Mountain Pine Beetle

Flux-tower measurements of carbon and water fluxes at mountain-pine-beetle-attacked sites in interior BC

In BC, an outbreak of the mountain pine beetle (MPB) (Dendroctonus ponderosae) recently reached an unprecedented extent. Since 1999 more than 710 million cubic metres of timber in an area of about 18.1 million hectares, which is more than five times the size of Vancouver Island, have been killed by the beetle. The current infestation peaked in 2007, but is still ongoing (B.C. Ministry of Forests, Lands and Natural Resource Operations, 2012).

Tree mortality due to the beetle outbreak significantly impacts the carbon (C) balance of forests (Brown et al., 2010). C uptake is reduced mainly because the rate of photosynthesis of the attacked stands decreases.  C loss may increase or decrease depending on how much heterotrophic respiration (R_h) (i.e., decaying roots, stems, branches and foliage) increases and autotrophic respiration (R_a) decreases. Modelling results suggest that stands that were once C sinks become C sources, i.e., the decline in gross primary productivity (GPP) exceeds the decrease in ecosystem respiration (R_e). Modelling the current MPB attack on lodgepole pine (Pinus contorta var. latifolia) stands in BC, Kurz et al. (2008) estimated its impact between 2000 and 2020 to be a loss of 270 teragrams (Tg) (or 270 megatons) C, which is equivalent to 36 g C m^-2 yr^-1 on average over 374,000 km² of forest. Salvage harvesting the beetle-killed and living trees adds another 50 Tg C to this loss. Thus, the forest becomes a source of C during and immediately after the outbreak. Since insect outbreaks like other disturbances influence the C uptake of forests, the effects should be included in C budget models in order to avoid overestimation of the forests’ ability to offset anthropogenic CO₂ emissions (Kurz et al., 2008).

Our group at UBC is making eddy-covariance (EC) measurements of C and water fluxes and climatic variables at three MPB-affected stands. These stands are located in the BC interior between Prince George and Mackenzie, and were attacked by the MPB between 2003 and 2006, hereafter named MPB-03 and MPB-06, respectively. These two non-invasively managed stands are important to compare, because they were attacked at different times and have different structure. At MPB-06 measurements started at the time of beetle attack so that immediate effects on the stand could be observed, while the other stand was in a later stage of attack and most of the lodgepole pine trees had already died. Differences in the development and recovery of the two stands result from the fact that MPB-06 is a pure lodgepole pine stand with little understory, while MPB-03 has a rich secondary structure consisting mainly of subalpine fir. In the recovery of the stand, the presence of secondary structure has a significant impact, because its photosynthetic CO₂ uptake plays an important role in the stand C balance (Bowler et al., 2012; Brown et al., 2010 and 2012; Emmel et al., 2012; Emmel et al., 2011a,b).

In order to see the effects of different management strategies on the C balance of forests, additional EC flux measurements were conducted in two clearcuts (ten years old and two years old) close to MPB-06 during the 2007 growing season. In the first year of measurements both MPB-03 and MPB-06 were weak annual C sources, but remained C sinks during the growing season (May – September). These stands recovered much faster than models have predicted. The C balance recovery was irrespective of differences in structure and composition but highly dependent on stand management (i.e., conventional clearcut harvesting, partial harvesting of MPB-affected trees only, and no intervention). MPB-03 and MPB-06 recovered at a rate of 20 g C m^-2 yr^-1, which is almost 8 times faster than the rate predicted by CBM-CFS3, and reached C neutrality 3-5 years after attack (Brown et al., 2012). Measurements in the two clearcuts, on the other hand, showed that during the growing season they were losing C, i.e., their average daily values of net ecosystem productivity (NEP) were negative. These clearcuts appear to remain C sources for as many as ten years (Brown et al., 2010).

Surprisingly, annual evapotranspiration (ET) in these stands changed little since the beetle attack (Brown et al., 2013). This indicates the important compensating effects of transpiration by the developing secondary structure and evaporation from the soil surface.

To study the effects of partial harvesting (the removal of all dead lodgepole pine trees), EC measurements were made between the fall of 2009 and December 2013 in a mixed conifer stand that was attacked in 2005-06 and partially harvested in February and March 2009 (MPB-09). During the first three years after harvesting, the stand was an annual C source, but it has been steadily recovering, i.e., the C source has weakened significantly. During the growing season of 2010, these measurements were supplemented with EC flux measurements in a nearby clearcut allowing a direct comparison of the effects of partial and clearcut harvesting. Results showed that MPB-09 was a small growing season C sink due to CO₂ uptake by the remaining trees and understory vegetation, whereas the clearcut remained a large C source. To fully investigate the effects of such management strategies on C and water balances following insect outbreaks, we are currently modifying the process-based model (3-PG), and evaluating the model using our flux measurements. In the model we are considering the changes in heterotrophic respiration due to decomposition of increasing numbers of fallen dead trees at the unharvested sites as well as wind throw at MPB-09.

This project has been funded by CFCAS, the BC Forest Science Program, the BC Ministry of Forests, Lands and Natural Resource Operations, the Pacific Institute for Climate Solutions, and an NSERC Strategic Project Grant.