Foliar water uptake in Amazonian trees: Evidence and consequences.

The absorption of atmospheric water directly into leaves enables plants to alleviate the water stress caused by low soil moisture, hydraulic resistance in the xylem and the effect of gravity on the water column, while enabling plants to scavenge small inputs of water from leaf-wetting events. By increasing the availability of water, and supplying it from the top of the canopy (in a direction facilitated by gravity), foliar uptake (FU) may be a significant process in determining how forests interact with climate, and could alter our interpretation of current metrics for hydraulic stress and sensitivity. FU has not been reported for lowland tropical rainforests; we test whether FU occurs in six common Amazonian tree genera in lowland Amazônia, and make a first estimation of its contribution to canopy-atmosphere water exchange. We demonstrate that FU occurs in all six genera and that dew-derived water may therefore be used to "pay" for some morning transpiration in the dry season. Using meteorological and canopy wetness data, coupled with empirically derived estimates of leaf conductance to FU (kfu ), we estimate that the contribution by FU to annual transpiration at this site has a median value of 8.2% (103 mm/year) and an interquartile range of 3.4%-15.3%, with the biggest sources of uncertainty being kfu and the proportion of time the canopy is wet. Our results indicate that FU is likely to be a common strategy and may have significant implications for the Amazon carbon budget. The process of foliar water uptake may also have a profound impact on the drought tolerance of individual Amazonian trees and tree species, and on the cycling of water and carbon, regionally and globally.

Limited acclimation in leaf anatomy to experimental drought in tropical rainforest trees.

Dry periods are predicted to become more frequent and severe in the future in some parts of the tropics, including Amazonia, potentially causing reduced productivity, higher tree mortality and increased emissions of stored carbon. Using a long-term (12 year) through-fall exclusion (TFE) experiment in the tropics, we test the hypothesis that trees produce leaves adapted to cope with higher levels of water stress, by examining the following leaf characteristics: area, thickness, leaf mass per area, vein density, stomatal density, the thickness of palisade mesophyll, spongy mesophyll and both of the epidermal layers, internal cavity volume and the average cell sizes of the palisade and spongy mesophyll. We also test whether differences in leaf anatomy are consistent with observed differential drought-induced mortality responses among taxa, and look for relationships between leaf anatomy, and leaf water relations and gas exchange parameters. Our data show that trees do not produce leaves that are more xeromorphic in response to 12 years of soil moisture deficit. However, the drought treatment did result in increases in the thickness of the adaxial epidermis (TFE: 20.5 ± 1.5 µm, control: 16.7 ± 1.0 µm) and the internal cavity volume (TFE: 2.43 ± 0.50 mm3 cm-2, control: 1.77 ± 0.30 mm3 cm-2). No consistent differences were detected between drought-resistant and drought-sensitive taxa, although interactions occurred between drought-sensitivity status and drought treatment for the palisade mesophyll thickness (P = 0.034) and the cavity volume of the leaves (P = 0.025). The limited response to water deficit probably reflects a tight co-ordination between leaf morphology, water relations and photosynthetic properties. This suggests that there is little plasticity in these aspects of plant anatomy in these taxa, and that phenotypic plasticity in leaf traits may not facilitate the acclimation of Amazonian trees to the predicted future reductions in dry season water availability.

Plasticity in leaf-level water relations of tropical rainforest trees in response to experimental drought.

The tropics are predicted to become warmer and drier, and understanding the sensitivity of tree species to drought is important for characterizing the risk to forests of climate change. This study makes use of a long-term drought experiment in the Amazon rainforest to evaluate the role of leaf-level water relations, leaf anatomy and their plasticity in response to drought in six tree genera. The variables (osmotic potential at full turgor, turgor loss point, capacitance, elastic modulus, relative water content and saturated water content) were compared between seasons and between plots (control and through-fall exclusion) enabling a comparison between short- and long-term plasticity in traits. Leaf anatomical traits were correlated with water relation parameters to determine whether water relations differed among tissues. The key findings were: osmotic adjustment occurred in response to the long-term drought treatment; species resistant to drought stress showed less osmotic adjustment than drought-sensitive species; and water relation traits were correlated with tissue properties, especially the thickness of the abaxial epidermis and the spongy mesophyll. These findings demonstrate that cell-level water relation traits can acclimate to long-term water stress, and highlight the limitations of extrapolating the results of short-term studies to temporal scales associated with climate change.

Variação temporal do efluxo de CO2do solo em sistemas agroflorestais com palma de óleo na Amazônia Oriental / Temporal variation of soil CO2 efflux in oil palm-based agroforestry systems in eastern Amazon

The soil carbon dioxide (CO2) efflux dynamics and its controlling factors of Amazonian agroforestry systems are poorly understood. The objective of this study was to evaluate the temporal variation of soil CO2 efflux in oil palm-based agroforestry systems and the relation between efflux and biotic (microbial and total soil carbon, microbial respiration, fine roots, individual components of agroforestry systems (AFS)) and abiotic factors (soil moisture and temperature). The measurements were taken during the less rainy (December 2010) and rainy (May 2011) periods. The soil CO2 efflux was highest during the rainy season, probably due to increased microbial activity influenced by climatic factors coupled with biotic factors. The soil CO2 efflux correlated positively with soil moisture and microbial biomass carbon and negatively with soil temperature and metabolic quotient, but these correlations were weak. The soil CO2 efflux was sensitive to the type of agroforestry system and to rainfall seasonality.

A dinâmica do efluxo de dióxido de carbono (CO2) do solo e seus fatores controladores em sistemas agroflorestais da Amazônia são pouco compreendidas. O objetivo deste estudo foi avaliar a variação temporal do efluxo de CO2 do solo em sistemas agroflorestais onde a palma de óleo é a cultura principal e sua relação com fatores bióticos (carbono microbiano do solo, carbono total do solo, respiração microbiana do solo, raízes finas do solo, indivíduos componentes dos sistemas agroflorestais (SAFs) ) e abióticos (umidade e temperatura do solo). As medições foram realizadas nos períodos menos chuvoso (dezembro de 2010) e chuvoso (maio de 2011). O efluxo de CO2 do solo foi mais alto no período chuvoso, provavelmente, devido à maior atividade microbiana nesse período influenciada por fatores climáticos aliados a fatores bióticos. O efluxo de CO2do solo se correlacionou positivamente com umidade do solo e carbono da biomassa microbiana e negativamente com temperatura do solo e quociente metabólico, porém as correlações foram fracas. O efluxo de CO2do solo foi sensível ao tipo de sistema agroflorestal e a sazonalidade da precipitação.

Frações de fósforo e simbiose micorrízica em floresta secundária em resposta a disponibilidade de água e nutrientes na Amazônia oriental / Soil phosphorus fractions and mycorrhizal symbiosis in response to the availability of moisture and nutrients at a secondary forest in eastern Amazonia

The availability of soil nutrients (especially phosphorus) and soil water strongly influence mycorrhizal symbiosis in forest ecosystems. The aim of this study was to evaluate the impact of litter removal and increased soil moisture availability on the spore density and mycorrhizal colonization of apogeotropic and non-apogeotropic roots of a secondary forest stand in eastern Amazonia. We determined the percentage of mycorrhizal colonization of apogeotropic and non-apogeotropic (0-10 cm soil depth) fine roots (diameter 2 mm), spore density, glomalin concentration, and soil phosphorus availability. Litter removal did not reduce soil phosphorus availability. The spore density was not affected by litter removal. In general, the variables varied on a seasonal basis, except the mycorrizal colonization of non-apogetropic roots and glomalin concentration, but were not affected by irrigation. Mechanical damage to the apogeotropic root system, inherent to the fortnightly litter removal, may have contributed to decrease mycorrhizal colonization of both apogeotropic and non-apogeotropic roots and, consequently, soil glomalin. Our results suggest that the reduction of soil cover may have negative impact over the fungus-plant symbiosis.

A disponibilidade de nutrientes, especialmente fósforo, e de água no solo influenciam fortemente a simbiose micorrízica em ecossistemas florestais. O objetivo do estudo foi avaliar o impacto da remoção de serapilheira e do aumento da disponibilidade de água no solo sobre a densidade de esporos e a colonização micorrízica em raízes apogeotrópicas e não-apogeotrópicas em floresta secundária na Amazônia oriental. Foram analisadas a porcentagem de colonização micorrízica de raízes finas (diâmetro ≤ 2 mm) apogeotrópicas e não-apogeotrópicas (presentes na camada de 0-10 cm do solo), a densidade de esporos, o teor de glomalina e a disponibilidade de fósforo no solo. A remoção de serapilheira não reduziu a disponibilidade de fósforo no solo. A densidade de esporos também não foi afetada pela remoção de serapilheira. De forma geral, as variáveis estudadas variaram sazonalmente, com exceção da colonização micorrízica em raízes não-apogeotrópicas e do teor de glomalina, mas não foram afetadas pela alteração na disponibilidade de água no solo decorrente da irrigação. Danos mecânicos ao sistema radicular apogeotrópico, inerentes à remoção quinzenal da serapilheira, devem ter contribuído para reduzir a colonização micorrízica em raízes apogeotrópicas e não-apogeotrópicas e, consequentemente, o teor de glomalina no solo. Os resultados deste estudo sugerem que a redução da cobertura do solo pode impactar negativamente a simbiose fungo-planta.

Nitrogênio mineral e microbiano do solo em sistemas agroflorestais com palma de óleo na Amazônia oriental / Soil mineral and microbial nitrogen in oil palm-based agroforestry systems in eastern Amazon

The success of oil palm (Elaeis guineensis Jacq.)-based agroforestry systems (oil palm-AFS) depends on sustainable soil management, especially of soil chemical and microbiological characteristics. Our objective was to evaluate the impact of oil palm-AFS on soil mineral and microbial nitrogen (N) in contrasting rainfall seasons. We evaluated different soil nitrogen (N) forms (microbial-N, nitrate, ammonium) and soil carbon concentration in oil palm-AFS with low and high diversity of species planted, which were compared with an adjacent 13-yr-old secondary forest. Most variables (total N, C:N ratio, microbial-N, microbial-N:total N ratio, ammonium, and net nitrification rate) varied only in response to rainfall seasonality. Soil C was significantly higher in the high diversity AFS (15.6 mg g-1) than in the secondary forest (13.0 mg g-1). In the rainy season, nitrate concentration (5.1 mg N kg-1 soil) was higher in the high diversity AFS than in other vegetation types; consequently, the average soil ammonium concentration (9.6 mg N kg-1 soil) was significantly lower in the high diversity AFS. Net N mineralization in the low diversity AFS (0.1 mg N kg-1 soil day-1) in the dry season was significantly lower than in other vegetation types. The soil variables were more sensitive to the rainfall seasonality than to the conversion of secondary forest to oil palm-based agroforestry systems.

O sucesso da produção de palma de óleo (Elaeis guineensis Jacq.) em sistemas agroflorestais (SAFs) na Amazônia está condicionado ao manejo sustentável do solo, em especial dos atributos químicos e microbiológicos. Nosso objetivo foi avaliar o impacto de SAFs com palma de óleo sobre os teores de nitrogênio (N) mineral e microbiano do solo, em função da sazonalidade pluviométrica. Avaliamos diferentes formas de N do solo (N microbiano, nitrato, amônio), além da concentração de carbono (C) do solo, em SAFs com baixa e alta diversidade de espécies cultivadas, os quais foram comparados com uma floresta secundária adjacente de 13 anos de idade. Para a maioria das variáveis analisadas (N total, relação C:N, N microbiano, relação N microbiano:N total, amônio) houve diferença significativa somente entre as épocas de amostragem. A concentração média de C no solo sob o SAF com alta diversidade (15,6 mg g-1) foi maior do que na floresta (13 mg g-1). Na época chuvosa a concentração de nitrato no SAF com alta diversidade foi 5,1 mg (N) kg-1 (solo), maior do que nos demais tipos de vegetação, resultando em menor concentração média de amônio no solo sob esse SAF, cujo valor foi 9,6 mg (N) kg-1 (solo). A mineralização do N no SAF com baixa diversidade, na época seca, foi 0,1 mg (N) kg-1 (solo) dia-1, menor do que nos demais tipos de vegetação. As variáveis analisadas foram mais sensíveis à sazonalidade da precipitação pluviométrica do que à conversão de floresta secundária em SAFs com palma de óleo.

Fine-root production in two secondary forest sites with distinct ages in Eastern Amazon / Produção de raízes finas em dois sítios de floresta secundária com diferentes idades na Amazônia Oriental

The objective of this work was to assess the fine-root (≤ 2 mm diameter) production dynamics of two forest regrowths at different ages. Fine-root production was monitored by the ingrowth core method in one 18-year-old site (2 ha) and one 10-year-old site (0.5 ha), both localized in the Apeú region, Northern Pará State, Brazil. The sites were abandoned after successive shifting cultivation, beginning in 1940. Monthly production of live fine-root was similar between sites and was influenced by rainfall seasonality, with higher production during the dry season than the wet season for mass and length. However, mortality in terms of mass was higher in the 10-year-old site than in the 18-year-old site. The seasonality influenced mortality only in the 18-year old site following the pattern observed for live fine-root. The influence seasonal on mortality in terms of length was different between sites, with higher mortality during the wet season in the 10-year-old site and higher mortality during the dry season in the 18-year-old site. Specific root length was higher during the wet season and at the 10-year-old site. Fine-root production was not influenced by the chronosequence of the sites studied, probably fine-root production may have already stabilized in the sites or it depended more on climate and soil conditions. The production of fine-roots mass and length were indicators that generally showed the same pattern.

O objetivo deste trabalho foi avaliar a dinâmica de produção de raízes finas (diâmetro ≤ 2 mm) em duas áreas de floresta secundária com diferentes idades. A produção de raízes finas foi monitorada utilizando a técnica de ingrowth core em um sítio com 18 anos de idade (2 ha) e um outro sítio com 10 anos de idade (0,5 ha), localizados na região de Apeú, nordeste do Estado do Pará. Os sítios foram abandonados depois de sucessivos ciclos agrícolas, iniciados em 1940. A produção mensal de raízes vivas foi semelhante entre os sítios e influenciada pela sazonalidade pluviométrica, com maior produção durante a estação seca para massa e comprimento. No entanto, a mortalidade, em termos de massa, foi maior no sítio de 10 anos. A sazonalidade influenciou a mortalidade somente no sítio de 18 anos, seguindo o padrão observado para as raízes vivas. A influência sazonal sobre a mortalidade em termos de comprimento foi diferente entre os sítios, com maior mortalidade durante a estação chuvosa no sítio de 10 anos e maior mortalidade durante a estação seca no sítio de 18 anos. O comprimento radicular específico foi maior durante a estação chuvosa e influenciado pelos sítios, sendo maior no sítio de 10 anos. A produção de raízes finas não foi influenciada pela cronossequência dos sítios estudados, provavelmente porque a produção de raízes finas pode ter estabilizado nos sítios ou depende mais das condições de clima e solo. A produção, em termos de massa e comprimento, foram indicadores que geralmente mostraram o mesmo padrão.

Effects of water and nutrient availability on fine root growth in eastern Amazonian forest regrowth, Brazil.

*Fine root dynamics is widely recognized as an important biogeochemical process, but there are few data on fine root growth and its response to soil resource availability, especially for tropical forests. *We evaluated the response of fine root dynamics to altered availability of soil water and nutrients in a 20-yr-old forest regrowth in eastern Amazonia. In one experiment the dry season reduction in soil moisture was alleviated by irrigation. In the other experiment, nutrient supply was reduced by litter removal. We used the ingrowth core technique to measure fine root mass growth, length growth, mortality and specific root length. *Dry-season irrigation had no significant effect on mass and length of live and dead roots, whereas litter removal reduced mass and length of live roots. For both irrigation and litter removal experiments, root growth was significantly greater in the dry season than in the wet season. *Increased root growth was associated with decreased soil water availability. However, root growth did not increase in response to nutrient reduction in litter removal plots. Overall, our results suggest that belowground allocation may differ according to the type of soil resource limitation.

Biomass equations for forest regrowth in the eastern Amazon using randomized branch sampling / Equações alométricas para estimativa de biomassa de floresta secundária na Amazônia Oriental usando amostragem aleatória de ramos

Forest regrowth occupies an extensive and increasing area in the Amazon basin, but accurate assessment of the impact of regrowth on carbon and nutrient cycles has been hampered by a paucity of available allometric equations. We develop pooled and species-specific equations for total aboveground biomass for a study site in the eastern Amazon that had been abandoned for 15 years. Field work was conducted using randomized branch sampling, a rapid technique that has seen little use in tropical forests. High consistency of sample paths in randomized branch sampling, as measured by the standard error of individual paths (14 percent), suggests the method may provide substantial efficiencies when compared to traditional procedures. The best fitting equations in this study used the traditional form Y=a×DBHb, where Y is biomass, DBH is diameter at breast height, and a and b are both species-specific parameters. Species-specific equations of the form Y=a(BA×H), where Y is biomass, BA is tree basal area, H is tree height, and a is a species-specific parameter, fit almost as well. Comparison with previously published equations indicated errors from -33 percent to +29 percent would have occurred using off-site relationships. We also present equations for stemwood, twigs, and foliage as biomass components.

Florestas secundárias ocupam uma área extensa e crescente na bacia Amazônica, porém determinações acuradas do impacto dessas florestas nos ciclos de carbono e nutrientes têm sido dificultadas pelo número reduzido de equações alométricas. Neste estudo, nós desenvolvemos equações em nível de comunidade e espécies individuais para estimar a biomassa total da parte aérea de uma floresta secundária com 15 anos de idade na Amazônia oriental. O trabalho de campo utilizou amostragem aleatória de ramos, que é uma técnica rápida, porém pouco utilizada em florestas tropicais. Baseada no erro padrão da série de segmentos individuais (14 por cento), a consistência da série de segmentos totais amostrados foi considerada elevada, sugerindo que o método pode ser eficiente em comparação com procedimentos tradicionais. Os melhores ajustes foram obtidos com a equação tradicional Y=a×DBHb, onde Y é a biomassa, DBH é o diâmetro à altura do peito, e a e b são parâmetros para cada espécie arbórea. Ajustes razoáveis também foram alcançados com equações da forma Y=a(BA×H), onde Y é a biomassa, BA é a área basal, H é a altura e a é um parâmetro específico para cada espécie arbórea. Comparações com equações disponíveis na literatura indicaram uma faixa de erro provável de -33 por cento a +29 por cento usando-se relações desenvolvidas para outros sítios. Nós também apresentamos equações para os seguintes componentes da biomassa da parte aérea: tronco, ramos e folhas.