Carbon in Canada’s boreal forest — A synthesis1

摘要

Canada’s managed boreal forest, 54% of the nation’s total boreal forest area, stores 28 Pg carbon (C) in biomass, dead organicmatter, and soil pools. The net C balance is dominated by the difference of two large continuous fluxes: C uptake (net primaryproduction) and release during decomposition (heterotrophic respiration). Additional releases of C can be high in years, or in areas,that experience large anthropogenic or natural disturbances. From 1990 to 2008, Canada’s managed boreal forest has acted as C sinkof 28 Tg C year~(-1), removing CO_2 from the atmosphere to replace the 17 Tg of C annually harvested and store an additional 11 Tg ofC year~(-1) in ecosystem C pools. A large fraction (57%) of the C harvested since 1990 remains stored in wood products and solid wastedisposal sites in Canada and abroad, replacing C emitted from the decay or burning of wood harvested prior to 1990 and contributingto net increases in product and landfill C pools. Wood product use has reduced emissions in other sectors by substituting foremission-intensive products (concrete, steel). The C balance of the unmanaged boreal forest is currently unknown. The futureC balance of the Canadian boreal forest will affect the global atmospheric C budget and influence the mitigation efforts required toattain atmosphericCO_2 stabilization targets. The single biggest threat toCstocks is human-caused climate change. LargeCstocks haveaccumulated in the boreal because decomposition is limited by cold temperatures and often anoxic environments. Increases intemperatures and disturbance rates could result in a large net C source during the remainder of this century and beyond. Uncertaintiesabout the impacts of global change remain high, but we emphasize the asymmetry of risk: sustained large-scale increases inproductivity are unlikely to be of sufficient magnitude to offset higher emissions from increased disturbances and heterotrophicrespiration. Reducing the uncertainties of the current and future C balance of Canada’s 270 Mha of boreal forest requires addressinggaps in monitoring, observation, and quantification of forest C dynamics, with particular attention to 125 Mha of unmanaged borealforest with extensive areas of deep organic soils, peatlands, and permafrost containing large quantities of C that are vulnerable toglobal warming.