ABSTRACT Intra-annual distribution of precipitation in central Amazonia often leads to a short mild dry season and an increase in irradiance, temperature, and vapor pressure deficit; however, the accurate effect of intra-annual microclimatic variability on stem growth is still under investigation. The objective of this study was to determine how stem growth responds to monthly variations in microclimatic factors in the central Amazon. During five years (2008-2012) we measured diameter stem growth of 109 trees (26 species) and used principal component regression to evaluate the effect of microclimatic variability on stem growth in diameter. We found that the mean stem growth in diameter across species increased in response to an increase in rainfall and reference evapotranspiration, but it decreased with a rise in mean and minimum vapor pressure deficit. A contribution of this study is to show that even when irradiance and temperature had no significant effect on stem growth, small changes in vapor pressure deficit significantly affect stem growth. If the dry season becomes longer, as predicted by models, trees currently more sensitive to microclimatic variability associated with droughts would be the most affected by climate changes.

A bacia amazônica tem mais de seis milhões de quilômetros quadrados e abriga a maior floresta tropical do mundo, sendo particularmente importante pela sua biodiversidade e pelo seu papel na ciclagem de água e carbono. Fotossíntese, condutância estomática e fluxo de seiva de espécies florestais da Amazônia apresentam variação ao longo do dia seguindo a variação diurna observada na irradiância, temperatura e o déficit de pressão de vapor. Em decorrência da fotorrespiração, cerca de 25% do carbono fixado é retornado para a atmosfera. Os aumentos na concentração de CO2 na atmosfera previstos para as próximas décadas poderão apresentar efeito positivo na assimilação de carbono deste ecossistema florestal. Em comparação à época chuvosa, redução da umidade do solo e aumento no déficit de pressão de vapor (associado à baixa umidade do ar e alta temperatura) favorecem o fechamento dos estômatos em detrimento da fotossíntese. Desse modo, em comparação com a estação chuvosa, a fotossíntese líquida é menor no período seco. No geral, na Amazônia as árvores que atingem o dossel da floresta crescem a taxas maiores na época chuvosa. Exceto em anos de menor precipitação, o ecossistema florestal atua como sumidouro de carbono na época chuvosa. Mais estudos são necessários para determinar como e de que forma fatores específicos do ambiente físico influenciam a assimilação de carbono e o crescimento de árvores nos diversos grupos funcionais na Amazônia.

The Amazon basin comprises more than six million square kilometers and holds the largest tropical forest in the world. It is particularly important for its biodiversity and for its role in the cycling of water and carbon. Photosynthesis, stomatal conductance and sap flow of Amazon tree species show variation throughout the day following the diurnal variation of irradiance, temperature and vapor pressure deficit. Due to photorespiration, at least 25% of the fixed carbon is returned to the atmosphere. Thus, increases in atmospheric CO2 concentration in the decades to come may have a positive effect on carbon assimilation of the forest ecosystem. Compared to the rainy season, low water availability in dry season and increased vapor pressure deficit (low humidity and high temperature) during the dry period induce stomata closure, which eventually leads to photosynthesis decline. Several studies have shown that Amazonian trees that reach the forest canopy grow at higher rates in the rainy season. Except in years with low rainfall, the forest ecosystem is a carbon sink in the rainy season. More studies are needed to determine how and in what extent specific factors of the physical environment influence carbon assimilation and growth of trees from different functional groups in the Amazon region.

Most Amazonian soils are highly weathered and poor in nutrients. Therefore, photosynthesis and plant growth should positively respond to the addition of mineral nutrients. Surprisingly, no study has been carried out in situ in the central Amazon to address this issue for juvenile trees. The objective of this study was to determine how photosynthetic rates and growth of tree saplings respond to the addition of mineral nutrients, to the variation in leaf area index of the forest canopy, and to changes in soil water content associated with rainfall seasonality. We assessed the effect of adding a slow-release fertilizer. We determined plant growth from 2010 to 2012 and gas exchange in the wet and dry season of 2012. Rainfall seasonality led to variations in soil water content, but it did not affect sapling growth or leaf gas exchange parameters. Although soil amendment increased phosphorus content by 60 %, neither plant growth nor the photosynthetic parameters were influenced by the addition of mineral nutrients. However, photosynthetic rates and growth of saplings decreased as the forest canopy became denser. Even when Amazonian soils are poor in nutrients, photosynthesis and sapling growth are more responsive to slight variations in light availability in the forest understory than to the availability of nutrients. Therefore, the response of saplings to future increases in atmospheric [CO2] will not be limited by the availability of mineral nutrients in the soil.

Estômatos são válvulas operadas a turgor que controlam a perda de água e a captura de CO2 durante a fotossíntese. Assim, as relações hídricas e o acumulo de biomassa vegetal são fortemente influenciadas pelo funcionamento estomático. Os objetivos deste trabalho foram: documentar como os estômatos estão distribuídos na superfície foliar e determinar se existe variação das características estomáticas entre espécies da Amazônia, estudar a relação entre densidade estomática (S D) e altura arbórea. Trinta e cinco árvores (>17 m de altura) de diferentes espécies foram selecionadas. Tipo de complexo estomático, S D, tamanho (S S) e distribuição na superfície foliar foram determinados utilizando impressões de ambas as superfícies foliares com esmalte incolor. Independente da espécie, os estômatos foram encontrados apenas na superfície abaxial (hipoestomatia) com ampla variação na S D e no S S entre espécies. A densidade estomática variou de 110 mm-2 em Neea altissima a 846 mm-2 em Qualea acuminata. Entretanto, a maioria das espécies apresentou S D entre 271 e 543 mm-2, com uma relação negativa entre S D e S S. Observou-se uma relação positiva entre S D e altura arbórea (r² = 0.14, p < 0.01), não havendo relação entre S D e espessura foliar. Os tipos estomáticos mais comuns foram: anomocíticos (37%), seguidos de paracíticos (26%) e anisocíticos (11%). Concluiu-se que em espécies da Amazônia, a distribuição de estômatos na superfície foliar está mais relacionada a fatores genéticos de cada espécie do que a variações ambientais. Entretanto, S D é fortemente influenciada por fatores ambientais concernentes à altura da árvore.

Stomata are turgor-operated valves that control water loss and CO2 uptake during photosynthesis, and thereby water relation and plant biomass accumulation is closely related to stomatal functioning. The aims of this work were to document how stomata are distributed on the leaf surface and to determine if there is any significant variation in stomatal characteristics among Amazonian tree species, and finally to study the relationship between stomatal density (S D) and tree height. Thirty five trees (>17 m tall) of different species were selected. Stomatal type, density (S D), size (S S) and stomatal distribution on the leaf surface were determined using nail polish imprints taken from both leaf surfaces. Irrespective of tree species, stomata were located only on the abaxial surface (hypostomaty), with large variation in both S D and S S among species. S D ranged from 110 mm-2 in Neea altissima to 846 mm-2 in Qualea acuminata. However, in most species S D ranges between 271 and 543 mm-2, with a negative relationship between S D and S S. We also found a positive relationship between S D and tree height (r² = 0.14, p < 0.01), but no correlation was found between S D and leaf thickness. The most common stomatal type was anomocytic (37%), followed by paracytic (26%) and anisocytic (11%). We conclude that in Amazonian tree species, stomatal distribution on the leaf surface is a response most likely dependent on the genetic background of every species, rather than a reaction to environmental changes, and that somehow S D is influenced by environmental factors dependent on tree height.