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

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 CO₂ exchange. Only a few studies thus far have investigated the contribution of understory to ecosystem-scale 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 CO₂ flux 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 below canopy fluxes were a net CO₂ source dominated by soil respiration, while the above canopy fluxes were dominated by tree photosynthesis leading to a net CO₂ sink. The understory was a CO₂ sink only in spring with the early emergence of understory plants before overstory canopy leaf-out, and below canopy respiration agreed well with soil respiration measurements. Overall, the understory contributed 66% to annual ecosystem respiration but only 9% to annual ecosystem photosynthesis. Moreover, below and above canopy fluxes became decoupled at full canopy closure, thus increasing the relevance of unaccounted EC fluxes when measured only above the canopy. CO₂ concentration profile measurements supported this finding. Our results showed that below canopy 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.