Effects of Interacting Disturbances on Landscape Patterns: Budworm Defoliation and Salvage Logging

Prior to European settlement, the 450 000-ha Pine Barrens region in northwestern Wisconsin, USA, was characterized by a landscape mosaic of large, open patches, savannas, and closed forest stands of jack pine (Pinus banksiana). Crown-fires created large open patches that persisted on the droughty soils, providing important habitat for a number of area-sensitive, open-habitat species. Insect outbreaks may have contributed to periodic fires by increasing the fuel load. Today, fires are suppressed in the managed landscape, but insect defoliation remains a major disturbance. Salvage logging commonly follows insect outbreaks. Our objective was to evaluate landscape pattern changes caused by this interaction of natural disturbances and forest management. We examined changes in landscape pattern during the most recent (1990–1995) outbreak of jack pine budworm (Choristoneura pinus pinus) in northwestern Wisconsin using four Landsat TM satellite images (1987, 1991, 1993, and 1995). The 1987 image provided the basis for a species-level forest classification identifying mature jack pine and open habitat prior to the budworm outbreak. Each subsequent image was used to identify clearcuts in mature jack pine. The 1995 image was also used to classify stand development in the open habitat of the 1987 image so that the overall availability of open habitat in 1995 could be assessed. GIS data layers were used to analyze logging rates and clearcut sizes separately for different classes of land ownership and soils. Approximately 12 500 ha were salvage logged during the 1990–1995 outbreak. Logging rates were highest on the most infertile soils, and on private, industrial forest land. Annual logging rates of different landowners were 3–6 times higher during the outbreak than previous to it. Salvage cut sizes were larger than clearcuts prior to the outbreak. New cuts were mostly located next to previous cuts, thus increasing the size of openings on the landscape. Initial survey results by N. Niemuth show that populations of one area-sensitive openhabitat species (Sharp-tailed Grouse, Tympanuchus phasianellus) thrive on the salvage cuts. Insect defoliation and subsequent salvage logging did create new habitat for open-habitat species that were declining. Landscape pattern changes due to multiple interacting disturbances have rarely been studied. The interaction of jack pine budworm defoliation and salvage logging substantially changed landscape pattern in the Pine Barrens. We speculate that interaction between insect defoliation and fire may have significantly shaped the presettlement landscape.