Carbon dioxide and water vapour exchange above two lodgepole pine stands recovering from mountain pine beetle attack in British Columbia

The mountain pine beetle (MPB) has killed more than 675 million m-3 of pine over an area estimated at 16.3 million hectares. This study used the eddy covariance (EC) technique to measure CO2, water vapour and sensible heat exchange above two MPB-attacked lodgepole stands. The first stand, located at Kennedy Siding (MPB-06) (55° 06' 43''N, 122° 50' 23''W) was first attacked in the summer of 2006 and by 2009 <20% of the canopy remained healthy. The second stand, located on the edge of Crooked River Provincial Park (MPB-03) (4° 28' 25''N, 122° 42' 43''W), had secondary structure consisting mainly of spruce trees, and was first attacked in 2003. Over 95% of the pine overstory trees were killed by the beetle when EC measurements began in 2007. MPB-06 was an annual carbon (C) source of 82 and 33 g C m-2 in 2007 and 2008 but became a C sink of 27 g C m-2 in 2009. Gross ecosystem photosynthesis (Pg) was 440, 521 and 550 g C m-2, and ecosystem respiration (R) was 521, 550 and 524 g C m-2 in 2007, 2008 and 2009, respectively. MPB-03 was a C source of 56 g C m-2 in 2007 but was a C sink in 2008 and 2009 of 4 and 11 g C m-2. Pg increased from 432 in 2007 to 515 in 2008 and 540 in 2009, while R was 489, 511 and 526 in 2007, 2008 and 2009, respectively. Net ecosystem productivity (NEP) recovered to C neutrality in the third and fifth year after attack at MPB-06 and at MPB-03, respectively. The recovery at MPB-06 resulted from an increase in photosynthesis by the remaining healthy trees and understory vegetation and at MPB-03 by the secondary structure. In both stands the non-attacked component of the vegetation benefitted from less competition for nutrients and water. In the years following attack there was little change in evapotranspiration (E) at both sites. E ranged from 231 to 240 mm y-1 at MPB-06, and from 286 to 308 mm y-1 at MPB-03, while growing season Priestley-Taylor α averaged 0.53 and 0.49 at MPB-06 and MPB-03, respectively, over the three years. In 2007, 2008, and 2009, values of average growing season water use efficiency were 2.4, 2.7, and 2.7 g C kg-1 H2O at MPB-03, and 1.7, 1.9 and 2.1 g C kg-1 H2O at MPB-06, respectively.