Unusual forest growth decline in boreal North America covaries with the retreat of Arctic sea ice

Abstract

The 20th century was a pivotal period at high northern latitudes as it marked the onset of rapid climatic warming brought on by major anthropogenic changes in global atmospheric composition. In parallel, Arctic sea ice extent has been decreasing over the period of available satellite data records. Here, we document how these changes influenced vegetation productivity in adjacent eastern boreal North America. To do this, we used normalized difference vegetation index (NDVI) data, model simulations of net primary productivity (NPP) and tree-ring width measurements covering the last 300 years. Climatic and proxy-climatic data sets were used to explore the relationships between vegetation productivity and Arctic sea ice concentration and extent, and temperatures. Results indicate that an unusually large number of black spruce (Picea mariana) trees entered into a period of growth decline during the late-20th century (62% of sampled trees; n = 724 cross sections of age >70 years). This finding is coherent with evidence encoded in NDVI and simulated NPP data. Analyses of climatic and vegetation productivity relationships indicate that the influence of recent climatic changes in the studied forests has been via the enhanced moisture stress (i.e. greater water demands) and autotrophic respiration amplified by the declining sea ice concentration in Hudson Bay and Hudson Strait. The recent decline strongly contrasts with other growth reduction events that occurred during the 19th century, which were associated with cooling and high sea ice severity. The recent decline of vegetation productivity is the first one to occur under circumstances related to excess heat in a 300-year period, and further culminates with an intensifying wildfire regime in the region. Our results concur with observations from other forest ecosystems about intensifying temperature-driven drought stress and tree mortality with ongoing climatic changes.

No growth stimulation of Canada’s boreal forest under half-century of combined warming and CO 2 fertilization

“…The high sensitivity to temperature observed during the late 20th century in eastern forests (Fig. 6A), parallel with rapid warming and growth decline, may reflect a temperature-dependent increase in respiration and depletion in carbon reserves, as was recently suggested by process-based modeling of productivity in black spruce stands (16). Such temperature responses are not static, and plants have the capacity to adjust to a warming environment via physiological acclimation (33).…”

“…Adding to this uncertainty is the discrepancy over recent changes in the productivity of boreal and other northern latitude forests. Some empirical evidence suggests increases in the forest productivity (12)(13)(14), whereas other studies suggest decreasing productivity over the last decades (7,8,(15)(16)(17). Furthermore, inversion and process-based ecosystem models indicate large carbon sinks (7,8), whereas field-based bottom-up approaches suggest smaller carbon sinks or small carbon sources (3, 18), or large sinks (19).…”

“…However, a recent comparison of commonly used NDVI datasets showed marked differences in their representation of mean seasonal vegetation productivity and decadal-long trends (21), which stresses the necessity for large-scale, ground-based observations. Ground-based observations of vegetation productivity may originate from permanent sample plots (13,17) or from tree-ring analyses (12,14,16,23), which differ in the temporal resolution (decadal vs. annual) and spatial scale (stand vs. tree). Both are prone to bias because of unbalanced spatial sampling.…”

Resistance of the boreal forest to high burn rates

“…As large fires are generally more severe than small ones (19), models predict than aspen abundance will increase in a future with more fires, thus potentially strengthening the negative feedback on burn rates (15,41,42). Boreal forest productivity may also increase or decrease directly with climate change (43)(44)(45), further suggesting the negative feedback on burn rates may not be stationary through time. Geographic differences may also exist across the boreal biome.…”

Climate Change and Decision-Making Under Uncertainty

“…Examples of these effects are shown for enhanced growth of Norway spruce (Picea abies) in forest productivity studies in Sweden and Finland [4]. However, negative effects have been observed on poor sites, decreasing growth [13] or increasing mortality of trees [14] especially under heat or drought stress [15], as a combined effect of temperature (increase) and precipitation (decrease).…”