Amazon Rainforest Exchange of Carbon and Subcanopy Air Flow: Manaus LBA Site—A Complex Terrain Condition

Abstract

On the moderately complex terrain covered by dense tropical Amazon Rainforest (Reserva Biologica do Cuieiras—ZF2—02°36′17.1′′ S, 60°12′24.4′′ W), subcanopy horizontal and vertical gradients of the air temperature, CO2 concentration and wind field were measured for the dry and wet periods in 2006. We tested the hypothesis that horizontal drainage flow over this study area is significant and can affect the interpretation of the high carbon uptake rates reported by previous works at this site. A similar experimental design as the one by Tóta et al. (2008) was used with a network of wind, air temperature, and CO2 sensors above and below the forest canopy. A persistent and systematic subcanopy nighttime upslope (positive buoyancy) and daytime downslope (negative buoyancy) flow pattern on a moderately inclined slope (12%) was observed. The microcirculations observed above the canopy (38 m) over the sloping area during nighttime presents a downward motion indicating vertical convergence and correspondent horizontal divergence toward the valley area. During the daytime an inverse pattern was observed. The micro-circulations above the canopy were driven mainly by buoyancy balancing the pressure gradient forces. In the subcanopy space the microcirculations were also driven by the same physical mechanisms but probably with the stress forcing contribution. The results also indicated that the horizontal and vertical scalar gradients (e.g., CO2) were modulated by these micro-circulations above and below the canopy, suggesting that estimates of advection using previous experimental approaches are not appropriate due to the tridimensional nature of the vertical and horizontal transport locally. This work also indicates that carbon budget from tower-based measurement is not enough to close the system, and one needs to include horizontal and vertical advection transport of CO2 into those estimates.

Wind Regimes above and below a Temperate Deciduous Forest Canopy in Complex Terrain: Interactions between Slope and Valley Winds

“…The daytime drainage flows under tall and dense canopies were also observed at other sites: down-slope at sites with flat terrain [8,30] or down-valley at complex sites [9,28,51]. Interestingly, the five sites with a daytime drainage flow below canopy (the MMSF site [10], the AEF site [29], the Manaus LBA site [30], the Xishuangbanna site [47] and the Maoershan site) had two common features: (1) a tall, dense canopy; (2) closed to the valley center or at the lower part of the sidewall with relative gentle slope. There is evidence that the radiative heating/cooling of dense canopy isolated the subcanopy layer with that above canopy [26,28], which plays a key role in the formation of these decoupled subcanopy flows.…”

“…These situations are quite different with those of open canopy forest or bare ground. Beneath such dense and high forest canopies, persistent subcanopy nighttime upward and daytime downward flow patterns are observed [10,30], which contradicts the typical thermally driven winds [15]. Another distinct feature of dense canopy wind is the core of drainage flow may be beyond the canopy height [29,31].…”

“…Unfortunately, most eddy flux towers were not tall enough to observe the jet-maximum of drainage flow above the canopy. Only a few studies (including this study) detected that the height of maximum wind velocity could occur over the canopy [21,28,30]. However, determining the whole profile of the valley-wind in any site needs high tower or other techniques (e.g., tethersonde balloon and sodar) [29].…”

“…However, because of the site-specific wind regime, it needs more researches to gain a general perspective on canopy winds in a complex topography. The combination of slope flows on the sidewalls and boundary layer processes on and above the valley floor is likely to cause valley winds [15], but the interaction between slope and valley winds in a forested area is poorly understood [10,22,30,33].…”

Stably stratified canopy flow in complex terrain

“…Sensors on the tower above the canopy cannot measure the fluxes conducted by drainage flow because the layer above the canopy is decoupled from the drainage flow by the isolating super-stable layer. This advection problem is a well-known issue that has not yet been solved using eddy-flux measurements (Goulden et al, 1996;Aubinet et al, 2003;Staebler and Fitzjarrald, 2004;Sun et al, 2007;Yi et al, 2008;Montagnani et al, 2009;Feigenwinter et al, 2010;Aubinet and Feigenwinter, 2010;Queck and Bernhofer, 2010;Tóta et al, 2012;Siebicke et al, 2012).…”