The effect of canopy gaps on subcanopy ventilation and scalar fluxes in a tropical forest

Forest gaps may provide conduits that preferentially vent moist, CO2-rich subcanopy air to the atmosphere. We measured the above-canopy fluxes of momentum, sensible heat (H), CO2, and water vapor (E-t), and the vertical profiles Of CO2 and water vapor, from two 67-m meteorological towers in a selectively logged Brazilian rainforest. The logging removed similar to 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 CO2, More water vapor, and was cooler than the air at comparable altitudes in the gap. Meanwhile, the daytime CO2 flux was less negative (reduced CO2 uptake) above the gap than above the intact forest, the daytime E, 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-CO2, 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 CO2 and energy exchange, as well as the flux footprint. (c) 2006 Elsevier B.V. All rights reserved.