Effects of fire and mechanical wounding on Pinus resinosa resin defenses, beetle attacks, and pathogens

Pinus resinosa is an ecologically and economically important native pine of the northern Great Lakes region, USA. The survival of adult trees can be challenged by fire, physical wounding, fungal pathogens, and bark beetles, which frequently occur in combination. We conducted experiments to test for interactions among these disturbance agents. Specifically, we measured responses of the oleoresin defense system to simulated ground fires and mechanical wounding, together and alone. Then we measured the attraction of study trees to ambient populations of Ips bark beetles, and their suitability as hosts for four species of necrotizing fungal pathogens commonly associated with Ips spp. P. resinosa responded to scorching and mechanical wounding with an initial decrease in resin defenses, which returned to pre-treatment levels in 7–10 days and continued to increase until resin flow was about twice that of control trees after 55 days. Fungal inoculation also produced an approximate doubling of resin flow after 55 days. Combinations of treatments did not increase resin flow more than single treatments. There were consistent differences among sympatric species of fungi in the size of lesions that were produced from standardized inoculations of the phloem, but lesion sizes were unaffected by previous exposure of trees to fire. Landings by Ips pini and I. grandicollis were unrelated to fire damage or physical wounding, but bark beetle attacks (entry into the phloem) were concentrated within the scorched region of trees exposed to fire. Fire, fungi, and bark beetle attacks, but not mechanical wounding, induced a strong localized reaction that rendered the affected phloem tissue dark and resin soaked; this tended to contain the tissue damage and usually killed the fungi and/or beetles, but was generally associated with death of the adjacent cambium. Our results suggest the possibility of fire-driven coevolution between P. resinosa and Ips.