Spatial distribution of carbon sources and sinks in Canada's forests

Abstract

Annual spatial distributions of carbon sources and sinks in Canada's forests at 1 km resolution are computed for the period from 1901 to 1998 using ecosystem models that integrate remote sensing images, gridded climate, soils and forest inventory data. GIS‐based fire scar maps for most regions of Canada are used to develop a remote sensing algorithm for mapping and dating forest burned areas in the 25 yr prior to 1998. These mapped and dated burned areas are used in combination with inventory data to produce a complete image of forest stand age in 1998. Empirical NPP age relationships were used to simulate the annual variations of forest growth and carbon balance in 1 km pixels, each treated as a homogeneous forest stand. Annual CO2 flux data from four sites were used for model validation. Averaged over the period 1990–1998, the carbon source and sink map for Canada's forests show the following features: (i) large spatial variations corresponding to the patchiness of recent fire scars and productive forests and (ii) a general south‐to‐north gradient of decreasing carbon sink strength and increasing source strength. This gradient results mostly from differential effects of temperature increase on growing season length, nutrient mineralization and heterotrophic respiration at different latitudes as well as from uneven nitrogen deposition. The results from the present study are compared with those of two previous studies. The comparison suggests that the overall positive effects of non‐disturbance factors (climate, CO2 and nitrogen) outweighed the effects of increased disturbances in the last two decades, making Canada's forests a carbon sink in the 1980s and 1990s. Comparisons of the modeled results with tower‐based eddy covariance measurements of net ecosystem exchange at four forest stands indicate that the sink values from the present study may be underestimated.

High carbon dioxide uptake by subtropical forest ecosystems in the East Asian monsoon region

“…Maximum NEP is observed in forests of less than 50 y, a phenomenon that is most likely a result of the high level of photosynthesis by young forests to produce biomass and structure that are consistently observed in tropical to boreal forests (18)(19)(20). In young forests, net primary productivity (NPP) exceeds heterotrophic respiration (R h ), resulting in high NEP, whereas in older stands, NPP may decline while R h continues to increase because of the accumulation of detritus and soil organic matter from earlier production (21). Fig.…”

Differentiating moss from higher plants is critical in studying the carbon cycle of the boreal biome

“…It is not likely that all overestimated NPP using the conventional approaches would be respired via heterotrophic respiration or completely retained by the ecosystems; the correct answer probably falls in the middle. If these considerations were incorporated in previous carbon sink estimates 25,28 , Canadian forests could be a source rather than a sink. In summary, the impacts of failing to differentiate the contributions of mosses and higher plants to the carbon cycle are substantial.…”

“…For example, a study based on a nested inverse modelling system indicates Canada's sink to be large (0.34 ± 0.14 Pg C per year) 28 . However, ecosystem modelling and inventory-based analysis found Canada's forest to be a weak sink of 0.05-0.07 Pg C per year 11,25 . Regional carbon balance strongly depends on the accuracy of estimated GPP.…”

“…Using the improved EC-LUE-2p model (see Methods), the relationship between the contribution of moss to the NDVI signal and forest age derived from 26 flux tower locations (Fig. 3), and the spatial distribution map of forest age 25 , we estimated GPP to be 2.44 Pg C per year for the Canadian boreal biome ( Fig. 4c; see Methods).…”

“…3) was used to calculate the spatial pattern of K_NDVI, based on the spatial distribution map of forest age in Canada 25,55 . We validated this large-scale model application at the site level.…”