Increasing potential NEP of eastern boreal North American forests constrained by decreasing wildfire activity

Abstract

Abstract. Wildfire activity has been decreasing over the past few centuries across North America's southeastern boreal forest. This change has caused a gradual shift in the age-class distribution toward a stronger representation of old-growth stands (age . 140 years) in unmanaged forest landscapes. Parallel to these changes, there has been an improvement in growth conditions driven by climate warming. Given the negative feedback of forest aging on net ecosystem production (NEP), a reasonable case can be made about an age-effect constraint on a climate change-induced increase in carbon assimilation. Here we combine empirical information (multicentury tree-ring records, time-since-fire map, and forest inventories) along with model simulations to develop a reconstruction of NEP of jack pine forests (Pinus banksiana Lambert) at the transition zone of the Spruce-feather moss and Balsam fir-white birch bioclimatic domains of eastern boreal North America covering the period A.D. 1830 to 1999. From this reconstruction, we test the hypothesis that currently improving carbon assimilation in unmanaged forests due to climate change is being offset by the aging of the forests resulting from the decreasing activity of wildfires. A sensitivity analysis of results to different assumptions about the carbon sink strength of old-growth forests is also made via a probability distribution derived from 10,000 ensemble members. Our simulation results suggest that increasing moisture availability and lengthening of the growing season contributed to a statistically significant upward trend in carbon assimilation of 0.39 g C m À2 yr À1 over 1901-2009. Meanwhile, mean age of jack pine stands has also changed, shifting from 87 years in the 1920s to 131 years in 1999. This forest aging contributed up to a 12% reduction in carbon assimilation from the 1930s to 1990s, an effect that is found to be sufficient to offset NEP increases due to climate change in 75% of our ensemble members. Great caution should therefore be applied when interpreting future carbon assimilation or forest growth projections that do not account for such second-order feedback from disturbances.

Multi‐model projections of tree species performance in Quebec, Canada under future climate change

“…Using site and species parameters (forest composition, aboveground biomass, tree density, mineral soil texture, aspect and slope), as well TA B L E 1 Characteristics of all models used in this study eastern Canada (Girardin et al, 2008(Girardin et al, , 2011(Girardin et al, , 2012(Girardin et al, , 2014(Girardin et al, , 2016.…”

Changes in growth of pristine boreal North American forests from 1950 to 2005 driven by landscape demographics and species traits

“…burning and disturbance regimes initiate new forest structures that constrain accelerating rates of potential carbon assimilation (Metsaranta et al, 2010;Girardin et al, 2011a).…”

“…In spite of the many factors that are occurring and known for positively affecting the growth of boreal forests, some forests across North America have felt the adverse impacts of environmental changes. Declines in forest production during the past three decades were notably reported in spruce forests in interior Alaska (Hicke et al, 2002;Beck et al, 2011) and much of the eastern boreal forests (Hicke et al, 2002;Girardin et al, 2011a). This paradigm between improving environmental conditions and declining production may occur as suitable climate envelopes of well-established species rapidly shift away and give rise to the onset of biome shifts (Michaelian et al, 2011;Beck et al, 2011), and as changes in biomass…”

Disturbance legacies and paludification mediate the ecological impact of an intensifying wildfire regime in theClayBelt boreal forest of easternNorthAmerica

“…Therefore, in the prolonged absence of fire, forests tend to converge to open or partially less productive spruce-Sphagnum forests regardless of the initial species composition (Harper et al 2003;Lecomte et al 2006a;Fenton et al 2007;Simard et al 2007;Lafleur et al 2010;Belleau et al 2011). A decrease in the fire activity in southeastern North American boreal forests during the past three millennia in association with an orbitally-driven climatic cooling Girardin et al 2013a,b) likely contributed to an acceleration of peat accumulation in the Clay Belt forest and a decrease in its productivity (Simard et al 2007;Girardin et al 2011).…”