Patterns of water and heat flux across a biome gradient from tropical forest to savanna in Brazil

Abstract

[1] We investigated the seasonal patterns of water vapor and sensible heat flux along a tropical biome gradient from forest to savanna. We analyzed data from a network of flux towers in Brazil that were operated within the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA). These tower sites included tropical humid and semideciduous forest, transitional forest, floodplain (with physiognomies of cerrado), and cerrado sensu stricto. The mean annual sensible heat flux at all sites ranged from 20 to 38 Wm À2, and was generally reduced in the wet season and increased in the late dry season, coincident with seasonal variations of net radiation and soil moisture. The sites were easily divisible into two functional groups based on the seasonality of evaporation: tropical forest and savanna. At sites with an annual precipitation above 1900 mm and a dry season length less than 4 months (Manaus, Santarem and Rondonia), evaporation rates increased in the dry season, coincident with increased radiation. Evaporation rates were as high as 4.0 mm d À1 in these evergreen or semidecidous forests. In contrast, ecosystems with precipitation less than 1700 mm and a longer dry season (Mato Grosso, Tocantins and São Paulo) showed clear evidence of reduced evaporation in the dry season. Evaporation rates were as low as 2.5 mm d À1 in the transitional forests and 1 mm d À1 in the cerrado. The controls on evapotranspiration seasonality changed along the biome gradient, with evaporative demand (especially net radiation) playing a more important role in the wetter forests, and soil moisture playing a more important role in the drier savannah sites.

Mechanisms of water supply and vegetation demand govern the seasonality and magnitude of evapotranspiration in Amazonia and Cerrado

“…Our use of the term "transitional forest" differs somewhat from other studies (e.g., da Rocha et al (2002Rocha et al ( , 2009), as it includes both the semideciduous forest RJA site which is proximal to but not within the forest-Cerrado ecotone and the seasonally flooded BAN site which is within the forest-Cerrado ecotone and contains both cerradão (tall ∼18-m trees) and cerrado sensu stricto (closed canopy of small 5 m-tall trees interspersed with taller 7-10 m trees). The tower at the PDG Cerrado site is situated within a zone of cerrado sensu stricto (da Rocha et al, 2002(da Rocha et al, , 2009. For additional site characteristics and ecosystem behavior, see Restrepo-Coupe et al (2013) and references therein and da Rocha et al (2002Rocha et al ( , 2009.…”

“…In this paper, "equatorial forest" is not intended to be representative of Amazonian equatorial forests in general, since the sites presented occur mostly on highly weathered, relatively nutrientpoor soils, in contrast to western Amazonia where soils are shallow and more nutrient-rich which support forests with higher rates of vegetation productivity and turnover (Quesada et al, 2012). Our use of the term "transitional forest" differs somewhat from other studies (e.g., da Rocha et al (2002Rocha et al ( , 2009), as it includes both the semideciduous forest RJA site which is proximal to but not within the forest-Cerrado ecotone and the seasonally flooded BAN site which is within the forest-Cerrado ecotone and contains both cerradão (tall ∼18-m trees) and cerrado sensu stricto (closed canopy of small 5 m-tall trees interspersed with taller 7-10 m trees). The tower at the PDG Cerrado site is situated within a zone of cerrado sensu stricto (da Rocha et al, 2002(da Rocha et al, , 2009.…”

“…The tower at the PDG Cerrado site is situated within a zone of cerrado sensu stricto (da Rocha et al, 2002(da Rocha et al, , 2009. For additional site characteristics and ecosystem behavior, see Restrepo-Coupe et al (2013) and references therein and da Rocha et al (2002Rocha et al ( , 2009. Table 1 also lists the temporal coverage and frequency of the climate measurements, eddy covariance data, and ancillary soil moisture data which are available at each site, in addition to the installation depths of soil moisture sensors.…”

“…The degree to which stomata regulate transpiration (E t ) independent of environmental conditions in the Amazon has been the topic of debate (Avissar and Werth, 2004;Costa et al, 2004). The conclusions of syntheses of eddy covariance measures of the seasonality of E in the Amazon have largely emphasized the secondary role of vegetation demand across a range of forest types (Costa et al, 2004;Juarez et al, 2007;da Rocha et al, 2002da Rocha et al, , 2009Fisher et al, 2009), but recent work suggests that forests indeed exhibit varying degrees of control on the seasonal exchange of water in their canopies (Costa et al, 2010). Much of what is known about the functioning of stomata remains phenomenological; at the leaf-level, attempts at forming a solid mechanistic basis of stomatal function have proven to be a challenge (Buckley, 2005;Peak and Mott, 2011).…”

“…Recent syntheses using data from eddy covariance measures of carbon, water, and energy exchange across Amazonia indicate a simple dependency of E on net radiation (R n ) for forest types ranging from seasonally wet to seasonally dry forests (Shuttleworth, 1988;Hasler and Avissar, 2007;Juarez et al, 2007;da Rocha et al, 2002da Rocha et al, , 2009Fisher et al, 2009). However, this stands in stark contrast to many model predictions which instead have historically simulated an annual E cycle in phase with precipitation (P) (Shuttleworth, 1991;Bonan, 1998;Dickinson et al, 2006), suggesting that E is limited by water availability.…”