Residual vegetation importance to net CO2 uptake in pine-dominated stands following mountain pine beetle attack in British Columbia, Canada

Abstract

An unprecedented epidemic of mountain pine beetle (MPB: Dendroctonus ponderosae) has resulted in extensive mortality of lodgepole pine (Pinus contorta var. latifolia) across British Columbia. The goal of this study was to quantify the contribution of residual live vegetation to forest-level daytime gross ecosystem photosynthesis (GEP) as measured by EC flux-towers in two lodgepole pine-dominated forests during and/or after MPB-attack. Diurnal foliar net photosynthesis (pn) of residual live herb, shrub, and tree foliage was measured periodically at both sites throughout the growing seasons of 2007, 2008, and 2009. There were no consistent trends in pn across years at either site for conifer or broadleaf components. The weighted average growing season pn of all components, normalized for LAI of the component, were similar to the average growing-season flux-tower estimates of GEP. Non-rectangular hyperbolic relationships between instantaneous values of pn and photosynthetically active radiation (Q) provided the best-fit models for pn. These relationships were used to scale up to stand level by using half-hourly values of Q, and measurements of the leaf area of vegetation components to provide modelled estimates of the vertical stand components and their summed ecosystem-scale net photosynthesis (PN ECOSYSTEM). PN ECOSYSTEM was similar to GEP at MPB-06 for both half-hourly and daily averages. At MPB-03, daily average estimates of PN ECOSYSTEM agreed very well with GEP estimates, but half-hourly averages were consistently underestimated in the morning compared to GEP in all 3years. Understory non-tree species (termed broadleaf vegetation component) were responsible for the majority (65–68%) of PNECOSYSTEM at MPB-03, and about one-third (33–35%) at MPB-06. Thus, residual vegetation, and particularly broadleaf species, is very important to growing-season CO2 uptake in MPB-attacked pine stands in central British Columbia and cannot be neglected in larger scale modelling of forest carbon dynamics in these locations. Management of forests in this region should take into account the importance of CO2 uptake by live residual vegetation in helping to mitigate forest management impacts on rising atmospheric CO2 concentrations.