title: Mountain pine beetle selectivity in old-growth ponderosa pine forests, Montana, USA.
abstract: A historically unprecedented mountain pine beetle (MPB) outbreak affected western Montana during the past decade. We examined radial growth rates (AD 1860-2007/8) of co-occurring mature healthy and MPB-infected ponderosa pine trees collected at two sites (Cabin Gulch and Kitchen Gulch) in western Montana and: (1) compared basal area increment (BAI) values within populations and between sites; (2) used carbon isotope analysis to calculate intrinsic water-use efficiency (iWUE) at Cabin Gulch; and (3) compared climate-growth responses using a suite of monthly climatic variables. BAI values within populations and between sites were similar until the last 20-30 years, at which point the visually healthy populations had consistently higher BAI values (22-34%) than the MPB-infected trees. These results suggest that growth rates two-three decades prior to the current outbreak diverged between our selected populations, with the slower-growing trees being more vulnerable to beetle infestation. Both samples from Cabin Gulch experienced upward trends in iWUE, with significant regime shifts toward higher iWUE beginning in 1955-59 for the visually healthy trees and 1960-64 for the MPB-infected trees. Drought tolerance also varied between the two populations with the visually healthy trees having higher growth rates than MPB-infected trees prior to infection during a multi-decadal period of drying summertime conditions. Intrinsic water-use efficiency significantly increased for both populations during the past 150 years, but there were no significant differences between the visually healthy and MPB-infected chronologies.
authors.affiliation: Carolina Tree-Ring Science Laboratory, Department of Geography, University of North Carolina Greensboro Greensboro, 27401, North Carolina.
authors.forename: Paul A
authors.initials: PA
authors.lastname: Knapp
authors.forename: Peter T
authors.initials: PT
authors.lastname: Soulé
authors.forename: Justin T
authors.initials: JT
authors.lastname: Maxwell
keywords: Basal area increment; Montana; Ponderosa pine; USA; growth divergence; intrinsic water-use efficiency; mountain pine beetle outbreak