Measurements and simulations using the 3-PG model of the water balance and water use efficiency of a lodgepole pine stand following mountain pine beetle attack

Abstract

Tree mortality due to the recent mountain pine beetle (MPB) (Dendroctonus ponderosae) outbreak in British Columbia (BC) is expected to impact evapotranspiration (E), gross primary productivity (GPP), and snow accumulation and melt, thereby influencing ecosystem hydrology. To quantify the impact on E and GPP, we have made eddy-covariance (EC) measurements of water vapor and carbon dioxide fluxes for nine years above a lodgepole pine (Pinus contorta var. latifolia) stand in northern interior BC that was not salvage harvested following beetle attack in 2006. To understand the processes and determine long-term recovery of the water and carbon balances of this stand following attack, we modified the 3-PG (Physiological Principles in Predicting Growth) model to include the effects of insect attack on the stand, and modelled monthly E, GPP and water use efficiency (WUE) of the understory and overstory. The modified model was validated using the flux and climate measurements made during the nine-year period. It generally worked well with modelled annual E and GPP agreeing reasonably well with their measured values. Modelled values of annual E and GPP decreased by about 62% and 52%, respectively, between 2005 and 2007. Annual measured E remained remarkably stable for five years after the attack, and then increased by ∼13% year−1 in the last four years; however, the model indicated a relatively steady increase of ∼13% year−1 over the 9years. On the other hand, EC-estimated GPP showed a steady recovery (∼9% year−1) almost returning to estimated pre-attack levels within nine years. Modelled annual GPP generally agreed with EC-estimated values as well as capturing much of the interannual variability observed in the last 4years. Projections for the following decade, depending on expected climate change, suggested that E would slightly increase, GPP would remain relatively constant, and WUE would decrease slightly. With the decrease in E, modelled drainage (Dr) increased significantly in the first year after attack compared to pre-attack estimates, but gradually decreased again in the following years. Measured and modelled Dr decreased between 2007 and 2015 by about 28% and 49%, respectively. Projections show Dr staying relatively constant for the following decade (2016–2025). Our observations that the recovery of E and GPP was faster than expected suggest that not to salvage harvest MPB-attacked stands with considerable remaining live canopy trees or understory should be a beneficial management strategy from carbon sequestration and hydrologic perspectives.