Contributions of Understory and Overstory to Ecosystem CO2 Fluxes in a Temperate Mixed Forest in Switzerland

Abstract:

Forests play an important role in the global carbon cycle by sequestering large amounts of atmospheric CO₂. The CO₂ sequestered by a forest varies depending on many factors including climate, species composition, growth strategy, stand age and structure. Forests are structurally complex ecosystems, both horizontally and vertically. In many cases, several canopy layers with distinct functional properties and sun exposure contribute differently to the ecosystem CO₂exchange. Only a few studies thus far have investigated the contribution of understory to overstory fluxes, and large variations have been found among sites.

Our study focused on partitioning the net ecosystem CO₂ flux of a mixed deciduous forest in Switzerland into its understory and overstory components using below and above canopy eddy-covariance (EC) measurements over two years. CO₂ concentration profile measurements made at eight levels within the canopy complemented those measurements. We quantified the CO₂flux contribution from the understory to the overstory, both in terms of photosynthesis and respiration, and assessed the differences between understory and overstory functional responses to environmental drivers.

On an annual basis, the understory was a CO₂ source, while the overstory was a CO₂ sink. The understory was a CO₂ sink only in spring with the early emergence of understory plants before overstory canopy leaf-out. Overall, the understory contributed 54% to annual ecosystem respiration but only 7% to annual ecosystem photosynthesis. Moreover, understory and overstory fluxes became decoupled at full canopy closure, thus leading to unaccounted EC fluxes when measured only above the canopy. CO₂ concentration profile measurements supported this finding. Our results showed that understory EC measurements are essential in this mixed deciduous forest, and likely in many other forests, to fully understand the carbon dynamics within structurally complex ecosystems.