Forest Development and Carbon Dynamics after Mountain Pine Beetle Outbreaks

Mountain pine beetles periodically infest pine forests in western North America, killing many or most overstory pine stems. The surviving secondary stand structure, along with recruited seedlings, will form the future canopy. Thus, even-aged pine stands become multiaged and multistoried. The species composition of affected stands will depend on the presence of nonpines and outbreak severity, among other factors, and can range from continued dominance by pines to hastened conversion to more shade-tolerant species. The loss of mature host trees results in reductions of ecosystem carbon productivity. The surviving and recruited stems, however, grow more quickly in response to the reduced competition, and carbon productivity and live basal area recover to preoutbreak levels within a few years or decades. Infestations may result in system carbon storage reductions, relative to storage among undisturbed developmental trajectories, mostly because of the temporary decrease in carbon productivity. Carbon losses in infested stands are slow as a result of recalcitrance of snags and coarse woody debris. Recalcitrant dead pools combined with recovering live pools results in fairly stable total ecosystem carbon storage among infested stands. Infested stands may switch from net carbon sinks to net carbon sources but typically recover within 5–20 years.