Fire-regime variability impacts forest carbon dynamics for centuries to millennia

Abstract

Abstract. Wildfire is a dominant disturbance agent in forest ecosystems, shaping important biogeochemical processes including net carbon (C) balance. Long-term monitoring and chronosequence studies highlight a resilience of biogeochemical properties to large, stand-replacing, high-severity fire events. In contrast, the consequences of repeated fires or temporal variability in a fire regime (e.g., the characteristic timing or severity of fire) are largely unknown, yet theory suggests that such variability could strongly influence forest C trajectories (i.e., future states or directions) for millennia. Here we combine a 4500-year paleoecological record of fire activity with ecosystem modeling to investigate how fire-regime variability impacts soil C and net ecosystem carbon balance. We found that C trajectories in a paleo-informed scenario differed significantly from an equilibrium scenario (with a constant fire return interval), largely due to variability in the timing and severity of past fires. Paleo-informed scenarios contained multi-century periods of positive and negative net ecosystem C balance, with magnitudes significantly larger than observed under the equilibrium scenario. Further, this variability created legacies in soil C trajectories that lasted for millennia. Our results imply that fire-regime variability is a major driver of C trajectories in stand-replacing fire regimes. Predicting carbon balance in these systems, therefore, will depend strongly on the ability of ecosystem models to represent a realistic range of fire-regime variability over the past several centuries to millennia.

Fixing a snag in carbon emissions estimates from wildfires

“…Our observation-based refinements utilize carbon stock datasets that span fire events, including new, detailed field observations from the 2013 Rim Fire in California (Lutz et al, 2017). We also simulate post-fire carbon cycle dynamics using an improved version of the globally recognized biogeochemical model DayCent (Hudiburg, Higuera, & Hicke, 2017;Parton, Hartman, Ojima, & Schimel, 1998) through addition of snag pools with varying combustion, decomposition, and fall rates ( Figure S1). We then estimate 2000-2016 fire emissions across the western United States with our improved methods.…”

The pyrogeography of eastern boreal Canada from 1901 to 2012 simulated with the LPJ-LMfire model

“…This initial condition assumes that past relationships between climate, fire, and vegetation have been stationary through time and that variability in modern climate is representative of all variability that has been recorded over the past 1200 years (time of spin-up phase + 112 years of simulation). However, it has been increasingly recognized that such an assumption is invalid and that modern observations are not a good analogue for prehistoric variability (Kelly et al, 2016;Hudiburg et al, 2017). For example, fire activity over much of the Holocene was higher in terms of frequency and fire size than the current levels across broad areas of eastern Canada (Girardin et al, 2013a;Remy et al, 2017).…”

“…For example, fire activity over much of the Holocene was higher in terms of frequency and fire size than the current levels across broad areas of eastern Canada (Girardin et al, 2013a;Remy et al, 2017). It is likely that not accounting for such variability may introduce biases in forest productivity dynamics and levels, more specifically on soil carbon dynamics (Hudiburg et al, 2017). This may be less problematic when studying fire and forest dynamics over the last century because the mean age of the major part of eastern boreal forest is less than 100 years (Bergeron et al, 2002).…”

Near‐future forest vulnerability to drought and fire varies across the western United States

“…Mortality events and vegetation state shifts can have long-term effects on carbon cycling and sequestration potential (Hudiburg, Higuera, & Hicke, 2017;McLauchlan et al, 2014) and important feedbacks to climate (Anderegg, Kane, et al, 2012;Bonan, 2008).…”