ABSTRACT
In this project, we incorporated a robust snag dynamics submodel into SORTIE-ND, a version of the spatially explicit, individual tree model SORTIE (Pacala et al. 1996; Coates et al. 2003), to predict impacts of silviculture strategies on mountain pine beetle (MPB) attacked stands.
For modelling purposes we selected four stand types from the SBSdk subzone to represent MPB susceptible stand types: Pine Minor Spruce, Mixed Pine–Spruce, Spruce Minor Pine, and Pine Dominant. We estimated light transmission by lodgepole pine snags, developed a model to predict snag fall down rates, and assumed levels of pine mortality, natural regeneration rates, and underplanting densities.
Simulation results showed that the light environment in the understory of recently killed lodgepole pine stands is too low for survival of pine seedlings. This is a very different regeneration environment than found after wildfire. Beetle killed pine snags block considerable light for at least 10 years after their death. After these first years, the growth rate of pine under the snags accelerates until it approaches the growth rate anticipated from pine in open conditions. This change in growth rate reflects the shift in light conditions as snags deteriorate with increasingly open conditions being created.
We were unable to predict the extent of natural regeneration in the four selected stand types due to lack of data to parameterize the recruitment submodel, however, we have a 2005/07 FSP funded study to address this shortcoming. Nonetheless, shading by MPB killed snags is severe in the first years after MPB attack and may greatly limit regeneration success.
SORTIE-ND predictions of growth of pine or spruce plantations after total salvage were very similar to TASS predictions (based on TIPSY v3.2 runs). SORTIE-ND subalpine fir plantations grew slower than those projected by TASS.
Three of the four selected stand types had variable levels of residual spruce in the overstory, the understory, or both. Simulation results showed that, after pine mortality, the spruce in these stand types released and grew well, resulting in well stocked stands with good basal areas. Two of the stand types recovered to preattack basal areas within 50 years, the
third by 80 years. These stand types can help mitigate midterm (3050 years) timber supply shortages if left unsalvaged or protected during partial salvage. The Pine Dominant stand type with little residual spruce did not recover; either salvage and planting or underplanting would be required.
Modeling planting of the Pine Dominant stand type with either spruce or subalpine fir shortly after MPB attack showed substantial yield increase at 100 years. The benefit of under-planting stand types with a greater residual spruce component depends on the distribution of canopy and understory trees at the time of planting.
Delaying underplanting until pine snags transmit more light to the understory (515 years after attack) may result in higher survival and subsequent volume development. This effect highlights the importance of considering changing overstory conditions when developing prescriptions to mitigate the effect of the mountain pine beetle. However when considering underplanting, the improved light environment must be balanced against the safety hazards posed by snags and the increased levels of brush and other competing natural regeneration that will also take advantage of steadily improving light levels.
