Increasing net ecosystem biomass production of Canada's boreal and temperate forests despite decline in dry climates

Abstract

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 (F ex ) 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 (F ex 0 ), which additionally accounted for potential negative feedback responses to anthropogenic growth enhancement. Simulations were performed over 1501-2012 to estimate F ex 0 . Overall, F ex 0 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.

Low Tree-Growth Elasticity of Forest Biomass Indicated by an Individual-Based Model

“…With respect to the analysis, the results only focused on how changes in tree growth resonate through an otherwise stationary system; everything else, including recruitment, background mortality, and disturbance, were held constant. While this allowed us to isolate the effect of tree growth, it ignored environmental changes that are simultaneously influencing biomass-dense forests through changes in the rates of tree mortality [8,54,74,75]. As the simulations consisted of single-species stands, potential effects of tree growth stimulation on rates of succession were also not considered.…”

Taxonomy, together with ontogeny and growing conditions, drives needleleaf species' sensitivity to climate in boreal North America

“…The annual growth performance of a tree is linked to its ability to access optimal amounts of water, light, and nutrients (Fritts, 1971), the availability of which is primarily controlled by site-specific abiotic factors, such as soil conditions (Hember et al, 2017).…”

Tree Ring Reconstructions of Stemwood Biomass Indicate Increases in the Growth Rate of Black Spruce Trees Across Boreal Forests of Canada

“…However, it is also supported by the average response of trees to increased nitrogen supply (LeBauer & Treseder, ; Xia & Wan, ), enriched carbon dioxide concentration (Idso & Idso, ; Kirschbaum & Lambie, ; Norby et al, ; Saxe et al, ; Wullschleger et al, ), and warming (Way & Oren, ). Studies of stand‐level biomass dynamics in humid boreal and temperate forests also consistently suggest that natural dynamics, such as change in stand age, cannot fully explain positive time trends in aboveground biomass production, implying a significant positive effect of environmental changes on tree growth (Erb et al, ; Fang et al, ; Hember et al, ; Hember et al, ; Kauppi et al, ; Pan et al, ; Pretzsch et al, ).…”