Numerical modeling of mountain pine beetle transport by the wind

The mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak in the central interior of British Columbia has reached epidemic proportions affecting 4.2 million hectares of forest and over 176 million cubic meters of timber based on trees killed prior to the 2003 flight. The mountain pine beetles emerge from the bark of host pine trees after they have reached biological maturity on days during July or August in which the temperature is between 18 and 30 C, with light winds and no precipitation. Peak emergence occurs over a few days when temperatures are greater than 25 C. Upon emergence, the beetles seek new host trees to colonize and produce larvae, relying on mass-attack to overwhelm the resistance of the pine tree. The atmosphere transports, disperses and can potentially concentrate the mountain pine beetle. The movements of mountain pine beetle (and other scolytids) associated with pheromone seeking activities within the forest canopy has received considerable attention. Movements above the canopy, in which the mountain pine beetle is largely advected by the mean wind, while recognized as being potentially important for long-range movement that can rapidly spread the infestation especially during epidemics, has been largely ignored. This study therefore focuses on the above canopy transport of the mountain pine beetle by the wind. The atmospheric conditions associated with peak emergence (light regional winds, high temperature, no precipitation) are also associated with the development of thermal circulations (anabatic flow, and valley winds), and with afternoon instability which mixes down more rapidly moving air from aloft, especially in the afternoon. The CSU RAMS mesoscale atmospheric numerical model is used to simulate the atmosphere at 1 km horizontal and 25 m vertical resolution during periods of peak emergence. RAMS simulated meteorological fields are assessed by comparison with surface wind observations. RAMS output is then used to develop forward trajectories from areas of known emergence and back-trajectories from areas of known new infestation. Additionally, mountain pine beetle movements are simulated using HYPACT, a lagrangian particle dispersion model that utilizes RAMS fields as input. The simulations give a better understanding of the between-stand movement of mountain pine beetles, and can therefore give a more complete picture of the historical and future redistribution of the mountain pine beetle population.