Humberto R. da Rocha

SEASONALITY OF WATER AND HEAT FLUXES OVER A TROPICAL FOREST IN EASTERN AMAZONIA

We used the eddy covariance technique from July 2000 to July 2001 to measure the fluxes of sensible heat, water vapor, and CO2 between an old-growth tropical forest in eastern Amazonia and the atmosphere. Precipitation varied seasonally, with a wet season from mid-December 2000 to July 2001 characterized by successive rainy days, wet soil, and, relative to the dry season, cooler temperatures, greater cloudiness, and reduced incoming solar and net radiation. Average evapotranspiration decreased from 3.96 ± 0.65 mm/d during the dry season to 3.18 ± 0.76 mm/d during the wet season, in parallel with decreasing radiation and decreasing water vapor deficit. The average Bowen ratio was 0.17 ± 0.10, indicating that most of the incoming radiation was used for evaporation. The Bowen ratio was relatively low during the early wet season (December–March), as a result of increased evaporative fraction and reduced sensible heat flux. The seasonal decline in Bowen ratio and increase in evaporative fraction coincided with an increase in ecosystem CO2 assimilation capacity, which we attribute to the growth of new leaves. The evaporative fraction did not decline as the dry season progressed, implying that the forest did not become drought stressed. The roots extracted water throughout the top 250 cm of soil, and water redistribution, possibly by hydraulic lift, partially recharged the shallow soil during dry season nights. The lack of drought stress during the dry season was likely a consequence of deep rooting, and possibly vertical water movement, which allowed the trees to maintain access to soil water year round.

Diel and seasonal patterns of tropical forest CO2 exchange

We used eddy covariance to measure the net exchange of CO2between theatmosphere and an old-growth tropical forest in Para , Brazil from 1 July 2000 to 1 July2001. The mean air temperature and daily temperature range varied little year-round; therainy season lasted from late December to around July. Daytime CO2uptake under highirradiance averaged 16–19mmol·m22·s21. Light was the main controller of CO2exchange,accounting for 48% of the half-hour-to-half-hour variance. The rate of canopy photosyn-thesis at a given irradiance was lower in the afternoon than the morning. This photosyntheticinhibition was probably caused by high evaporative demand, high temperature, an intrinsiccircadian rhythm, or a combination of the three. Wood increment increased from Januaryto May, suggesting greater rates of carbon sequestration during the wet season. However,the daily net CO2exchange measured by eddy covariance revealed the opposite trend, withgreater carbon accumulation during the dry season. A reduction in respiration during thedry season was an important cause of this seasonal pattern. The surface litter was desiccatedin the dry season, and the seasonal pattern of respiration appears to be a direct result ofreduced forest floor decomposition during drought. In contrast, canopy photosynthesis wasnot directly reduced by the dry season, probably because deep rooting allows the forest toavoid drought stress

BIOMETRIC AND MICROMETEOROLOGICAL MEASUREMENTS OF TROPICAL FOREST CARBON BALANCE

We used two independent approaches, biometry and micrometeorology, to determine the net ecosystem production (NEP) of an old growth forest in Para, Brazil. Biometric inventories indicated that the forest was either a source or, at most, a modest sink of carbon from 1984 to 2000 (+0.8 ± 2 Mg C·ha−1·yr−1; a positive flux indicates carbon loss by the forest, a negative flux indicates carbon gain). Eddy covariance measurements of CO2 exchange were made from July 2000 to July 2001 using both open- and closed-path gas analyzers. The annual eddy covariance flux calculated without correcting for the underestimation of flux on calm nights indicated that the forest was a large carbon sink (−3.9 Mg C·ha−1·yr−1). This annual uptake is comparable to past reports from other Amazonian forests, which also were calculated without correcting for calm nights. The magnitude of the annual integral was relatively insensitive to the selection of open- versus closed-path gas analyzer, averaging time, detrending, and high-frequency correction. In contrast, the magnitude of the annual integral was highly sensitive to the treatment of calm nights, changing by over 4 Mg C·ha−1·yr−1 when a filter was used to replace the net ecosystem exchange (NEE) during nocturnal periods with u* < 0.2 m/s. Analyses of the relationship between nocturnal NEE and u* confirmed that the annual sum needs to be corrected for the effect of calm nights, which resulted in our best estimate of the annual flux (+0.4 Mg C·ha−1·yr−1). The observed sensitivity of the annual sum to theu* filter is far greater than has been previously reported for temperate and boreal forests. The annual carbon balance determined by eddy covariance is therefore less certain for tropical than temperate forests. Nonetheless, the biometric and micrometeorological measurements in tandem provide strong evidence that the forest was not a strong, persistent carbon sink during the study interval.

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

[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 2 , 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 1 in these evergreen or semidecidous forests. In contrast, ecosystems with precipitation less than 1700 mm and a longer dry season (Mato Grosso, Tocantins

ECOLOGICAL RESEARCH IN THE LARGE-SCALE BIOSPHERE– ATMOSPHERE EXPERIMENT IN AMAZONIA: EARLY RESULTS

The Large-scale Biosphere-Atmosphere Experiment in Amazonia (LBA) is a multinational, interdisciplinary research program led by Brazil. Ecological studies in LBA focus on how tropical forest conversion, regrowth, and selective logging influence carbon storage, nutrient dynamics, trace gas fluxes, and the prospect for sustainable land use in the Amazon region. Early results from ecological studies within LBA emphasize the var- iability within the vast Amazon region and the profound effects that land-use and land- cover changes are having on that landscape. The predominant land cover of the Amazon region is evergreen forest; nonetheless, LBA studies have observed strong seasonal patterns in gross primary production, ecosystem respiration, and net ecosystem exchange, as well as phenology and tree growth. The seasonal patterns vary spatially and interannually and evidence suggests that these patterns are driven not only by variations in weather but also by innate biological rhythms of the forest species. Rapid rates of deforestation have marked the forests of the Amazon region over the past three decades. Evidence from ground-based surveys and remote sensing show that substantial areas of forest are being degraded by logging activities and through the collapse of forest edges. Because forest edges and logged forests are susceptible to fire, positive feedback cycles of forest degradation may be initiated by land-use-change events. LBA studies indicate that cleared lands in the Amazon, once released from cultivation or pasture usage, regenerate biomass rapidly. However, the pace of biomass accumulation is dependent upon past land use and the depletion of nutrients by unsustainable land-management practices. The challenge for ongoing research within LBA is to integrate the recognition of diverse patterns and processes into general models for prediction of regional ecosystem function.

Nocturnal cold air drainage and pooling in a tropical forest

The usefulness of eddy covariance for understanding terrestrial carbon exchange has been hampered by uncertainty over the magnitude and causes of a systematic underestimation of CO2 efflux on calm nights. We combined in situ measurements of the temperature, wind and CO2 profile with nocturnal Land Surface Temperature (LST) imagery from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) to infer the patterns of cold air drainage in an Amazonian tropical forest. The meteorological tower was located on a flat plateau that sloped gently to the southwest. The vertical profile from 0 to 64 m above ground level was divisible into two air parcels at night: a warm, low CO2, upper parcel and a cool, high CO2, lower parcel that was stable with respect to the upper parcel. The nocturnal wind direction of the upper parcel was from the east, a pattern that is consistent with the general circulation, whereas the wind direction of the lower parcel was from the northeast, a pattern that implies drainage down the local topographic gradient. The nocturnal patterns of LST were closely related to local topography and land use. In general, a nearby river was warm, gullies were cold, plateau centers were cold, stream drainages were cold, pastures were particularly cold, and upper slopes and plateau edges were warm. The in situ temperature and wind observations, combined with the observed relationship between elevation and nocturnal LST and the occurrence of warm thermal belts extending inward from the edges of plateaus, imply that cold air drainage occurs on clear nights. The slope of the relationship between LST and elevation varied between nights, indicating that the degree of thermal stratification, and possibly the extent of cold air drainage, also varied. The night-tonight variation in stratification across the landscape was correlated with the vertical temperature gradient at the tower but not the above-canopy friction velocity (u*). Criteria associated with vertical temperature gradients may prove better than u* for screening nocturnal eddy covariance observations to eliminate periods that underestimate CO2 flux.

Reduced impact logging minimally alters tropical rainforest carbon and energy exchange

We used eddy covariance and ecological measurements to investigate the effects of reduced impact logging (RIL) on an old-growth Amazonian forest. Logging caused small decreases in gross primary production, leaf production, and latent heat flux, which were roughly proportional to canopy loss, and increases in heterotrophic respiration, tree mortality, and wood production. The net effect of RIL was transient, and treatment effects were barely discernable after only 1 y. RIL appears to provide a strategy for managing tropical forest that minimizes the potential risks to climate associated with large changes in carbon and water exchange.

Measurements of CO2 exchange over a woodland savanna (Cerrado Sensu stricto) in southeast Brasil

A tecnica de correlacao dos vortices turbulentos (eddy correlation) foi utilizada para se estimar a produtividade liquida do ecossistema (PLE) em uma area de Cerrado Sensu stricto, no sitio experimental da Gleba Pe de Gigante, no sudeste do Brasil. O conjunto de dados coletados incluiu tambem medidas de variaveis climatologicas e de respiracao do solo com câmaras estaticas, no periodo de 10 de Outubro de 1999 a 30 de Marco de 2002. A respiracao do solo media anual foi de 4.8 molCO2m-2s-1, com diferencas sazonais que variaram entre 2 a 8 molCO2 m-2s-1 durante a estacao seca (Abril a Agosto) e na estacao chuvosa, respectivamente, por um padrao de sensivel correlacao com a temperatura (Q10=4.9) e umidade do solo. Com base nos fluxos atmosfericos de CO2, a PLE mostrou uma variabilidade no ciclo diurno grandemente controlada pela radiacao solar, umidade e temperatura do ar. Na escala sazonal, a umidade do solo foi uma variavel de alta correlacao com a PLE, que aparentemente induziu a queda de folhedo, reducao da atividade fotossintetica e da respiracao do solo. O sinal da PLE foi negativo (sumidouro) na estacao chuvosa e no inicio da estacao seca, com taxas de -25 kgCha-1dia-1, e maximos de ate 40 kgCha-1dia-1. Na estacao seca o sinal foi positivo (emissao), o que foi revertido logo no inicio das chuvas. No fim da estacao seca, em dias de grande estresse hidrico, ainda observou-se a resposta da fotossintese na escala do ecossistema, mesmo tendo sido positiva a PLE. Paralelamente ao decorrer da estacao seca, a PLE progressivamente aumentou de 5 ate 50 kgCha-1dia-1. A soma annual da PLE mostrou-se aproximadamente balanceada, tendo sido no entanto, sob um vies de maior precisao, um pequeno mas significativo sumidouro de 0.1 0.3 tCha-1ano-1. Consideramos a hipotese de um pequeno sumidouro como possivelmente realista, dadas as restringentes correcoes impostas no calculo dos fluxos turbulentos, e algumas hipoteses favoraveis de sucessao de estagios do Cerrado, fertilizacao de CO2 atmosferico e variabilidade climatica.

Circadian rhythms constrain leaf and canopy gas exchange in an Amazonian forest

We used a controlled-environment leaf gas-exchange system and the micrometeorological technique eddy covariance to determine whether circadian rhythms constrain the rates of leaf and canopy gas exchange in an Amazonian forest over a day. When exposed to continuous and constant light for 20 to 48 hours leaves of eleven of seventeen species reduced their photosynthetic rates and closed their stomata during the normally dark period and resumed active gas exchange during the normally light period. Similarly, the rate of whole-forest CO2 uptake at a predetermined irradiance declined during the late afternoon and early morning and increased during the middle of the day. We attribute these cycles to circadian rhythms that are analogous to ones that have been reported for herbaceous plants in the laboratory. The importance of endogenous gas exchange rhythms presents a previously unrecognized challenge for efforts to both interpret and model land-atmosphere energy and mass exchange.

Can the Deforestation Breeze Change the Rainfall in Amazonia? A Case Study for the BR-163 Highway Region

Abstract The authors simulated the effects of Amazonian mesoscale deforestation in the boundary layer and in rainfall with the Brazilian Regional Atmospheric Modeling System (BRAMS) model. They found that both the area and shape (with respect to wind incidence) of deforestation and the soil moisture status contributed to the state of the atmosphere during the time scale of several weeks, with distinguishable patterns of temperature, humidity, and rainfall. Deforestation resulted in the development of a three-dimensional thermal cell, the so-called deforestation breeze, slightly shifted downwind to large-scale circulation. The boundary layer was warmer and drier above 1000-m height and was slightly wetter up to 2000-m height. Soil wetness affected the circulation energetics proportionally to the soil dryness (for soil wetness below ∼0.6). The shape of the deforestation controlled the impact on rainfall. The horizontal strips lined up with the prevailing wind showed a dominant increase in rainfall, signific...