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

点击翻译按钮获取中文摘要

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).