[1] The seasonal pattern of evapotranspiration (expressed as latent heat flux Q e ) for a 28-to 30-m-tall tropical transitional (ecotonal) forest was quantified over an annual cycle using eddy covariance measurement and micrometeorological estimation techniques. The study was conducted near the city of Sinop, in northern Mato Grosso, Brazil, which is located within the ecotone of tropical rain forest and savanna (cerrado). Although the majority of net radiation (Q*) was consumed by Q e (50-90%), seasonal variations in Q e were large and positively correlated with precipitation. Total daily Q e for the dry season (June-August) was on average 6.0 MJ m À2 d À1 , while daily Q e for the transition (October-November and April-May) and wet (December-March) season periods were 7.5 and 10.0 MJ m À2 d À1 , respectively. The seasonal variation in midday (0900-1500 LT) surface conductance (g s ) was also positively correlated with precipitation. Analysis of the ''decoupling factor'' ( ) indicated that the forest was strongly coupled to the atmosphere ( = 0.1-0.3) over the dry season and transition periods, suggesting that Q e was under relatively strong stomatal control. Although rainfall during the study period was above the long-term (30-year) average, our results indicate that the seasonal dynamics of Q e for the tropical transitional forest were more comparable to tropical savanna than to rain forest.

[1] Deforestation and climate change have the capacity to alter rainfall regimes, water availability, and surface-atmosphere flux of water and energy of tropical forests, especially in ecotonal, semi-deciduous tropical forests of the southern Amazon Basin, which have experienced rapid regional warming and deforestation over the last three decades. To reduce uncertainty regarding current and future energy and water flux, micrometeorological measurements of latent (Q e ) and sensible heat flux (Q h ) and canopy conductance (G c ) were combined with measurements of sap flux density (F d ) and maximum leaf conductance (g smax ) to characterize the seasonal controls on mass (H 2 O) and energy exchange of an ecotonal, semi-deciduous forest in northern Mato Grosso, Brazil over the 2005-2006 annual cycle. Average diel patterns and daily rates of energy flux and conductance declined during the dry season; however, the decline in F d and Q e was smaller and/or more gradual than G c and g smax . Weekly averages of transpiration calculated from sap flow measurements during the dry-wet season transition period were positively correlated (r 2 = 0.47; p < 0.05; n = 11) with estimates of leaf area index (LAI) derived from the Modis-Aqua satellite platform while estimates of evapotranspiration ET derived from eddy covariance were not, presumably because these estimates also include an evaporation component. Overall, our results suggest that access to deep water reserves can support high rates of F d and Q e during the dry season, but because of high evaporative demand, declines in plant water potential lead to a corresponding decline in G c . Furthermore, seasonal variations in LAI, that are likely to be controlled in part by plant water status and phenology, constrain tree and stand transpiration. Thus the consistency of Q e over the annual cycle appears to be the result of trade-offs between water availability (rainfall, soil moisture, water potential), canopy structural properties (LAI), and meteorological conditions including vapor pressure deficit and net radiation.

Summary 1.Tower-based eddy covariance and measurements of the vertical CO 2 concentration gradient within the canopy were used to quantify the seasonal variations in the net ecosystem CO 2 exchange ( NEE ) of a 28 -30 m tall transitional tropical forest (cerradão). The study was conducted near the city of Sinop, Mato Grosso, Brazil (11 ° 24·75 ′ S; 55 ° 19·50 ′ W), which is located in the ecotone of two major regional ecosystem types of South America (tropical rainforest and savanna). 2. The NEE during the dry season (August-September) was in balance, but during the transition period between the dry and wet seasons (October-November) the cerradão stand became a net source of 50-150 mmol m -2 day -1 CO 2 to the atmosphere. Measurements during the wet season (February, April) indicate that the forest was a net sink of between -55 and -102 mmol m -2 day -1 . 3. The NEE of the transitional tropical forest was more similar to that of tropical rainforest during the wet season, but during the dry season the NEE of the transitional forest was more similar to that reported for tropical savanna. The data suggest that seasonal variations in rainfall have important implications for the seasonal pattern of NEE in cerradão.

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[1] Tropical savanna (locally known as cerrado) composes 24% of Brazil and is characterized by high climatic variation; however, patterns of energy exchange are poorly understood, especially for mixed grasslands (locally known as campo sujo). We used eddy covariance to measure latent (L e ) and sensible (H) heat flux of a mixed grassland and linked meteorological and remote sensing data to determine the controls on these fluxes. We hypothesized that (1) seasonal variations in H and L e would be large due to variations in precipitation; (2) ecosystem phenology, estimated using the enhanced vegetation index (EVI), would be the best predictor of seasonal variation in L e ; and (3) cerrado, transitional, and humid evergreen forests would have similar rates of average annual L e despite large seasonal variation in cerrado L e . Our data suggest that campo sujo exhibits large seasonal fluctuations in energy balance that are driven by rainfall and that responses to rainfall pulses are rapid and dynamic, especially during the dry season. Path analysis indicated that temporal variations in the EVI did not significantly affect L e or G c , but this was because all three variables (EVI, L e , and G c ) responded similarly to temporal variations in surface water availability. Compared to other tropical ecosystems, wetter sites had higher rates of L e during the dry season but similar rates during the wet season when water was not limiting. Over annual time periods, average rates of L e increased significantly as average annual rainfall increased, due to dry-season water limitations in the more seasonal tropical ecosystems.

This research utilized tower-based eddy covariance to quantify the trends in net ecosystem mass (CO 2 and H 2 O vapor) and energy exchange of important land-cover types of NW Mato Grosso during the March-December 2002 seasonal transition. Measurements were made in a mature transitional (ecotonal) tropical forest near Sinop, Mato Grosso, and a cattle pasture near Cotriguaçú , Mato Grosso, located 500 km WNW of Sinop. Pasture net ecosystem CO 2 exchange (NEE) was considerably more variable than the forest NEE over the seasonal transition, and the pasture had significantly higher rates of maximum gross primary production in every season except the dry-wet season transition (September-October). The pasture also had significantly higher rates of whole-ecosystem dark respiration than the forest during the wetter times of the year. Average ( AE 95% CI) rates of total daily NEE during the March-December 2002 measurement period were 26 AE 15 mmol m À2 day À1 for the forest (positive values indicate net CO 2 loss by the ecosystem) and À38 AE 26 mmol m À2 day À1 for the pasture. While both ecosystems partitioned more net radiation (R n ) into latent heat flux (L e ), the forest had significantly higher rates of L e and lower rates of sensible heat flux (H) than the pasture; a trend that became more extreme during the onset of the dry season. Large differences in pasture and forest mass and energy exchange occurred even though seasonal variations in micrometeorology (air temperature, humidity, and radiation) were relatively similar for both ecosystems. While the short measurement period and lack of spatial replication limit the ability to generalize these results to pasture and forest regions of the Amazon Basin, these results suggest important differences in the magnitude and seasonal variation of NEE and energy partitioning for pasture and transitional tropical forest.

O presente trabalho teve como objetivo verificar a variação da produção de serrapilheira de diferentes biomas: Cerrado (com as fitofisionomias Cerrado sensu stricto e Cerradão) e Floresta de Transição Amazônia-Cerrado, em clima tropical. Para a determinação da produção de serrapilheira foram utilizados coletores de tela em náilon. Dados micrometereológicos foram coletados nas áreas de estudo. A produção de serrapilheira nos dois biomas mostrou acentuada sazonalidade, com as maiores produções ocorrendo durante a estação seca e menor durante a estação chuvosa. A maior produção de serrapilheira ocorreu na Floresta de Transição, seguida do bioma Cerrado. A fração de folhas foi mais representativa do que as frações de galhos, flores, frutos em ambas as áreas estudadas.

Leaf area index (LAI) is a key driver of forest productivity and evapotranspiration; however, it is a difficult and labor-intensive variable to measure, making its measurement impractical for large-scale and long-term studies of tropical forest structure and function. In contrast, satellite estimates of LAI have shown promise for large-scale and long-term studies, but their performance has been equivocal and the biases are not well known. We measured total, overstory, and understory LAI of an Amazon-savanna transitional forest (ASTF) over 3 years and a seasonal flooded forest (SFF) during 4 years using a light extinction method and two remote sensing methods (LAI MODIS product and the Landsat-METRIC method), with the objectives of (1) evaluating the performance of the remote sensing methods, and (2) understanding how total, overstory and understory LAI interact with micrometeorological variables. Total, overstory and understory LAI differed between both sites, with ASTF having higher LAI values than SFF, but neither site exhibited year-to-year variation in LAI despite large differences in meteorological variables. LAI values at the two sites have different patterns of correlation with micrometeorological variables. ASTF exhibited smaller seasonal variations in LAI than SFF. In contrast, SFF exhibited small changes in total LAI; however, dry season declines in overstory LAI were counteracted by understory increases in LAI. MODIS LAI correlated weakly to total LAI for SFF but not for ASTF, while METRIC LAI had no correlation to total LAI. However, MODIS LAI correlated strongly with overstory LAI for both sites, but had no correlation with understory LAI. Furthermore, LAI estimates based on canopy light extinction were correlated positively with seasonal variations in rainfall and soil water content and negatively with vapor pressure deficit and solar radiation; however, in some cases satellite-derived estimates of LAI exhibited no correlation with climate variables (METRIC LAI or MODIS LAI for ASTF). These data indicate that the satellite-derived estimates of LAI are insensitive to the understory variations in LAI that occur in many seasonal tropical forests and the micrometeorological variables that control seasonal variations in leaf phenology. While more ground-based measurements are needed to adequately quantify the performance of these satellite-based LAI products, our data indicate that their output must be interpreted with caution in seasonal tropical forests.