Bark beetles, drought and forest conditions in the Southwest

Outline

John Anhold and Joel McMillin, USFS-FHP, Flagstaff Arizona

• History and status of pine bark beetles in the Southwest

• Major issues related to bark beetle outbreaks

• Predicted extent and intensity of outbreaks

Pine Bark Beetles in Arizona

•Pine engraver beetles (Ips)

–11 species in Arizona

•Western pine beetle

•Roundheaded pine beetle

•Mountain pine beetle

•Southern and Mexican pine beetle (Chiricahua Mts.)

•Larger Mexican pine beetle

•Red turpentine beetle

Historical status of pine bark beetle outbreaks in SW

• Several localized outbreaks of various bark beetles in the Southwest since the 1800’s
• Typically insufficient records of historical outbreaks to compare with present events
• 1950’s drought and beetle-caused pine mortality

• 490,000 ac beetle activity in Arizona in 1957 (Yasinski & Pierce 1958)

• Million ac of pine mortality in the SW

• 95% of all age classes of pine at Bandelier

• Largest impacts on low elevation, dry sites (info from Allen and Breshears)

• History of mountain pine beetle outbreaks on the North Kaibab

• 7 outbreaks back to 1837. Largest killed 12% of ponderosa pine growing stock (1917 – 1926).

• Roundheaded pine beetle

• 6 outbreaks back to 1937 in the Sacramento Mtns. Largest killed 11 to 54% of ponderosa pine.

• Western pine beetle

• Periodic outbreaks with Ips throughout SW, i.e. Jemez Mtns, often associated with drought (Allen, 1989)

Current status of pine mortality in the Southwest

• More than 2 million ponderosa pine died in 2002 in AZ & NM (600,000+ac)
• Millions of piñon pine died across SW (undetermined acreage)
• Most initial attacks are by Ips species
• Other bark beetle species are common in lower portion of large ponderosa pine

What are the major issues?

• Is the drought or the bark beetles causing the tree mortality?
• How does drought affect pine resistance to bark beetles? How do we measure resistance?
• How important is tree density or stand density?
• What sites are experiencing the highest mortality and are there predictive models?
• What other factors regulate beetle populations?
• When will the beetle populations collapse?

Stand density & bark beetles

• Density of trees has increased more than 35 times in the last 100 – 150 years
• High stand density leads to inter-tree competition for resources & weakened defense mechanisms
• Density also affects micro-site conditions and pheromone interactions that may be more favorable to beetles
• Close association between stand conditions and Dendroctonus bark beetles, less clear for most Ips

Other factors affecting bark beetles

• Stress

• Fire

• Defoliation

• Disease – i.e., root disease, mistletoe

• Food

• Storm damage

• Logging residue

What is the beetle forecast?

• Uncertain, but high levels of additional tree mortality probable

Ø 8-fold increase in area affected each of last 2 years for Arizona

Ø more than 150-fold increase last year on some forests in Arizona

• Weather – unpredictable

Ø Precipitation – the big unknown

Ø Cold temperature – hasn’t happened yet