Altered Fire Regimes and Forest Thinning in Major Biomes

Tom Kolb and Pete Fule

NAU School of Forestry

The main points of our presentation are:

• Evidence of frequent, low-intensity fires in Southwestern ponderosa pine forests prior to Euro-American settlement, disruption of this fire regime in the late 1800s, and large increases in tree density, are irrefutable.
• Increasing fuel loads and drought in these forest cause large, synchronous crown fires
• The frequency of these large synchronous crown fires will likely increase in the future because present fuel loads and future increases in fuels overwhelm current capacity for fuels reduction treatments
• Effects of these large synchronous crown fires include: increased opportunities for invasion by exotic, noxious weeds; massive soil erosion; flooding; damage to downslope riparian areas; deforestation for decades to centuries; degradation of aesthetics; degradation of recreation opportunities; and many others
• Heavy crown damage to ponderosa pine reduces tree resistance to bark beetle attack, and may promote landscape scale beetle outbreaks
• Thinning that reduces ponderosa pine tree density can increase tree resistance against bark beetles and reduce crown fire occurrence
• Heavy ponderosa pine mortality from bark beetles, an effect of the 2002 drought, will likely affect fire incidence and behavior, but data is scarce. A hypothesis is: As long as dead needles remain on the dead trees the hazard for an intense crown fire is high. When the needles fall to the ground the probability of an intense crown fire decreases, but increases for a low-intensity ground fire until they deteriorate. When the dead trees fall, the probability of crown fire is nil, and the residence time of ground fires should increase because of more slow-burning fuels.
• Beneficial effects of thinning on ponderosa pine water uptake and growth are most pronounced in drought years; thinning ameliorates effects of drought on tree stress
• Severe drought decreases herbaceous productivity and diversity, with the following possible effects: increased soil erosion, increased opportunities for exotic plant invasion, reduced energy transfers through food webs, lower NPP and carbon sequestration, altered food webs (simplification?), less animal forage
• Thinning causes small, ephemeral increases in downslope water runoff and drainage; effects diminish as vegetation recovers.
• Much less is known about effects of thinning and fuels treatments on pinyon-juniper woodlands than ponderosa pine forests. Current thinning experiments in P-J offer research opportunities
• Landscape scale thinning in the future will be limited by the high costs of treatment, low wood value, and lack of regional markets; inadequate resources for quick NEPA has been suggested as a constraint. Solutions include: 1) coordinated/consistent wood supply, 2) incentives to stimulate local markets/biomass energy, 4) “super NEPA” teams, 3) large state/federal subsides/public works programs