱动。本研究结果表明，北极碳循环的多十年尺度轨迹存在显著空间异质性，因此亟需构建更多机理模型以优化预测能力。本研究使用的模型已公开获取（https://github.com/jinyun1tang/ECOSYS），本次提交的内容包含用于分析模型输出结果的Python/Matlab代码（附带README文件对代码进行说明）。本次附带的基准数据集源自一系列已发表及公开可用的数据源（使用GRABIT工具提取：https://www.mathworks.com/matlabcentral/fileexchange/7173-grabit），这些数据源描述了冻原（tundra）/北方（boreal）生态系统中增温诱导的变化。