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.
Highlights
The ongoing mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) epidemic affecting pine species in western North America is the largest in recorded history (Mitton and Ferrenberg 2012)
At Cabin Gulch, there was a large difference between chronologies, but less difference existed between chronologies at Kitchen Gulch
30-year intervals, mean basal area increment (BAI) for visually healthy trees at Cabin Gulch was significantly less during 1890–1919 and significantly greater during 1980–2007 when compared with the MPB-infected sample (Table 2)
Summary
The ongoing mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) epidemic affecting pine species in western North America is the largest in recorded history (Mitton and Ferrenberg 2012). During 2001–2010, over 2.3 million hectares of forests in Montana were affected by the MPB (Chaney 2011), which caused mortality to numerous species of the Pinus genus including lodgepole (P. contorta), ponderosa (P. ponderosa), and whitebark (P. albacaulis) pines (Logan et al 2010). The MPB is native to western North America, with occasional years marked by outbreak populations (Brunelle et al 2008) triggered by favorable periodic climatic and/or stand conditions (Gibson et al 2009), but typically only low levels of infestation and subsequent tree mortality occur in a given year. The magnitude of the current epidemic has been attributed to multi-decadal climate change (Logan and Powell 2001; Akuma et al 2008; Bentz et al 2010) directly through a reduction in low temperature-induced mortality caused by extended cold-air incursions, during the autumn and spring, and indirectly through increased drought frequency.
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