Interactions between fire and bark beetles in an old growth pine forest

Management strategies for old growth pine forests have recently begun to include prescribed burns. Fire could influence interactions between bark beetles and mature pine trees, but we cannot predict the effects because we know too little about the numeric and functional responses of bark beetle populations to fire, and because we do not know how fire affects the oleoresin defense system of pine trees. We estimated population abundance of Ips spp. (Coleoptera: Scolytidae), and the resin flow of mature red pines (Pinus resinosa), before and after a prescribed burn, inside and outside the burn, in an old growth forest at Itasca State Park, Minnesota. Following a prescribed burn in April, the local abundance of I. pini increased by two-fold during May, decreased by a comparable amount during 6 weeks starting in mid-July, and was otherwise unchanged. The abundance of I. grandicollis and I. perroti were unaffected, while that of a specialist predator, Thanasimus dubius (Coleoptera: Cleridae) increased by 30-90% during May. Many mature trees that sustained no visible crown damage from the fire were attacked by Ips within the scorched region of the lower bole. Oleoresin flow increased substantially in trees with scorched boles, which may limit the probability that trees will be killed by bark beetles following a ground fire. We tested whether fire increases the probability that a healthy tree will sustain bark beetle attacks by locating beetle-infested trees inside and outside of the burned area, and comparing their growth history (from growth rings) with paired, unattacked trees. Surprisingly, there was no indication of recently declining growth, or chronically slow growth, in beetle-infested trees, either inside or outside the prescribed burn. Half of the trees attacked by Ips in 1998 were dead in 1999 and the remainder were partly girdled by the attacks, which increases their subsequent vulnerability to fires, insects, and pathogens. Ips bark beetles can exert meaningful effects on the survivorship of red pine populations, and their demographic impact is probably increased by ground fires.