Multiple site tower flux and remote sensing comparisons of tropical forest dynamics in Monsoon Asia

The spatial and temporal dynamics of tropical forest functioning are poorly understood, partly attributed to a weak seasonality and high tree species diversity at the landscape scale. Recent neotropical rainforest studies with local tower flux measurements have revealed strong seasonal carbon fluxes that follow the availability of sunlight in intact forests, while in areas of forest disturbance, carbon fluxes more closely tracked seasonal water availability. These studies also showed a strong seasonal correspondence of satellite measures of greenness, using the Enhanced Vegetation Index (EVI) with ecosystem carbon fluxes in both intact and disturbed forests, which may enable larger scale extension of tower flux measurements. In this study, we investigated the seasonal patterns and relationships of local site tower flux measures of gross primary productivity (Pg) with independent Moderate Resolution Imaging Spectroradiometer (MODIS) satellite greenness measures across three Monsoon Asia tropical forest types, encompassing drought-deciduous, dry evergreen, and humid evergreen secondary tropical forests. In contrast to neotropical forests, the tropical forests of Monsoon Asia are more extensively degraded and heterogeneous due to intense land use pressures, and therefore, may exhibit unique seasonal patterns of ecosystem fluxes that are more likely water-limited and drought-susceptible. Our results show significant phenologic variability and response to moisture and light controls across the three tropical forest sites and at the regional scale. The drier tropical forests were primarily water-limited, while the wet evergreen secondary forest showed a slight positive trend with light availability. Satellite EVI greenness observations were generally synchronized and linearly related with seasonal and inter-annual tower flux Pg measurements at the multiple sites and provided better opportunities for tower extension of carbon fluxes than other satellite products, such as the MODIS Pg product. Satellite EVI-derived Pg images revealed strong seasonal variations in photosynthetic activity throughout the Monsoon Asia tropical region. © 2008 Elsevier B.V. All rights reserved

Multiple timescale variations and controls of soil respiration in a tropical dry dipterocarp forest, western Thailand

This study aims to improve our understanding of temporal variations and controlling factors of soil respiration (R-s) and its components (microbial respiration or R-m and root respiration or R-b) in an Asian tropical seasonal forest at diurnal, seasonal and annual timescales in relation to biotic and abiotic controls. R-s was studied by the trenching method in a seasonal dry diptercarp forest, western Thailand. An automated soil chamber system was used to produce hourly data of R-s during 2008-2011. Analysis of R-s in relation to both biotic and abiotic factors was carried out to understand its temporal variations at different timescales. R-m was the main contributor to overall magnitude and variability of R-s. Soil temperature alone was the main driver of diurnal variation, while the combination of soil moisture and soil temperature determined the seasonal variations. The amount of R-s was also related to the fine root (< 2 mm) and microbial biomass at seasonal timescales. Due to the small inter-annual variations in soil temperature and moisture, total soil respiration among the 4 years was not significantly different (p < 0.05). The annual totals for R-s during 2008-2011 were 3.20, 3.89, 3.52, 4.14 kgCO(2) m(-2) years(-1), respectively. The 4-year average ratio of R-m (trenched) to R-s (untrenched) was 66 +/- 4 %. R-m plays a crucial role in determining the magnitude (large ratio between R-m and R-s) and temporal variations of R-s. In this forest ecosystem, high seasonal variations in R-s were observed and were mainly attributed to the response of R-m to moisture

The land-atmosphere water flux in the tropics

Tropical vegetation is a major source of global land surface evapotranspiration, and can thus play a major role in global hydrological cycles and global atmospheric circulation. Accurate prediction of tropical evapotranspiration is critical to our understanding of these processes under changing climate. We examined the controls on evapotranspiration in tropical vegetation at 21 pan-tropical eddy covariance sites, conducted a comprehensive and systematic evaluation of 13 evapotranspiration models at these sites, and assessed the ability to scale up model estimates of evapotranspiration for the test region of Amazonia. Net radiation was the strongest determinant of evapotranspiration (mean evaporative fraction was 0.72) and explained 87% of the variance in monthly evapotranspiration across the sites. Vapor pressure deficit was the strongest residual predictor (14%), followed by normalized difference vegetation index (9%), precipitation (6%) and wind speed (4%). The radiation-based evapotranspiration models performed best overall for three reasons: (1) the vegetation was largely decoupled from atmospheric turbulent transfer (calculated from Ω decoupling factor), especially at the wetter sites; (2) the resistance-based models were hindered by difficulty in consistently characterizing canopy (and stomatal) resistance in the highly diverse vegetation; (3) the temperature-based models inadequately captured the variability in tropical evapotranspiration. We evaluated the potential to predict regional evapotranspiration for one test region: Amazonia. We estimated an Amazonia-wide evapotranspiration of 1370mmyr-1, but this value is dependent on assumptions about energy balance closure for the tropical eddy covariance sites; a lower value (1096mmyr-1) is considered in discussion on the use of flux data to validate and interpolate models. © 2009 The Authors Journal compilation © 2009 Blackwell Publishing Ltd