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

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 À1, 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. #

Reduced impact logging minimally alters tropical rainforest carbon and energy exchange

“…and decreased canopy coverage from 96% to 88% (18). The extent of canopy destruction because of RIL was far less than the 30% loss reported for CL (12).…”

“…The shifts in sensible and latent heat flux following RIL were smaller than the 20-40% changes reported for conversion to pasture in the southwestern Amazon (2), and, importantly, the changes following RIL persisted for just 1 or 2 y, whereas changes associated with deforestation were sustained. Similarly, although measurements of canopy gap microclimate following RIL showed warmer and drier conditions near the ground compared with undisturbed forest (18), the rapid reestablishment of the hydrological cycle suggests that any increased fire risk was small and transient, as fire susceptibility at TNF is determined primarily by relative humidity at the forest floor rather than fuel loading (28).…”

“…After the logging, gap location, size, and shape were mapped in a 600-m × 300-m area that extended 500 m to the northeast of the loggedsite flux tower. The logging created 44 new gaps, which ranged in size from single tree falls, which were ∼10-m across, to log landings used to temporarily store boles, which were ∼50-m across (18).…”

The influence of soils on heterotrophic respiration exerts a strong control on net ecosystem productivity in seasonally dry Amazonian forests

“…2a). Both sites have been extensively documented da Rocha et al, 2004Miller et al, 2004;Keller et al, 2004;. Field data for these sites cover the periods: 2001-2003 for K83 and 2000-2002 for RJA.…”

“…The site was selectively logged starting in 2001 (Miller et al, 2007), coincident with the time period investigated. This selective logging at K83 is reported to not significantly influence the energy and carbon fluxes when compared to a nearby undisturbed site (Tapajós National Forest site 67 km; Miller et al, 2007Miller et al, , 2011. Soil depth at K83 is > 12 m .…”