Fire History And Tree Growth Responses In The Tenmile South Watershed, Helena, MT

Abstract

The Tenmile South watershed, southwest of Helena, Montana, is the subject of a controversial fuel reduction project initiated by the surrounding community. Historic mining activities have resulted in the accumulation of heavy metal contaminated sediments in the upper watershed. These toxic sediments could be mobilized following a catastrophic fire and pose a risk to Helena’s water supply. Dead trees from beetle infestations in 2006-2009 now pose a significant fire hazard across the watershed. A proactive fuels reduction project has been proposed by the United States Forest Service (USFS) to reduce the risk of fire and potential impact on the water supply. Our study is a reconstruction of fire history and will be part of an environmental impact statement being prepared by the USFS. Cross-section ‘cookies’ were obtained from fire scarred trees located across the watershed from five biophysical settings identified by the USFS. Old growth ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) with multiple fire scars were the primary targets for analysis, but a dominance of fire scarred beetle-killed lodgepole pine (Pinus contorta) comprise a large portion of the data set. Assessment of fire scars suggests a fire return interval of 70 to 80 years from a record extending back to the early 1700s. Initial analysis of ring width measurements indicate that trees are not highly responsive and that growth rates in the sampled trees is more highly correlative with precipitation than temperature.