Forest recovery from mountain pine beetle attack : synthesis and simulations of stand carbon and water balances using a modified version of the 3-PG model

The most recent mountain pine beetle (MPB) (Dendroctonus ponderosae) outbreak in British Columbia (BC), which began in the late 1990s, killed ~54% of the mature merchantable lodgepole pine volume and was expected to impact gross primary productivity (GPP), ecosystem respiration (R) and thus net ecosystem productivity (NEP), as well as evapotranspiration (E), snow accumulation and melt in infested stands due to tree mortality. To quantify these effects, eddy-covariance (EC) measurements of carbon (C) and water vapour fluxes have been made above two not-salvage-harvested MPB-attacked pine stands, one with little understory (MPB-06) and another with considerable understory for ten and six years, respectively, and for three years in a partial-salvage-harvested stand, complemented with short-term EC measurements in nearby clearcuts. To determine long-term recovery of the C and water balances following attack, I modified the 3-PG (Physiological Principles Predicting Growth) model to simulate the effects of MPB attack on MPB-06. Modifications included a 2-layer canopy with a partly-dying overstory and growing understory, water availability from snowmelt, and a heterotrophic respiration sub-model. Modelled monthly and annual fluxes at MPB-06 agreed well with the respective EC-estimated values during the decade following attack. Modelled annual GPP, R, NEP and E decreased by about 52%, 35%, 126% and 62%, respectively, in the first year following attack compared to pre-attack values in 2005. While modelled GPP and R, as well as EC-estimated GPP, showed a relatively steady increase over the following decade, EC-estimated R changed little in the first eight years after attack and then increased in the last two years. Both modelled and measured NEP increased significantly over the decade with MPB-06 becoming C neutral within three to four years following attack. EC-measured annual E remained remarkably stable for five years after the attack, and then increased in the last five years, whereas the model indicated a relatively steady increase over the decade. Model projections for five climate change scenarios show 2026 average GPP, R, NEP and E being 14%, 1%, 65% and 5%, respectively, lower than pre-attack values. The quick recovery suggests that not-salvage-harvesting can be a beneficial management practice for C sequestration and hydrology.