Canadian Forest Service Publications
Increasing net ecosystem biomass production of Canada's boreal and temperate forests despite decline in dry climates. 2017. Hember, R.A., Kurz, W.A., Coops, N.C. Global Biogeochem. Cycles, 31, 134–158.
Issued by: Pacific Forestry Centre
Catalog ID: 38903
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Repeated measurements of tree biomass at field plots describe recovery from disturbances, sampling artifacts, and potential effects of environmental change on forest ecosystems. Challenges in differentiating between intrinsic and extrinsic sources of variation, both in theory and in practice, continue to confound claims of an anthropogenic carbon sink in forest biomass. Here we analyzed observations at 10,307 plots across southern ecozones of Canada to investigate temporal trends in stand-level biomass growth (G), biomass loss due to mortality (M), and net ecosystem biomass production (NEBP) of intact stands. Net extrinsic forcing (Fex) was expressed by the collective dependence of biomass fluxes on climate anomalies, nitrogen deposition (N), and atmospheric carbon dioxide concentration (C). Inferences drawn directly from linear mixed-effects model coefficients only reflect the static behavior of the model specifically at field plot locations. We, therefore, defined a dynamic landscape-scale net extrinsic forcing (Fex′), which additionally accounted for potential negative feedback responses to anthropogenic growth enhancement. Simulations were performed over 1501–2012 to estimate Fex′. Overall, Fex′ was positive, suggesting that environmental changes drove a 90% increase in NEBP. The increase in NEBP was confined to wet regions, while the biomass sink in dry regions decreased, suggesting that large expanses of northern forests, historically located near the boundary between wet and dry climates, may be at high risk of decline under continued increases in evaporative demand. These results have important implications for the greenhouse gas balance of Canada's forest sector.
Plain Language Summary
Over 10,000 field plots were used to estimate trends in stand-level net ecosystem biomass production (NEBP) across southern ecozones of Canada. Statistical models were developed to predict NEBP based on stand age, stand competition, climate and atmospheric composition. The models were applied over 1951-2012 to assess whether positive sensitivity to environmental change would be counteracted by increasing intensity of drought and competition for resources. NEBP increased in wet climates (i.e., the Pacific Maritime, Boreal Shield, and Atlantic Maritime ecozones) in response to changes in climate and atmospheric composition, while NEBP declined in dry climates (i.e., the Montane Cordillera and Boreal Plain ecozones) due to preclusion of growth enhancement and increased drought-related mortality. Overall, NEBP increased by 90 percent, because the area of wet climates make up a greater proportion of southern Canadian forests. The evidence suggests that yield curve-based models are likely under-predicting NEBP in wet climates and over-predicting NEBP in dry climates during the last two decades.