Towards a constrained estimate of boreal forest productivity across northwestern North America

北方森林是气候系统的一个组成部分，影响着陆地-大气相互作用和大尺度环流模式。世界上约80%的北方森林含有永久冻土，即常年冰冻的土地。在北美西北部，最北端的北方森林生长在相对寒冷的永久冻土上，从连续的永久冻土过渡到不连续的永久冻土。沿着永久冻土的南部边界，北方森林出现在与当前气候不平衡的相对温暖和稀薄的永久冻土地区。在那里，永久冻土的存在是由非生物和生物生态系统特性(例如地形、植被、雪)驱动的。北半球的永久冻土地区变暖的速度是全球的两倍，但我们对变暖可能如何影响北方森林生产力，以及北方森林如何作为气候系统的一部分发挥作用的了解仍然有限。通过为NNA提供受限的北方森林生产力估计(COPE)，通过整合树叶、整棵树、景观特征和生态系统水平的测量来解决这一知识差距，以增强地球系统模型(ESM)中的北方森林生态现实性。COPE(2018-2022年)的四个增量短期目标通过五个项目加以解决。中心部分是正在进行的生态系统级别净碳(C)、水和能量通量、整个树木级别树液通量密度和树干直径变化的涡旋协方差(EC)测量，这些测量在加拿大西北部沿2000公里纬度永久冻土梯度的四个北方林地进行。首先，结合稳定的碳同位素比率分析和封闭动态小室测量，后者将被用来了解北方乔木物种水分利用和储存的物种间和物种内的变异性，并帮助限制目前由EC衍生的北方森林生产力。这些努力将得到重复调查的补充，这些调查描述了植物功能特征和叶水平气体交换的可变性，以及沿坡度的永久冻土条件。最后，将一种基于概率植物功能特征的建模方法应用于社区土地模型(CLM)中。使用针对COPE中收集的观测数据进行评估的CLM改进版本，将检验受不同永久冻土条件控制的NNA未来北方森林生产力将显著不同的假设。COPE是对FLUXNET等全球倡议以及美国宇航局北极-北冰洋脆弱性实验的重要高纬度贡献。根据加拿大审计长最近的一份报告(2017年10月18日)，COPE在制定森林管理适应战略方面至关重要，这是西北地区迫切需要的能力。

Boreal forests constitute an integral component of the climate system, affecting land surface-atmosphere interactions and large-scale circulation patterns. About 80% of the world's boreal forests contain permafrost, perennially cryotic ground. In northwestern North America (NNA), the northernmost boreal forests grow on relatively cold permafrost at the transition from continuous to discontinuous permafrost. Along the southern limit of permafrost, boreal forests occur in areas with relatively warm and thin permafrost in disequilibrium with the current climate. There, permafrost presence is driven by abiotic and biotic ecosystem properties (e.g., topography, vegetation, snow). The permafrost region of the Northern Hemisphere has been warming at twice the global rate yet our understanding of how warming may affect boreal forest productivity, and thus how boreal forests function as part of the climate system, remains limited. By providing a COnstrained boreal forest Productivity Estimate (COPE) for NNA, this knowledge gap is addressed by integrating leaf-, whole tree-, landscape feature-, and ecosystem-level measurements to enhance boreal forest ecological realism in Earth System Models (ESMs). The four incremental short-term objectives of COPE (2018-2022) are addressed through five projects. The central component is ongoing eddy covariance (EC) measurements of ecosystem-level net carbon (C), water and energy fluxes, whole tree-level sap flux density and stem diameter variation measurements at four boreal forest sites along a 2000-km latitudinal permafrost gradient across northwestern Canada. First, in combination with stable C isotope ratio analysis and closed-dynamic chamber measurements, the latter be will used to understand inter- and intra-species variability in boreal tree species water use and storage, and to help constraining present EC-derived boreal forest productivity. These efforts will be complemented by repeat surveys that characterize plant functional trait and leaf-level gas exchange variability, and permafrost conditions along the gradient. Last, a probabilistic plant functional trait-based modeling approach will be adopted and implemented in the Community Land Model (CLM). Using the refined version of CLM evaluated against the observations collected as part of COPE, the hypothesis that there will be pronounced different in future boreal forest productivity across NNA as controlled by different permafrost conditions will be tested. COPE constitutes an important high-latitude contribution to global initiatives such as FLUXNET, and to NASA's Arctic-Boreal Vulnerability Experiment. COPE is of fundamental importance in the development of forest management adaptation strategies, an urgently needed capacity that is lacking in the Northwest Territories according to a recent report by the Auditor General of Canada (October 18th, 2017).