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Kumagai T.,Nagoya University | Mudd R.G.,University of Hawaii at Manoa | Miyazawa Y.,Kyushu University | Liu W.,University of Hawaii at Manoa | And 9 more authors.
Ecological Modelling | Year: 2013

We developed a soil-vegetation-atmosphere transfer (SVAT) model applicable to simulating CO2 and H2O fluxes from the canopies of rubber plantations, which are characterized by distinct canopy clumping produced by regular spacing of plantation trees. Rubber (Hevea brasiliensis Müll. Arg.) plantations, which are rapidly expanding into both climatically optimal and sub-optimal environments throughout mainland Southeast Asia, potentially change the partitioning of water, energy, and carbon at multiple scales, compared with traditional land covers that are being replaced. Describing the biosphere-atmosphere exchange in rubber plantations via SVAT modeling is, therefore, important to understanding the impacts on environmental processes. The regular spacing of plantation trees creates a peculiar canopy structure that is not well represented in most SVAT models, which generally assume a non-uniform spacing of vegetation. Herein we develop a SVAT model applicable to a rubber plantation and an evaluation method for its canopy structure, and examine how the peculiar canopy structure of rubber plantations affects canopy CO2 and H2O exchanges. Model results are compared with measurements collected at a field site in central Cambodia. Our findings suggest that it is crucial to account for intensive canopy clumping in order to reproduce observed rubber plantation fluxes. These results suggest a potentially optimal spacing of rubber trees to produce high primary productivity and water use efficiency. © 2013 Elsevier B.V.

Giambelluca T.W.,University of Hawaii at Manoa | Giambelluca T.W.,Nagoya University | Mudd R.G.,University of Hawaii at Manoa | Liu W.,University of Hawaii at Manoa | And 11 more authors.
Water Resources Research | Year: 2016

To investigate the effects of expanding rubber (Hevea brasiliensis) cultivation on water cycling in Mainland Southeast Asia (MSEA), evapotranspiration (ET) was measured within rubber plantations at Bueng Kan, Thailand, and Kampong Cham, Cambodia. After energy closure adjustment, mean annual rubber ET was 1211 and 1459 mm yr-1 at the Thailand and Cambodia sites, respectively, higher than that of other tree-dominated land covers in the region, including tropical seasonal forest (812-1140 mm yr-1), and savanna (538-1060 mm yr-1). The mean proportion of net radiation used for ET by rubber (0.725) is similar to that of tropical rainforest (0.729) and much higher than that of tropical seasonal forest (0.595) and savanna (0.548). Plant area index (varies with leaf area changes), explains 88.2% and 73.1% of the variance in the ratio of latent energy flux (energy equivalent of ET) to potential latent energy flux (LE/LEpot) for midday rain-free periods at the Thailand and Cambodia sites, respectively. High annual rubber ET results from high late dry season water use, associated with rapid refoliation by this brevideciduous species, facilitated by tapping of deep soil water, and by very high wet season ET, a characteristic of deciduous trees. Spatially, mean annual rubber ET increases strongly with increasing net radiation (Rn) across the three available rubber plantation observation sites, unlike nonrubber tropical ecosystems, which reduce canopy conductance at high Rn sites. High water use by rubber raises concerns about potential effects of continued expansion of tree plantations on water and food security in MSEA. © 2016. American Geophysical Union. All Rights Reserved.

Kim C.,Cambodian Rubber Research Institute | Deratani A.,Montpellier University | Bonfils F.,Montpellier University
Journal of Liquid Chromatography and Related Technologies | Year: 2010

The refractive index increment (dn/dc) of a solution is a constant that indicates the variation of the refractive index with the solute concentration. It is used in the multi-angle light scattering technique to determine the concentration and the weight-average molar mass of polymers. It depends on many parameters, including the structure of the polymer, the solvent, the wavelength of the light and the temperature of the solution. To obtain accurate results, it is necessary to determine the dn/dc at the same condition as for the multi-angle light scattering measurement. In this work, the dn/dc at 633nm of standard synthetic monodispere poly(cis-1,4-isoprene) (PI) and natural rubber (NR) solutions in THF were determined at 25 and 40C using an Optilab DSP refractometer. The mean value of the dn/dc obtained for NR and PI was 0.13mL/g. No significant difference between different types of samples and temperatures (25°C and 40°C) were observed. The nanoaggregates in NR solution that were not retained after filtration through 1m filter had no effect on the dn/dc. The intercept value of the conformation plot increased in line with the dn/dc, but the Flory exponent remained unchanged.

Kim C.,Cambodian Rubber Research Institute | Morel M.-H.,Montpellier University | Beuve J.S.,Montpellier University | Guilbert S.,Montpellier University | Bonfils F.,Montpellier University
Polymer Engineering and Science | Year: 2010

The objective of our work was to characterize natural rubber (NR) samples with different macromolecular structures by measuring Mooney viscosities (V R) at variable rotor speeds ≤2 rpm, called variable speed Mooney viscosity (MVS). Model samples of technically specified rubbers of constant Mooney viscosity (TSR5CV) were prepared with chosen specific clones. The structures of the samples were characterized by size-exclusion chromatography coupled with an online multi-angle light-scattering detector (SEC-MALS). Rheological properties of the samples were also characterized by a dynamic moving die rheometer. Measuring monoclonal model samples by M VS showed three types of VR flow curves. The VR at high rotor speed (2 rpm) was correlated with number-average molar mass (Mn), whereas VR at low rotor speed (0.05 rpm) was correlated with weight-average molar mass (Mw). Measuring M VS revealed the rheological behaviors of samples and enabled discrimination between samples with different macromolecular structures and should thus help in predicting processability. POLYM. ENG. SCI., 50:240-248, 2010. Copyright © 2009 Society of Plastics Engineers.

Kumagai T.,Nagoya University | Mudd R.G.,University of Hawaii at Manoa | Giambelluca T.W.,University of Hawaii at Manoa | Kobayashi N.,Nagoya University | And 9 more authors.
Agricultural and Forest Meteorology | Year: 2015

Delineating the characteristics of biosphere-atmosphere exchange in rubber (Hevea brasiliensis Müll. Arg.) plantations, which are rapidly expanding throughout mainland Southeast Asia, is important to understanding the impacts of the land-use change on environmental processes. In attempt to shed new light on the impacts of conversion to rubber, we have conducted eddy flux measurements over a 3-year period in two rubber plantation sites: (1) Som Sanuk, located in northeastern Thailand; and (2) Cambodian Rubber Research Institute (CRRI), located in central Cambodia. Both sites have a distinct dry season. We used a combination of actual evapotranspiration (ET) flux measurements and an inverted version of a simple 2-layer ET model for estimating the mean canopy stomatal conductance (gs). The potential water balance (precipitation (P)-potential evaporation (ET_POT)) for each season (i.e., December-February: DJF, March-May: MAM, June-August: JJA, and September-November: SON) revealed when and how the water use is controlled. In the seasons when actual water balance (P-ET) was negative (DJF and MAM), the deficit was compensated with soil water from the previous season at depths of 0-2m (Thailand site) and 0-3m (Cambodia site). At both sites, the reference value of gs (gsref) and the sensitivity of gs to atmospheric demand (m) appeared to be less in DJF and MAM than each in the other two 3-month periods (seasons). On average, in a whole year, m/gsref was less in Thailand (<<0.6) than in Cambodia (near 0.6 for part of the year), suggesting that there was less sufficient stomatal regulation at the Thailand site, where there might be little risk of water stress-induced hydraulic failure because of its higher annual rainfall amount. In comparison, at CRRI where annual P-ET_POT was negative, there was stricter stomatal regulation, preventing excessive xylem cavitation. © 2015 Elsevier B.V.

Kobayashi N.,Nagoya University | Kumagai T.,Nagoya University | Miyazawa Y.,Kyushu University | Matsumoto K.,University of Ryukyus | And 7 more authors.
Tree Physiology | Year: 2014

The rapid and widespread expansion of rubber plantations in Southeast Asia necessitates a greater understanding of tree physiology and the impacts of water consumption on local hydrology. Sap flow measurements were used to study the intra-and inter-annual variations in transpiration rate (Et) in a rubber stand in the low-elevation plain of central Cambodia. Mean stand sap flux density (JS) indicates that rubber trees actively transpire in the rainy season, but become inactive in the dry season. A sharp, brief drop in JS occurred simultaneously with leaf shedding in the middle of the dry season in January. Although the annual maxima of JS were approximately the same in the two study years, the maximum daily stand E t of ∼2.0 mm day-1 in 2010 increased to ∼2.4 mm day-1 in 2011. Canopy-level stomatal response was well explained by changes in solar radiation, vapor pressure deficit, soil moisture availability, leaf area, and stem diameter. Rubber trees had a relatively small potential to transpire at the beginning of the study period, compared with average diffuse-porous species. After 2 years of growth in stem diameter, transpiration potential was comparable to other species. The sensitivity of canopy conductance (gc) to atmospheric drought indicates isohydric behavior of rubber trees. Modeling also predicted a relatively small sensitivity of gc to the soil moisture deficit and a rapid decrease in gc under extreme drought conditions. However, annual observations suggest the possibility of a change in leaf characteristics with tree maturity and/or initiation of latex tapping. The estimated annual stand Et was 469 mm year-1 in 2010, increasing to 658 mm year-1 in 2011. Diagnostic analysis using the derived gc model showed that inter-annual change in stand Et in the rapidly growing young rubber stand was determined mainly by tree growth rate, not by differences in air and soil variables in the surrounding environment. Future research should focus on the potentially broad applicability of the relationship between Et and tree size as well as environmental factors at stands different in terms of clonal type and age. © 2014 The Author.

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