Restoring forest structure and process stabilizes forest carbon in wildfire‐prone southwestern ponderosa pine forests

Abstract

Changing climate and a legacy of fire-exclusion have increased the probability of high-severity wildfire, leading to an increased risk of forest carbon loss in ponderosa pine forests in the southwestern USA. Efforts to reduce high-severity fire risk through forest thinning and prescribed burning require both the removal and emission of carbon from these forests, and any potential carbon benefits from treatment may depend on the occurrence of wildfire. We sought to determine how forest treatments alter the effects of stochastic wildfire events on the forest carbon balance. We modeled three treatments (control, thin-only, and thin and burn) with and without the occurrence of wildfire. We evaluated how two different probabilities of wildfire occurrence, 1% and 2% per year, might alter the carbon balance of treatments. In the absence of wildfire, we found that thinning and burning treatments initially reduced total ecosystem carbon (TEC) and increased net ecosystem carbon balance (NECB). In the presence of wildfire, the thin and burn treatment TEC surpassed that of the control in year 40 at 2%/yr wildfire probability, and in year 51 at 1%/yr wildfire probability. NECB in the presence of wildfire showed a similar response to the no-wildfire scenarios: both thin-only and thin and burn treatments increased the C sink. Treatments increased TEC by reducing both mean wildfire severity and its variability. While the carbon balance of treatments may differ in more productive forest types, the carbon balance benefits from restoring forest structure and fire in southwestern ponderosa pine forests are clear.

Quantifying the Carbon Balance of Forest Restoration and Wildfire under Projected Climate in the Fire-Prone Southwestern US

“…The model requires the landscape be subdivided into abiotically similar ecoregions and that an initial forest communities layer be developed that includes the spatial distribution of age-cohorts of species. Following Hurteau et al [8] I used the same 150m grid, six ecoregions, and initial communities layer (comprised of ponderosa pine and Gambel oak) that were developed based on soil properties, topographic variables, forest inventory data, and age-size distributions from Fulé et al [19] and Mast et al [25] for all simulations.…”

“…The extension simulates above and belowground C and nitrogen pools and fluxes as influenced by species-specific parameters, climate, soils, and their interaction [21–22]. The Century succession extension was parameterized by Hurteau et al [8] using the SSURGO database (NRCS 2013) and soil samples from the installation. Model spin-up is conducted for the length of time equivalent to the age of the oldest tree cohort and is used to initialize biomass and soil organic matter pools.…”

“…Soil organic matter decay rates were calibrated following Loudermilk et al [29] and Martin et al [30] such that after model spin-up, soil C values fell within the field-sampled range. Species-specific parameter values for the Century succession extension were obtained from the CENTURY user guide, published literature, and US Government databases [9, 27, 31–35] and the distribution of grid cell C values across the landscape compared well with the distribution of empirical values derived from inventory data (see [8]). The only change in Century succession parameterization from Hurteau et al [8] was a decrease in the probability of establishment from 1 to 0.5 to improve representation of the episodic nature of ponderosa pine regeneration [36].…”

“…However, the remaining C can be held in a more stable form that is resistant to loss from wildfire because forest restoration treatments are effective at modifying fire behavior and reducing overstory mortality [6–7]. Previous research in southwestern ponderosa pine ( Pinus ponderosa ) forest has demonstrated that a restored condition that is maintained by regular surface fire can store more C than the fire-suppressed condition when the effects of stochastic wildfire are incorporated [8]. However, this result is predicated on reduced tree mortality during wildfire and continued tree growth following wildfire in the restored forest condition.…”

Developing and Implementing Climate Change Adaptation Options in Forest Ecosystems: A Case Study in Southwestern Oregon, USA

“…Carbon emissions from mechanical treatments and prescribed fire are significant, but these treatments can reduce subsequent fire severity, potentially reducing impacts on soil and wildlife habitat, and reducing carbon emissions in subsequent wildfires [87][88][89][90]. There are uncertainties associated with the long-term effects of thinning and fuel treatment regimes on carbon storage in a changing climate, and further research is needed to evaluate potential short-term and long-term costs and benefits.…”