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Abstract
Forest gaps may provide conduits that preferentially vent moist, CO 2-rich subcanopy air to the atmosphere. We measured the above-canopy fluxes of momentum, sensible heat (H), CO 2, and water vapor (E t), and the vertical profiles of CO 2 and water vapor, from two 67-m meteorological towers in a selectively logged Brazilian rainforest. The logging removed ∼3.5 trees ha −1, and increased the incidence of gaps by a factor of 3 over nearby undisturbed forest. One tower was located in an intact patch of forest within the selectively logged area; the other was 400 m upwind in a large gap created by the logging. During daytime the subcanopy air in the intact patch of forest had more CO 2, more water vapor, and was cooler than the air at comparable altitudes in the gap. Meanwhile, the daytime CO 2 flux was less negative (reduced CO 2 uptake) above the gap than above the intact forest, the daytime E t was greater above the gap than the intact forest, and the daytime H was lower above the gap than the intact forest. These patterns cannot be explained fully by the local loss of canopy gas exchange in the gap, but are consistent with the horizontal transport into the gap, and subsequent vertical transport out of the gap, of high-CO 2, humid, cool air from the forest understory. The understory was drier and warmer during daytime after the logging, which would be expected to increase flammability. Further measurement and modeling efforts are needed to better understand the effect of canopy gaps on the local CO 2 and energy exchange, as well as the flux footprint.
Highlights
Forest gaps, caused by the death of canopy trees due to disease, stress, age, strong winds, lightning, or selective logging, may provide conduits that preferentially vent moist, CO2-rich subcanopy air to the atmosphere
Key issues for the use of eddy covariance above forest are whether canopy gaps create local circulations that violate the assumption of horizontal homogeneity, and whether and how locating eddy covariance towers close to or within a gap effects turbulent fluxes (Sun et al, 1998)
We investigated the possibility that a large gap created by selective logging in an Amazonian tropical forest was a preferential vent for the evacuation of subcanopy air during the daytime
Summary
Forest gaps, caused by the death of canopy trees due to disease, stress, age, strong winds, lightning, or selective logging, may provide conduits that preferentially vent moist, CO2-rich subcanopy air to the atmosphere. Venting has the potential to redistribute the flux of CO2, with implications for the use of eddy covariance to determine ecosystem carbon balance (Acevedo, 2001). Key issues for the use of eddy covariance above forest are whether canopy gaps create local circulations that violate the assumption of horizontal homogeneity, and whether and how locating eddy covariance towers close to or within a gap effects turbulent fluxes (Sun et al, 1998). We investigated the possibility that a large gap created by selective logging in an Amazonian tropical forest was a preferential vent for the evacuation of subcanopy air during the daytime. Our analysis focused on comparing the daytime fluxes of momentum, heat, carbon dioxide and water vapor, and the vertical profiles of carbon dioxide and water vapor between the towers
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