Increasing Seasonal CO2 Fluxes and the Potential Role of Changing Plant Functional Types

Abstract:

Previous work by Graven et al. (Science, 2013) has shown that the seasonal cycle of CO₂ has increased throughout the troposphere by 50% north of 45°N since the 1950s. This suggests large-scale ecological changes centered on the boreal and temperate forests, and a contribution from the Arctic. Graven et al. indicated a comparable 30-50% increase in seasonal net ecosystem exchange (NEE) fluxes is required to explain the observed change in CO₂ amplitude. We examine changes in NEE fluxes in boreal and Arctic regions for the period 1986-2012 using two time-varying atmospheric inversions, RIGC and Jena. The inversions show that, over the last few decades, the largest percent increases in seasonal NEE occur between 50-70°N. They also provide evidence that larger summertime uptake is the main driver of CO₂ amplitude increases.

One way that summer uptake may have increased is by changes in plant functional type (PFT), away from evergreen toward more deciduous forest coverage. Eddy covariance NEE measurements are helpful in determining the seasonality of net CO₂ exchange for different PFTs. The seasonal amplitude in net CO₂ uptake by deciduous boreal forests can easily be twice as large as for evergreen forests. In this presentation we will explore the relative influence of deciduous and evergreen plant functional types on the seasonal cycle of CO₂ using FluxNet data and atmospheric transport modeling, and the potential effect of changing forest composition on CO₂ amplitude trends. Understanding the processes affecting the seasonal cycle of NEE fluxes will allow better predictions of the net carbon-uptake or release in this climatically sensitive region.