Foliar moisture content variations in lodgepole pine over the diurnal cycle during the red stage of mountain pine beetle attack

Widespread outbreaks of the mountain pine beetle (Dendroctonus ponderosae Hopkins) in the lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) forests of North America have produced stands with significant levels of recent tree mortality. The needle foliage from recently attacked trees typically turns red within one to two years of attack indicating successful colonization by the beetle and tree death. Attempts to model crown fire potential in these stands have assumed that the moisture content of dead foliage responds similarly to changes in air temperature and relative humidity as other fine, dead surface fuels. However, this assumption has not been verified. In this exploratory study we sampled the moisture content of dead foliage on an hourly basis through two different diurnal cycles during the fire season and compared the results to measurements of 10-h fuel moisture indicator sticks and predictions made from models used to estimate dead fuel moisture in the USA, Canada, and Australia. The observed degree of variation in dead foliar moisture content was small (6.9-14.5%) with a mean value of ~10%. All existing models performed poorly, but measurements of 10-h fuel moisture and a modified version of an existing model where timelags were extended to ~20-h had the best fit to the data. The results from our study suggest that the dead foliage on attacked trees does not respond similarly to changing environmental conditions as other fine, dead surface fuels as has been assumed. This in turn has important implications for wildland fire suppression operations, including firefighter safety, and in modeling fire behavior, and solicits the need for further research. Diurnal changes in foliage moisture of MPB-attacked trees are presented.Moisture content of red needles displays little variability across the diurnal cycle.Common models of dead fuel moisture are not appropriate.A modified model and 10-h fuel moisture measurements had the best fit to the data.