Impacts of Climate Change on Fire Regimes and Carbon Stocks of the U.S. Pacific Northwest

The vegetation types and carbon groupings present in the U.S. Pacific Northwest (PNW) are closely tied to fire seasons that rely on fire suppression and climate change. To evaluate the effects of current climate change on PNW carbon and fire relationships, the team created a new fire suppression rule for the MC1 general vegetation model and ran simulations under three climate change scenarios (Rogers et al. 2011). Moderately moist forests may be vulnerable to future fires and emit large amounts of carbon while drier forests displayed carbon sequestration despite projections of increased fire frequencies under climate change. The simulations showed substantial increases in area burned and burn severity suggesting that fire suppression will become less effective leaving ecosystems vulnerable to larger fires in the future. –Loren Stutts

Rogers, B. M., R. P. Neilson, R. Drapek, J. M. Lenihan, J. R. Wells, D. Bachelet, and B. E. Law 2011. Impacts of limate change on fire regimes and carbon stocks of the U.S. Pacific Northwest, J. Geophys. Res., 116, G03037, oi:10.1029/2011JG001695.

Given the diverse vegetation of the PNW, Rogers and his team use a dynamic general vegetation model (DGVM) with specific suppression rules on regional domains to understand the future impact of fire in the PNW. To assess which processes may control the PNW’s carbon budget (or the balance between spring precipitation, CO2 fertilization, summer drought and intensity of fire season) and fire regimes, the team used the MAPSS-CENTURY 1 model (MC1), a DGVM that captures the feedbacks and interactions between major ecosystem processes. They ran the MC1 with their created fire suppression rule over the PNW on a fine scale grid a grid that represents geological variation on very fine scales, in this case using 30 arc-seconds resolution. They ran this MC1 over the PNW under historical climates and three projected future climates, and performed sensitivity analyses to emphasize potential changes.

For the historical vegetation types and climates, the team used three regions in western Oregon and Washington: Western Forests, Eastern Forests, and the Columbia Plateau. The Western Forests experience high rainfall with long intervals of no fire, the Eastern Forests are drier and burn more often, while the Columbia Plateau is the driest region. Historical climate data were obtained from these regions and modeled at a 30 arc- second resolution. Three general circulation models (GCMs) were used to obtain the three future climate projections: CSIRO Mk3, MIROC 3.2 medres, and Hadley CM3 (CSIRO, MIROC, and Hadley) chosen for their range of temperature changes.

The team then used the MC1 to simulate the most common vegetation types with full fire for the historical and future periods under the three climate projections. In terms of historical comparisons, MCI results were favorable compared to historical observations although there was some disagreement. In terms of the three climate projections and their precipitation, the CSIRO projection is cool and wet, MIROC is hot and wet, while Hadley is hot and dry. With MIROC and Hadley, the growing season was lengthened and exaggerated the already strong seasonal cycles which increased net primary productivity (NPP) during the rainy season and decreased summer NPP by amplifying summer drought. Under the milder conditions of CSIRO, NPP increases. Under all climate projections simulated fire increased across the domain. Under CSIRO and MIROC these increases surface late in the twenty first century while with Hadley’s conditions create large fires in the early to mid-twenty-first century. Due to larger and more intense fires in the Western and Eastern Forests and the woodiness of the Columbia Plateau, under all three scenarios burn severities increase across the domain through the twenty first century. The simulated twenty first century PNW carbon budget balances biomass losses from summer drought and fire and biomass gains from higher rainy season NPP. The domain gains carbon emissions under CSIRO and gains less under MIROC but loses emissions under Hadley.

Sensitivity analyses were run to assess the influence of fire and fire suppression on the carbon balance. MC1 was first run with fire suppression off (full fire) and with all fires off (no fire). Fire suppression creates less burn area and biomass consumed than full fire yet when compared to results for historical periods with the identical fire rules, fire suppression results in greater increases in burn area and biomass consumption than does full fire under all scenarios. Current fire suppression efforts may not be as effective against future fires. Simulated, observed, and historical fire suppression causes elevated fuel loads which suggests intensification of future PNW fire regimes yet because biomass consumption is less and carbon is gained after the initiation of simulated fire suppression, suppression creates smaller losses than full fire simulations (except for under Hadley).

Research suggests that fire regimes will be amplified during the twenty first century. The MC1 illustrates an increase in fire intensity and severity, worsened by the history of fire suppression.

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