Through litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order to understand the controls on the terrestrial carbon transfer to the atmosphere. However, previous studies were mostly based on site-specific litter and methodologies, adding major uncertainty to syntheses, comparisons and meta-analyses across different experiments and sites. In the TeaComposition initiative, the potential litter decomposition is investigated by using standardized substrates (Rooibos and Green tea) for comparison of litter mass loss at 336 sites (ranging from -9 to +26 °C MAT and from 60 to 3113 mm MAP) across different ecosystems. In this study we tested the effect of climate (temperature and moisture), litter type and land-use on early stage decomposition (3 months) across nine biomes. We show that litter quality was the predominant controlling factor in early stage litter decomposition, which explained about 65% of the variability in litter decomposition at a global scale. The effect of climate, on the other hand, was not litter specific and explained <0.5% of the variation for Green tea and 5% for Rooibos tea, and was of significance only under unfavorable decomposition conditions (i.e. xeric versus mesic environments). When the data were aggregated at the biome scale, climate played a significant role on decomposition of both litter types (explaining 64% of the variation for Green tea and 72% for Rooibos tea). No significant effect of land-use on early stage litter decomposition was noted within the temperate biome. Our results indicate that multiple drivers are affecting early stage litter mass loss with litter quality being dominant. In order to be able to quantify the relative importance of the different drivers over time, long-term studies combined with experimental trials are needed.

[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.

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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.

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.

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 (Amax) and stomatal conductance (gmax) 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 CO2 concentration (Ci) was >30% lower during the dry season suggesting that declines in Amax were caused in part by stomatal limitations to CO2 diffusion. For other species, such as Brosimum lactescens growing at the top of the canopy, Tovomita schomburgkii growing in the mid-canopy, and Dinizia excelsa growing in the understorey, dry season Ci declined by <20% suggesting that factors independent of CO2 diffusion were more important in limiting Amax. Dry-season declines in gmax 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.

[1] This study analyzed how seasonal and interannual variations in climate alter litter dynamics, including production, decomposition, and accumulation. Monthly measurements of leaf, stem, and reproductive (flower plus fruit) litter and the forest floor litter mass were combined with a mass balance model to determine rates of litter decomposition for a semideciduous tropical forest located in the rain forest-savanna ecotone of the southern Amazon Basin for [2001][2002][2003][2004][2005][2006][2007]. Annual rates of litter production varied between 8 and 10.5 Mg ha À1 a À1 , and leaf litter production accounted for the majority ($70%) of the total litter production. Leaf litter production peaked at the end of the May-August dry season while stem litter production peaked during the wet season and reproductive litter production peaked during the dry-wet season transition. Forest floor litter mass ranged between 5 and 8 Mg ha À1 over the study period and generally declined as litter inputs declined. Litter decomposition rates were remarkably stable from year-toyear and varied between 10.8 and 12.4 Mg ha À1 a À1 . On average, rates of litter decomposition were highest during the dry-wet season transition. Overall, our results suggest that rainfall variability directly altered litter production dynamics and indirectly altered forest floor litter mass and decomposition kinetics through its effect on litter production. Future changes in seasonal and/or interannual rainfall patterns, whether in response to El Niño or to anthropogenic climate change, will likely have important consequences for the litter dynamics of Amazonian semideciduous forest.

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Variations in Stand Structure and Diversity along a Soil Fertility Gradient in a Brazilian Savanna (Cerrado) in Southern Mato Grosso Forest, Range & Wildland Soils B razilian savanna, locally known as cerrado, covers approximately 20 to 25% of the total land cover of Brazil and is the second largest vegetation type following Amazonian forest (Furley and Ratter, 1988). Cerrado is composed of distinctive physiognomies that vary as a function of height, cover, and/ or density of trees (Goodland, 1971; Eiten, 1972; Furley and Ratter, 1988). The factors that affect the physiognomy and distribution of cerrado remain a subject of debate; however, seasonal variation in rainfall, soil fertility and drainage, and fire are considered the most important (Eiten, 1972; Furley and Ratter, 1988; Lopes and Cox, 1977). In terms of soil properties, variations in soil texture, water holding capacity, and chemical properties, such as pH and Al 3+ concentration, have been found to be important variables affecting cerrado physiognomy and tree species distribution (Lopes and Cox, 1977; Furley and Ratter, 1988; de Souza et al., 2007; de Assis et al., 2011). Nutrient limitation has been implicated as a primary factor inhibiting the development of forests in tropical savanna, and across large-scale fertility gradients, an increase in soil fertility can lead to an increase in the production of woody vegetation, and the density and cover of trees (Goodland and Pollard, 1973; Lopes