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，当地地形、植被组成和结构、有机层厚度、雪深以及土壤温度和水分)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提供受约束的北方森林生产力估计(COP)，我们将通过在基于植物功能特征的概率建模方法中整合叶、整棵树、景观特征和生态系统水平的测量来解决这一重要的知识差距，以增强地球系统模型中的北方森林生态现实主义。这项提案要求提供资金，以便在加拿大西北部北部沿2000公里纬度永久冻土梯度的五个北方森林地点完成独特的仪器设置(连续的全年涡旋协方差，并辅之以树液通量密度和树干直径变化的测量)：自动圆周树形计、自制的热耗散树干通量密度探测器和微型气象站。这些测量被用来了解北方树种水分使用和储存的物种间和物种内的变异性，并帮助限制生态系统规模的总初级生产力，这些测量是从沿坡度的四个北方林地进行的涡旋协方差测量获得的。COPE是美国国家航空航天局北极-北方脆弱性实验的重要贡献。此外，COPE对于制定森林管理适应战略至关重要，根据加拿大审计长最近的一份报告(2017年10月18日)，西北地区迫切需要这一能力。

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., local topography, vegetation composition and structure, organic layer thickness, snow depth, and soil temperature and moisture). 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, we will address this important knowledge gap by integrating leaf-, whole tree-, landscape feature-, and ecosystem-level measurements in a probabilistic plant functional trait-based modeling approach to enhance the boreal forest ecological realism in Earth System Models. This proposals asks for funds to complete a unique instrumental set-up (continuous year-round eddy covariance complemented by sap flux density and stem diameter variation measurements) at five boreal forest sites along a 2000-km latitudinal permafrost gradient across the boreal forest of northwestern Canada: automatic circumference dendrometers, self-manufactured thermal dissipation sap flux density probes and micrometeorological stations. These measurements are used to understand inter- and intra-species variability in boreal tree species water use and storage, and to help constraining ecosystem-scale gross primary productivity obtained from eddy covariance measurements made at four boreal forest sites along the gradient. COPE constitutes an important contribution to NASA’s Arctic-Boreal Vulnerability Experiment. In addition, 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).