Changes in the spatial distribution of tree species in a terra firme rain forest in Brazilian Amazonia after logging.

The objective of this paper was to determine changes in the spatial distribution of tree species in a logged compared to an unlogged forest of the Tapajos National Forest in the municipality of Belterra, State of Para, Brazil, over an eight-year period. The distribution pattern was determined for trees> 5 cm dbh and, also, for trees > 30 cm dbh. The relationship (a quadrate method) discussed by McGinnis was selected to be used in this study. Forty-seven percent of species with trees > 5 cm dbh showed clumped distribution in the studied forests. Geissospermwn sericeunz Benth & Hook., Minquartia guianensis Aubl., Poureria bilocularis (H. Winkler) Bachni, Protium guacayantan Cuatrec, Sclerolobium chrysophyllunz Poepp. et Endl. and the Sapotaceae family (9 species) occurred in clumps of small trees (5 cm 5 dbh < 30 cm) and big trees (dbh > 30 cm) in both the logged and undisturbed forest. Trees in all sizes of these species certainly have aggregation characteristics in different light condition's during the whole growth-cycle. Only Sclerolobium cizzysophylltan out of fourteen species that occurred aggregated in all forest conditions was light demanding. The shade-tolerant Lecythis lurida (Miers) Mori and Manilkara huberi (Ducke) Stand!. showed also aggregated distribution for small and big trees in the unlogged forest. An aggregated distribution is not always directly correlated to abundance, considering that most of the clumped species had less than seven trees per hectare.

Amazon forest structure generates diurnal and seasonal variability in light utilization.

The complex three-dimensional (3-D) structure of tropical forests generates a diversity of light environments for canopy and understory trees. Understanding diurnal and seasonal changes in light availability is critical for interpreting measurements of net ecosystem exchange and improving ecosystem models. Here, we used the Discrete Anisotropic Radiative Transfer (DART) model to simulate leaf absorption of photosynthetically active radiation (lAPAR) for an Amazon forest. The 3-D model scene was developed from airborne lidar data, and local measurements of leaf reflectance, aerosols, and PAR were used to model lAPAR under direct and diffuse illumination conditions. Simulated lAPAR under clear-sky and cloudy conditions was corrected for light saturation effects to estimate light utilization, the fraction of lAPAR available for photosynthesis. Although the fraction of incoming PAR absorbed by leaves was consistent throughout the year (0.80?0.82), light utilization varied seasonally (0.67?0.74), with minimum values during the Amazon dry season. Shadowing and light saturation effects moderated potential gains in forest productivity from increasing PAR during dry-season months when the diffuse fraction from clouds and aerosols was low. Comparisons between DART and other models highlighted the role of 3-D forest structure to account for seasonal changes in light utilization. Our findings highlight how directional illumination and forest 3-D structure combine to influence diurnal and seasonal variability in light utilization, independent of further changes in leaf area, leaf age, or environmental controls on canopy photosynthesis. Changing illumination geometry constitutes an alternative biophysical explanation for observed seasonality in Amazon forest productivity without changes in canopy phenology.

Periodicidade do crescimento de espécies arbóreas da Floresta Estacional Semidecidual no Sul do Brasil.

Embora a sazonalidade climática seja menos evidente nas regiões tropicais que nas temperadas, muitas espécies tropicais apresentam crescimento rítmico. A avaliação do crescimento em circunferência do tronco (CCT) permitirá obter informações sobre o desenvolvimento dos indivíduos de espécies arbóreas desta região. Esse estudo teve como objetivo avaliar o CCT de 11 espécies arbóreas de uma floresta estacional semidecidual no sul do Brasil, visando testar duas hipóteses: (i) existe sazonalidade no CCT das árvores e este crescimento está relacionado às variações climáticas da região de estudo; (ii) as características dos indivíduos (posição sociológica, altura da árvore, grau de ocupação por lianas, deciduidade e diâmetro à altura do peito) influenciam no incremento acumulado em circunferência. Para detectar a periodicidade do CCT foram implantadas faixas dendrométricas permanentes no tronco (altura do DAP) de 156 indivíduos. O acompanhamento do crescimento foi realizado mensalmente por um período de 18 meses. Foram feitas correlações do CCT mensal com a precipitação, temperatura e fotoperíodo para verificar a influencia dos fatores ambientais e análise de covariância para averiguar se as características dos indivíduos também interferiram no incremento acumulado em circunferência do tronco. As espécies apresentaram um padrão sazonal de CCT, com as maiores taxas de crescimento de outubro a dezembro, sendo que os parâmetros ambientais avaliados atuaram em conjunto - principalmente o fotoperíodo e a temperatura - promovendo essa sazonalidade e confirmando a primeira hipótese. Considerando as características dos indivíduos, somente o diâmetro do tronco correlacionou-se negativamente com o incremento acumulado em circunferência (b = -0,32; p = 0,02), aceitando parcialmente a segunda hipótese.

Temperatura em solo de floresta equatorial úmida.

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Climate seasonality limits leaf carbon assimilation and wood productivity in tropical forests.

The seasonal climate drivers of the carbon cy- cle in tropical forests remain poorly known, although these forests account for more carbon assimilation and storage than any other terrestrial ecosystem. Based on a unique combina- tion of seasonal pan-tropical data sets from 89 experimental sites (68 include aboveground wood productivity measure- ments and 35 litter productivity measurements), their asso- ciated canopy photosynthetic capacity (enhanced vegetation index, EVI) and climate, we ask how carbon assimilation and aboveground allocation are related to climate seasonal- ity in tropical forests and how they interact in the seasonal carbon cycle. We found that canopy photosynthetic capacity seasonality responds positively to precipitation when rain- fall is < 2000 mm yr-1 (water-limited forests) and to radia- tion otherwise (light-limited forests). On the other hand, in- dependent of climate limitations, wood productivity and lit- terfall are driven by seasonal variation in precipitation and evapotranspiration, respectively. Consequently, light-limited forests present an asynchronism between canopy photosyn- thetic capacity and wood productivity. First-order control by precipitation likely indicates a decrease in tropical forest pro- ductivity in a drier climate in water-limited forest, and in cur- rent light-limited forest with future rainfall < 2000 mm yr-1.