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RMRS-GTR-31: Abundance and characteristics of snags in western Montana forests

Harris, Richard B. 1999. Abundance and characteristics of snags in western Montana forests. Gen Tech. Rep. RMRS-GTR-31. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 19 p.

Plot data from the U.S. Forest Service's Forest Inventory and Analysis program was used to characterize the abundance and selected characteristics of snags from forests in western Montana. Plots were grouped by whether they had a history of timber harvest, and the U.S. Forest Service classifications of forest type, habitat type, and potential vegetation group were used to characterize plot conditions. Snag abundance was classified by d.b.h. class and species. On uncut plots (no evidence of previous timber harvest or other mechanized human activity), total snag (9 inches or greater) density varied from under three per acre on dry Ponderosa pine ( Pinus ponderosa) types to approximately 21 per acre on mesic spruce and fir types as well as warmer sites supporting grand fir ( Abies grandis) and western redcedar ( Thuja plicata). Higher snag densities occurred in dry subalpine types (over 30 per acre), but these figures may be anomalous because of recent, uncharacteristically high mortality of whitebark pine ( Pinus albicaulis). Abundance of large (21 inches or greater) snags was much lower, but showed similar trends, varying from as low as 0.4 per acre on dry ponderosa pine sites and 0.2 per acre on lodgepole pine (Pinus contorta) dominated sites to 2 per acre on warm, mesic sites. Similar trends were observed when stands were categorized by habitat type groups and potential vegetation groups. Snag abundance on young, recently disturbed stands was only slightly lower than on older, sawtimber stands, and in general, snag dynamics differed from those of live trees during the process of stand aging. Stands lacking a history of timber harvest had significantly higher snag abundance than those with a history of timber harvest. The generally higher snag abundances in uncut stands reflect not only an unharvested condition, but also a lack of fire and probably an attendant excess of mortality from insects and disease, which would be manifested more strongly in smaller d.b.h. classes. Snag abundances in larger d.b.h. classes of uncut stands should closely mimic natural conditions on the landscape in the absence of timber harvest if accounting for a small upward bias caused by fire suppression. These estimates for larger d.b.h. classes can be used as rough targets for landscapes where managing for biodiversity or emulating natural disturbance patterns is an important objective.

Keywords: cover type, habitat type, logging, snag, snag abundance

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Title: RMRS-GTR-31: Abundance and characteristics of snags in western Montana forests
Publish Date: November 13, 2003
Last Update:
August 15, 2008

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