The carbon balance of two lodgepole pine stands recovering from mountain pine beetle attack in British Columbia

Disturbances play an important role in determining the C balance of North American forests. This study used the eddy covariance (EC) technique to examine the impact of mountain pine beetle (MPB) (Dendroctonus ponderosae) outbreak on the gross ecosystem photosynthesis (Pg), ecosystem respiration (Re) and net ecosystem production (NEP), of two lodgepole pine-dominated stands in the central interior of British Columbia. MPB-06, an 85-year-old stand, was first attacked in 2006 and by 2010 only ∼16% of the trees remained healthy. MPB-03, a 110-year-old stand with a developed secondary structure (tree seedlings and saplings, sub-canopy and canopy trees that survive the attack), was first attacked in 2003 and by 2007 had >95% pine canopy mortality. Annual NEP increased each year at MPB-06, from −81 g C m−2 in 2007 to 64 g C m−2 in 2010, due to an increase in Pg from 440 g C m−2 in 2007 to 576 g C m−2 in 2010. Annual Re was more conservative, ranging from 512 g C m−2 in 2010 to 557 g C m−2 in 2008. At MPB-03, NEP increased from −57 g C m−2 in 2007 to 3 and 6 g C m−2 in 2008 and 2009 before falling to −26 g C m−2 in 2010. Between 2007 and 2010, Pg and Re ranged from 430 to 516 g C m−2 and from 487 to 513 g C m−2, respectively. Variation in NEP at both sites was mainly a result of changes in Pg. At MPB-03, Pg was reduced by drought during the 2009 and 2010 growing seasons. An empirical logistic equation best described the relationship between Re and soil temperature at 5-cm depth (Ts), and the relationship varied significantly over the four years at both sites. At both sites, over the 4 years the ratio of the growing season totals of modelled Re using nighttime and daytime NEP data was well within 15% with the exception of 2010 at MPB-06. Growing season averages of foliar net assimilation followed a similar trend as average growing season Pg, increasing from 2007 to 2009 at MPB-06, and decreasing in response to drought in 2009 and 2010 at MPB-03. Growing season water use efficiency (Pg/evapotranspiration) increased from 2.2 to 2.8 g C (kg H2O)−1 from 2007 to 2010 at MPB-06, and ranged from 1.7 to 2.0 g C (kg H2O)−1 between 2007 and 2010 at MPB-03. Spring NEP was an important determinant of the annual C balance at both sites.