Partitioning forest carbon fluxes with overstory and understory eddy-covariance measurements: a synthesis based on FLUXNET data.

摘要

Forests are complex ecosystems characterized by several distinctive vertical layers with different functional properties. Measurements of CO2 fluxes by the eddy-covariance method at different heights can be used to separate sources and sinks in these layers. We used meteorological and eddy-covariance flux data gathered at 10 sites (Jackpine and Aspen in Saskatchewan, Canada; Hyytiala, Finland; Wind River, Washington; Le Bray, France; Metolius, Oregon; Blodgett, California; Hesse, France; Walker Branch, Tennessee; Tonzi, California) in the FLUXNET network across a wide range of forest type, structure and climate. We showed that eddy-covariance flux measurements made in the understorey are problematic at night in open forests because of the build up of a strong inversion layer, but are more reliable during the day. Denser forests have higher turbulence at night in the understorey because the inversion is weaker. However, the flux footprint above and below canopy is less similar than in more open forests, partly because wind direction is more deflected while entering the canopy. We showed that gross primary productivity (GPP) of the understorey can reach 39% of the total canopy GPP, with an average of 14% across the studied sites. Both understorey leaf area index (LAI) and light penetration through the canopy are important for understorey GPP. We found that understorey respiration contributed an average of 55% to ecosystem respiration, with a range of 32-79%. Understorey in deciduous forests (62%) had higher contributions to ecosystem respiration than in evergreen forests (49%). Boreal and temperate forests had a mean understorey respiration contribution of 61%, while semi-arid forests showed lower values (44%). The normalized understorey respiration fluxes at 20 degrees C were negatively related to soil temperature, when differences in soil moisture across sites are taken into account. We showed evidence that drought limited the efficiency of microbial metabolic activity. Understorey respiration fluxes were positively correlated with gross ecosystem primary productivity..