Print PageForest ecosystems play an important role in sequestering and storing atmospheric carbon; globally they hold 60% of all terrestrial carbon. Disturbances, such as insect outbreaks, can disrupt carbon dynamics and alter species composition in forests. Following the recent severe outbreak of mountain pine beetle (Dendroctonus ponderosae; MPB) in Rocky Mountain lodgepole pine (Pinus contorta) forests, we used field-based measurements and vegetation simulation modeling to quantify changes in species composition and carbon storage. In 2010 we collected field data at 119 plots in Grand County Colorado, which experienced peak MPB activity in 2005-8. Plot locations were chosen using stratified random sampling across a gradient of MPB mortality. In each 1/20 acre plot, we measured individual trees, seedlings, saplings and fuels using FIREMON protocols. These data were used in the Forest Vegetation Simulator (FVS) to model changes in vegetation composition and carbon storage through the year 2210, and then compared to a control simulation that assumed no MPB mortality had occurred. We modeled vegetation change conservatively, using only trees, saplings, and seedlings present in 2010.
Results/Conclusions
Our plots averaged 10 tons of carbon/acre in live biomass and 30 tons of carbon/acre in dead biomass. In contrast, if all MPB-killed trees had remained alive, there would have been 25 tons of carbon/acre in live biomass and 15 tons of carbon/acre in dead biomass. The post-MPB simulation predicted that carbon in live biomass would recover in 40 years to pre-outbreak levels of 25 tons of carbon/acre. After 200 years, both simulations had approximately 60 tons/acre and 40 tons/acre of live and dead carbon respectively. Before MPB mortality in our plots, canopy vegetation was dominated by lodgepole pine with 9 inch mean diameter at breast height (DBH). After 200 years of simulation and with no additional major disturbance, canopies were dominated by subalpine fir (Abies lasiocarpa) with 11 inch mean DBH. In the control simulation, plots had an even mix of lodgepole pine and subalpine fir with a mean DBH of 13 inches. Our results indicate that MPB disturbance caused a considerable redistribution of carbon among live and dead biomass pools, but the modeled effects only persisted for 40 years. However, vegetation composition and structure were predicted to change substantially over the longer term. These results may represent how large insect disturbances can introduce greater uncertainty in evaluating and managing forested ecosystems for their ecosystem services, such as offsetting greenhouse gas emissions.
