Francisco de Almeida Lobo

Seasonal variation in the leaf gas exchange of tropical forest trees in the rain forest-savanna transition of the southern Amazon Basin

The photosynthetic light response of Amazonian semi-deciduous forest trees of the rain forest-savanna transition near Sinop Mato Grosso, Brazil was measured between July 2000 and September 2003 to test the hypothesis that the photosynthetic capacity of trees acclimated to different growth light environments will decline during the dry season. Maximum photosynthesis (A max ) and stomatal conductance (g max ) were significantly higher during the wet season; however, the physiological response to drought was not a clear function of growth light environment. For some species, such as Psychotria sp. growing in the mid-canopy, internal leaf CO 2 concentration (C i ) was > 30% lower during the dry season suggesting that declines in A max were caused in part by stomatal limitations to CO 2 diffusion. For other species, such as Brosimum lactescens growing at the top of the canopy. Tovornita schomburgkii growing in the mid-canopy, and Dinizia excelsa growing in the understorey, dry season C i declined by < 20% suggesting that factors independent of CO 2 diffusion were more important in limiting A max . Dry-season declines in g max appeared to be important for maintaining a more consistent leaf water potential for some species (T. schomburgkii and D. excelsa) but not others (Psychotria sp.). These results indicate that while seasonal drought exerts an important limitation on the physiological capacity of semi-deciduous Amazonian forest trees, the mechanism of this limitation may differ between species.

The Sensitivity of Diel CO2 and H2O Vapor Exchange of a Tropical Transitional Forest to Seasonal Variation in Meteorology and Water Availability

Abstract Measurements of ecosystem gas exchange, meteorology, and hydrology (rainfall and soil moisture) were used to assess the seasonal patterns of, and controls on, average diel (24 h) net ecosystem CO2 exchange (NEE), evapotranspiration (E), and bulk canopy water vapor conductance (Gc) of a tropical transitional (ecotonal) forest in the Brazilian Amazon. Diel trends in E and NEE were almost completely explained by the diel variation in photosynthetic photon flux density (QPPFD), and while the QPPFD response of E varied little over the annual cycle, the QPPFD response of NEE declined substantially during the dry season, and the magnitude of hysteresis in the NEE–QPPFD response increased as well. The magnitude of the residuals for the QPPFD versus NEE response was significantly negatively correlated with total monthly rainfall and surface soil moisture and positively correlated with the maximum daily temperature and atmospheric vapor pressure deficit (V). Average daily Gc was also significantly correlat...

Contribuição de folhas na formação da serrapilheira e no retorno de nutrientes em floresta de transição no norte de Mato Grosso

RESUMO Este estudo foi desenvolvido numa area de floresta de transicao Amazonia-Cerrado no norte de Mato Grosso. Teve como objetivo quantificar a dinâmica da producao e acumulo e as partes constituintes da serrapilheira, bem como a participacao das folhas no retorno do nitrogenio, fosforo e carbono ao solo por meio da decomposicao. Foram coletados mensalmente durante um ano dados de serrapilheira total produzida, serrapilheira total acumulada no solo e avaliada a decomposicao de folhas no periodo de seca e de chuvas. A producao de serrapilheira apresentou variacoes sazonais com maiores valores no periodo seco e predominio da fracao foliar durante todo o ano. Por outro lado, a serrapilheira acumulada nao apresentou diferencas significativas entre o periodo de seca e o chuvoso como tambem correlacao significativa com a serrapilheira produzida. A decomposicao da serrapilheira exclusiva de folhas foi mais intensa no periodo chuvoso e em ordem decrescente de especies, em T. schomburgkii, Thyrsodium sp, B. lactescens e P. sagotianum. A especie P. sagotianum tanto no periodo seco quanto no chuvoso apresentou maior resistencia a decomposicao. O potencial de retorno de nitrogenio ao solo foi da ordem de 70,3 kg.ha -1 .ano -1 , para o fosforo, de 76,1 kg.ha -1 .ano -1 e para o carbono, de 3,5 ton.ha -1 .ano -1 . Houve uma correlacao entre a decomposicao exclusiva de folhas e o retorno de nitrogenio e fosforo para todas as especies estudadas no periodo de seca. No periodo chuvoso foi registrada correlacao significativa somente para nitrogenio e o carbono para a especie P. sagotianum. PALAVRAS-CHAVE: ciclagem de nutrientes, serrapilheira, floresta de transicao.

Variations in aboveground vegetation structure along a nutrient availability gradient in the Brazilian pantanal

Background and aimsForest expansion into seasonally flooded (hyperseasonal) savanna of the Brazilian Pantanal has been occurring for decades. Our goal was to evaluate how ecosystem physiognomy varied across a nutrient availability gradient and if hyperseasonal savanna had adequate nutrient stocks to support forest expansion.MethodsWe quantified soil properties, aboveground ecosystem structure, and nutrient stocks of three savanna and three forest stands in the Pantanal of Mato Grosso, Brazil, and used correlation analysis to assess how aboveground vegetation structure varied across a soil nutrient availability gradient.ResultsWood and foliage carbon storage and leaf area index were positively correlated with soil extractable phosphorus (P), calcium (Ca2+), and magnesium (Mg2+) concentrations but not soil organic matter or texture. Soil profiles indicated that vegetation enriched surface P and K+ availability but not Ca2+ and Mg2+. Savanna ecosystems had adequate K+, Ca2+, and Mg2+ to support gallery and riparian forests but not palm forest, while the savanna P stock was inadequate to support forest expansion.ConclusionsHyperseasonal savanna has adequate nutrients (except P) to support forest expansion. Forest trees likely invade P-deficient savanna by surviving in P-rich microsites. Over time, biotic enrichment of soil may accelerate forest expansion into P-poor savanna.

Temporal Patterns of Energy Balance for a Brazilian Tropical Savanna under Contrasting Seasonal Conditions

The savanna of Central Brazil (locally known as cerrado) has a long history of land cover change due to human activity. These changes have led to the degradation of cerrado forests and woodlands, leading to the expansion of grass-dominated cerrados and pastures. Thus, the aim of this study was to evaluate the temporal variation in energy flux in areas of degraded, grass-dominated cerrado (locally known as campo sujo) in Central Brazil. The amount of partitioned into H declined as monthly rainfall increased and reached a level of approximately 30% during the wet season, while the amount of partitioned into increased as monthly rainfall increased and reached a level of approximately 60% during the wet season. As a result, H was significantly higher than during the dry season, resulting in a Bowen ratio (β = H/) of 3-5, while Le was higher than H during the wet season, resulting in a . These data indicate that the energy partitioning of grass-dominated cerrado is relatively more sensitive to water availability than cerrado woodlands and forests, and have important implications for local and regional energy balance.

Seasonal variation in the maximum rate of leaf gas exchange of canopy and understory tree species in an Amazonian semi-deciduous forest

abstract Leaf gas exchange, water potential, and specific leaf area of two tropical semi-deciduous tree species, Brosimum lactescens S. Moore and Tovomita schomburgkii Planch & Triana, were quantified to establish how these properties were affected by seasonal variations in rainfall and leaf canopy position. The study was conducted at a site near Sinop Mato Grosso, Brazil, which is located within the ecotone of savanna and tropical rain forest. Both species exhibited significant declines in leaf water potential (Ψ L ), specific leaf area, area- and mass-based light saturated photosynthesis and dark respiration, and maximum stomatal conductance during the dry-season, suggesting that leaf structural properties and gas exchange are significantly altered by drought that develops during the 4-month dry season. Internal leaf CO 2 concentrations (C i ) were consistently lower during the dry season suggesting that the decline in maximum photosynthesis was due in part to a decline in stomatal conductance. However, seasonal variations in leaf gas exchange were larger for upper-canopy leaves, indicating an important interaction between drought stress and canopy position. The seasonal variation in leaf gas exchange and morphology was presumably due to a combination of drought stress and leaf lifespan. The results of this study suggest that drought has important implications for the leaf physiology and morphology of semi-deciduous Amazonian forest trees.

Leaf and fruiting phenology and gas exchange of Mangabeira in response to irrigation

The mangabeira (Hancornia speciosa Gomez) is a native species of Brazil occurring in the savanna (Cerrado) and Atlantic Coastal forests. It is a promising species for large-scale horticulture due to the high-protein properties of its fruits, but extensive cultivation has not yet been accomplished. Research is still needed to determine cultivation techniques that will optimize productivity. As irrigation is considered to be the most important agronomic technique to enhance productivity, the goal of this investigation was to evaluate the effects of irrigation on leaf gas exchange (net photosynthesis rate and transpiration rate) and foliar and fruiting phenology. Our results suggest that irrigation acts by increasing the growth and fruit production of mangabeira due to its effects on phenology. Gas exchange of adult plants was only marginally affected by irrigation; however, irrigated plants retained leaves longer than non-irrigated plants during the dry season, which increased the C gain over an annual period. The increase in C gain presumably led to irrigated plants having a higher relative frequency of fruited plants than the non-irrigated. A significant positive correlation between net photosynthesis rate per unit dry mass and specific leaf area was found for irrigated plants, which suggests that irrigation can promote the development of new leaf cells with no direct contribution to

Large net CO2 loss from a grass‐dominated tropical savanna in south‐central Brazil in response to seasonal and interannual drought

The savanna vegetation of Brazil (Cerrado) accounts for 20–25% of the land cover of Brazil and is the second largest ecosystem following Amazonian forest; however, Cerrado mass and energy exchange is still highly uncertain. We used eddy covariance to measure the net ecosystem CO2 exchange (NEE) of grass-dominated Cerrado (campo sujo) over 3 years. We hypothesized that soil water availability would be a key control over the seasonal and interannual variations in NEE. Multiple regression indicated that gross primary production (GPP) was positively correlated (Pearsons r = 0.69; p < 0.001) with soil water content, radiation, and the Moderate Resolution Imaging Spectroradiometer (MODIS)-derived enhanced vegetation index (EVI) but negatively correlated with the vapor pressure deficit (VPD), indicating that drier conditions increased water limitations on GPP. Similarly, ecosystem respiration (Reco) was positively correlated (Pearsons r = 0.78; p < 0.001) with the EVI, radiation, soil water content, and temperature but slightly negatively correlated with rainfall and the VPD. While the NEE responded rapidly to temporal variations in soil water availability, the grass-dominated Cerrado stand was a net source of CO2 to the atmosphere during the study period, which was drier compared to the long-term average rainfall. Cumulative NEE was approximately 842 gC m−2, varying from 357 gC m−2 in 2011 to 242 gC m−2 in 2012. Our results indicate that grass-dominated Cerrado may be an important regional CO2 source in response to the warming and drying that is expected to occur in the southern Amazon Basin under climate change.

DINÂMICA DOS ATRIBUTOS FÍSICO-QUÍMICOS E VARIAÇÃO SAZONAL DOS ESTOQUES DE CARBONO NO SOLO EM DIFERENTES FITOFISIONOMIAS DO PANTANAL NORTE MATO-GROSSENSE

The Pantanal biome may be considered a hyperseasonal environment where there is a flooded period followed by an extended dry period with very high temperatures. The diversity of vegetation types modify the input of carbon into the soil in different quantity and quality. With this focus, this study aimed to verify the effect of the hydrological Pantanal Cycle (rising, flood, low tide and dry season), on the dynamics of soil organic carbon stock in different vegetation types of Cambarazal, Riparian Forest, Mountain Range and Thorn-Scrub. Principal component analysis (PCA) identified three distinct groups, characterized by nutrient reserves and by soil acidity. The carbon stock of the soil is conditioned by seasonal factors and equally dependent on the local vegetation type that occurred in vegetation types that showed the highest level of natural fertility, respectively, Cambarazal and Thorn-Scrub. The highest carbon content was observed during the flood period, for most vegetation types and the lowest carbon stock occurred during the period of low tide in 2008, just after a period of anaerobiosis due to the flood and before the dry season became predominant.

Influência da escarificação e da temperatura sobre a germinação de sementes de Samanea tubulosa (Benth.) Barneby & J.W. Grimes (sete cascas)

Samanea tubulosa (Benth.) Barneby & J.W Grimes (sete cascas) e uma planta arborea nativa do Pantanal Matogrossense, cujas sementes possuem dormencia provavelmente causada pela impermeabilidade do tegumento a agua. O presente trabalho foi conduzido com o objetivo de comparar a eficiencia da escarificacao mecânica e quimica para superar a dormencia tegumentar e o efeito da temperatura sobre a germinacao de sementes de sete cascas. Foram empregados quatro diferentes procedimentos de escarificacao: testemunha (sem escarificacao); a escarificacao mecânica; a escarificacao com acido sulfurico durante cinco minutos e a escarificacao com acido sulfurico durante dez minutos. Os niveis de temperatura empregados foram de 20, 25, 30 e 35oC. A semeadura foi realizada em papel toalha germitest, na forma de rolo. O delineamento experimental adotado foi o inteiramente casualizado com os tratamentos em esquema fatorial 4x4. Para cada tratamento, foram utilizadas quatro repeticoes de 25 sementes. Foi avaliada a porcentagem de germinacao das sementes, utilizado como criterio emissao da raiz primaria com 2 mm de comprimento. Foi observado que a especie S. tubulosa possui dormencia tegumentar causada pela impermeabilidade do tegumento a agua. As escarificacoes quimica com acido sulfurico por cinco e dez minutos foram eficientes para superacao da dormencia e as combinacoes de escarificacao com temperatura que promoveram maiores porcentagens de germinacao para a especie foram a escarificacao com imersao em acido sulfurico durante cinco e dez minutos e as temperaturas de 25, 30 e 35oC.