Piyachat Ratana

Amazon rainforests green‐up with sunlight in dry season

Received 23 December 2005; revised 6 February 2006; accepted 8 February 2006; published 22 March 2006. [1] Metabolism and phenology of Amazon rainforests significantly influence global dynamics of climate, carbon and water, but remain poorly understood. We analyzed Amazon vegetation phenology at multiple scales with Moderate Resolution Imaging Spectroradiometer (MODIS) satellite measurements from 2000 to 2005. MODIS Enhanced Vegetation Index (EVI, an index of canopy photosynthetic capacity) increased by 25% with sunlight during the dry season across Amazon forests, opposite to ecosystem model predictions that water limitation should cause dry season declines in forest canopy photosynthesis. In contrast to intact forests, areas converted to pasture showed dry-season declines in EVI-derived photosynthetic capacity, presumably because removal of deep-rooted forest trees reduced access to deep soil water. Local canopy photosynthesis measured from eddy flux towers in both a rainforest and forest conversion site confirm our interpretation of satellite data, and suggest that basin-wide carbon fluxes can be constrained by integrating remote sensing and local flux measurements. Citation: Huete, A. R., K. Didan, Y. E. Shimabukuro, P. Ratana, S. R. Saleska, L. R. Hutyra, W. Yang, R. R. Nemani, and R. Myneni (2006), Amazon rainforests green-up with sunlight in dry season, Geophys. Res. Lett., 33, L06405, doi:10.1029/2005GL025583.

Large seasonal swings in leaf area of Amazon rainforests

Despite early speculation to the contrary, all tropical forests studied to date display seasonal variations in the presence of new leaves, flowers, and fruits. Past studies were focused on the timing of phenological events and their cues but not on the accompanying changes in leaf area that regulate vegetation–atmosphere exchanges of energy, momentum, and mass. Here we report, from analysis of 5 years of recent satellite data, seasonal swings in green leaf area of ≈25% in a majority of the Amazon rainforests. This seasonal cycle is timed to the seasonality of solar radiation in a manner that is suggestive of anticipatory and opportunistic patterns of net leaf flushing during the early to mid part of the light-rich dry season and net leaf abscission during the cloudy wet season. These seasonal swings in leaf area may be critical to initiation of the transition from dry to wet season, seasonal carbon balance between photosynthetic gains and respiratory losses, and litterfall nutrient cycling in moist tropical forests.

Analysis of Cerrado Physiognomies and Conversion in the MODIS Seasonal–Temporal Domain

Abstract The “cerrado” biome in central Brazil is rapidly being converted into pasture and agricultural crops with important consequences for local and regional climate change and regional carbon fluxes between the atmosphere and land surface. Satellite remote sensing provides an opportunity to monitor the highly diverse and complex cerrado biome, encompassing grassland, shrubland, woodland and gallery forests, and converted areas. In this study, the potential of Terra Moderate Resolution Imaging Spectroradiometer (MODIS) data is analyzed to discriminate among these diverse cerrado physiognomies and converted pastures based on their seasonal dynamics and phenology. Four years (2000–03) of MODIS 16-day composited, 250-m resolution vegetation index (VI) data were extracted over a series of biophysically sampled field study sites representing the major cerrado types. The temporal VI profiles over the cerrado formations exhibited high seasonal contrasts with a pronounced dry season from June to August and a w...

Indices of Vegetation Activity

In this chapter we explain satellite-based vegetation indices (VIs) as dynamic spectral measures of vegetation activity. VIs are among the most widely used satellite products in monitoring ecosystems and agriculture, resource management, and estimations of many biophysical canopy properties. A theoretical basis for their formulation is presented and we describe how VIs are processed and composited from satellite imagery. Recent trends in their validation and quality assessment using in situ tower measurements are also discussed. Finally, a cross section of major findings involving the use of satellite VIs in ecological and climate science is presented and we conclude with research challenges and environmental issues that will drive future uses of satellite VIs.

Interrelationship among among MODIS vegetation products across an Amazon Eco-climatic gradient

The Moderate Resolution Imaging Spectroradiometer (MODIS) has been found to be a useful tool in spatial and temporal terrestrial biosphere monitoring. In this study, we investigated the spatial and temporal interrelationships among MODIS vegetation products across a north-south Amazon eco-climatic transect, encompassing tropical forest, forestsavanna transition zone, and cerrado. MODIS vegetation index (VI), leaf area index (LAI), fraction of absorbed photosynthetically active radiation (FPAR), gross primary production (GPP), land surface water indices (i.e. LSWI) and land surface temperature (LST) products were extracted over both natural and converted areas along the climatic gradient. Our results showed strong spatial and temporal variations in vegetation dynamics over the climatic gradient with natural and converted areas responding differently. The relationships between VI and LAI/FPAR were unique with biome type. The VI and LSWI seasonal profiles matched fairly well in the cerrado region and conversion areas but not primary forest areas. A positive relationship was found between VI and LST in tropical forest, however the cerrado showed negative VI-LST relationships. These differences were related to the unique seasonal water and carbon patterns within each ecosystem. The interrelationships between MODIS land data products and land surface water indices yield important information useful in land cover characterization and prediction of vegetation responses to climate change and land cover conversions. Keywords-: MODIS, inter-relationship, Eco-climatic transect, Brazil, Amazon

Use of MODIS enhanced vegetation index to detect seasonal patterns of leaf phenology in central Amazon várzea forest

MODIS 16-day composite EVI, NDVI, and VI Quality Analysis values for 2000–2005 were extracted for 21 várzea forest sites along the Solimões-Amazon floodplain west of Manaus, Brazil. VI values were filtered to exclude dates with VI-QA values greater than 3, and time series of median values of the remaining pixels were examined in conjunction with river stage levels recorded at the Manacapuru gauge. All sites showed a regular seasonal variation in EVI, ranging from a mean low for all sites of 0.41 to a mean high of 0.61. The amplitude of variability in NDVI was about 50% that of EVI. Minimum EVI, corresponding to minimum leaf area, occurred in late May, about 40 days preceding maximum river stage; EVI peaked in mid-October, about 30 days before lowest river levels. These temporal patterns are in general agreement with field observations of leaf phenology at várzea stands near Manaus.

Técnicas avançadas de sensoriamento remoto aplicadas ao estudo de mudanças climáticas e ao funcionamento dos ecossistemas amazônicos

This paper aims to assess the contribution of remote sensing technology in addressing key questions raised by the Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA). The answers to these questions foster the knowledge on the climatic, biogechemical and hydrologic functioning of the Amazon, as well as on the impact of human activities at regional and global scales. Remote sensing methods allow integrating information on several processes at different temporal and spatial scales. By doing so, it is possible to perceive hidden relations among processes and structures, enhancing their teleconnections. Key advances in the remote sensing science are summarized in this article, which is particularly focused on information that would not be possible to be retrieved without the concurrence of this technology.

Seasonal dynamics of native and converted cerrado physiognomies with MODIS data

The Cerrado or Brazilian savanna represents 23% of the land surface of the country. This important bid me, however, has been subjected to rapid rates of land conversion to agriculture and pasture. This has important environmental consequences to local and regional climate change and carbon fluxes. Therefore, a study of seasonal cerrado dynamics, including forest and converted areas, was conducted with four years of data (February 2000 to December 2003) from the Moderate Resolution Imaging Spectroradiometer (MODIS). The 16-day composite vegetation index (VI) data were used to analyze the seasonal patterns of photosynthetic vegetation activity and examine the separability of cerrado formations of varying physiognomies in Brasilia National Park and surrounding areas. The results showed that the cerrado formations exhibited a high seasonality contrast with a pronounced dry season from June through August and wet season from November to March. Discrimination within cerrado formations was difficult due to similarities in their seasonal dynamic behavior. Maximum contrast among all the cerrado formations occurred during dry season, suggesting this as the best time for cerrado physiognomy discrimination. The converted agricultural areas had a higher contrast than the native cerrado, and the forest formation had the lowest seasonal contrast. This enabled an operational method to discriminate the cerrado formation from the converted areas and adjoining forests. Thus, MODIS offers a useful tool to monitor the threatened cerrado biome