Fire Regimes in the Pacific Northwest: Top-Down vs. Bottom-Up Influences, Study notes of Forestry

Download Fire Regimes in the Pacific Northwest: Top-Down vs. Bottom-Up Influences and more Study notes Forestry in PDF only on Docsity! 1 Fire Regimes of the Pacific Northwest 11-17-08 Modified from Historic Jim Agee lectures Outline • Examples of PNW Fire Regimes: – High (TSHE) – Mixed (moderate)—central OR Cascades-Northern CA – Low (non-lethal)—ponderosa pine • Does climate control fire (Top down)? Or does physiography and fuels control fire (Bottom up) • Which regimes are top-down and which bottom up? Top-down or Bottom-up influence on fire and fire regimes? “Whether the weather is cold Or whether the weather is hot We must weather the weather Whatever the weather Whether we like it or not” (unknown limerick—Agee 1997. Northwest Science. 71:153- 156. • Top-down: coarse-scale influences, usually climate • Bottom-up: fine-scale influences, usually fuels and/or topography Published by AAAS A. L. Westerling et al., Science 313, 940 -943 (2006) Fig. 1. (A) Annual frequency of large (>400 ha) western U.S. forest wildfires (bars) and mean March through August temperature for the western United States (line) (26, 30) Published by AAAS A. L. Westerling et al., Science 313, 940 -943 (2006) Fig. 2. (A) Pearson's rank correlation between annual western U.S. large (>400 ha) forest wildfire frequency and streamflow center timing Fire Regime Influences • Fire regime: description of role that fire plays in an ecosystem (Agee. 1993. Fire Ecology of the PNW) • Fire Regime Characters – Frequency (how often) – Intensity (how much energy is released – Seasonality (what time of year – Extent (how large – Severity (ecological) • In the same fire regime: weather, topography and fuels have an influence 2 Fire Regimes Vary by Environment Wet Dry Warm Cold Agee, J.K. 1998. Northwest Science. 72:24-34 Fire Regimes • Fire Characters: frequency, intensity, extent, season, synergism • Three major fire regimes Historical Fire Regimes of the Pacific Northwest • Severity is the effect of Fire on system – Function of plant adaptations + fire characters – High (Lethal) Severity • Infrequent (100+ yrs) and stand-replacing – Mixed (Moderate) Severity • Less frequent (25-75 yrs) and a mix of severities – Low (Non-lethal) Severity • Frequent (5-15 yrs) but low intensity Agee, J.K. 1998. Northwest Science www.fs.fed.us/fire/fuelman/firereg.htm Three Forest Examples 5 Fire Regime Shifts to more moderate South • Mesic forest – ABLA – ABGR • Dry forests – ABGR – PSME – PIPO Heyerdahl et al. 2001. Ecology 82:660-678. Heyerdahl et al. 2001. Ecology 82:660-678. Fire suppression Fires burned 2x more frequently south--drier Topographic influence important at local scale -steep slopes -barriers to spread Moderate Severity Mosaic • High -Black • Moderate – vertical stripe • Low – dotted – Over a century near H.J. Andrews forest Morrison an Swanson. 1990 PNW GTR-254 Fire Regime Shifts to South • Central Oregon Cascades into Northern California • Mixed Severity Fire Regime • Note patch size smaller and severity is variable across the landscape Warner Creek Fire Douglas-fir forest Agee Warner Creek Fire Low Moderate High Agee Fuel Limits on Fire Spread Mixed severity fire regime – red fir, not Douglas-fir Fires butt into Each other and Go out 6 Mixed severity regimes: Top-down or Bottom-up? • Regional gradient in drought top-down • Local influences topographically influenced, so bottom-up • Fuel differences a bottom-up influence but subordinate to topography and weather Ponderosa Pine - Low Severity Historical Conditions • Small Even-aged Groups • 50-150 trees/hectare • Oldest groups (see arrow) killed by bark beetles Western Pine Beetle • Small Group Tree Killer • Attacks Trees of Low Resistance • In historical forests, these were the oldest groups Examples of Historic Pine Forest Tall Crowns Thick Bark Wide Spacing Small trees are a result of fire exclusion What Maintained Open Structure? • Frequent, Low Intensity Forest Fires • Leave History with Fire Scars • Up to 30 Scars on One Tree Thick bark protects most of cambium 7 Convergence of Stand Structure in PICO stands in Yellowstone • 1988 fire burned 1.5 million ac • Is the uniform structure on the landscape the result of catastrophic wildfire • Stratified sample (50-100, 125-175, 200- 250 and 300-350 yr) • Strip transects mapped • Tree cores and stand reconstruction Reconstructions show decrease in density • Large catastrophic fire initiates stands of various densities • Convergence to high density, high fuel level systems