Spruce Beetle-Induced Changes to Engelmann Spruce Foliage Flammabiliiy

Intermountain Engelmann spruce (Picea engelmannii Parry ex Engelm) stands affected by the spruce beetle (Dendroctonus rufipennis Kirby) represent a unique and growing fuel complex. In this study, we quantified and compared the changes in moisture content, chemistry, and flammability of foliage from trees in three crown condition classes: unattacked (green [G]), currently mass attacked (green-infested [Cl]), and mass attacked the previous year (yellow (1]) over the course of a fire season. Cl trees displayed highly variable decreases in moisture content both between trees and within individual tree crowns that produced variable increases in flammability. The foliage on Y trees had significantly lower moisture contents, higher proportions of lignin and cellulose, and lower proportions of carbohydrate-based compounds than G foliage, which resulted in increased flammability. This increase in crown flammability was short-lived because the foliage on Y trees dropped abruptly approximately 14 months after mass attack (by late July). Given the observed changes in flammability, increased crown fire potential may occur in spruce beetle-infested forests during the spring when G and Y foliage flammability is highest, provided sufficiently dry conditions, and in late summer when the combination of peak GI foliage flammability coincides with the peak in seasonal drying.