C. Impact of climatic variation on the relationships between variations of GPP, ET and WUE The monthly patterns of total GPP and ET had similar relationships with net radiation, relative humidity (% RH), soil water content (SWC), vapor pressure deficit (VPD), air temperature and precipitation (Fig.4). GPP and ET had strongly significant relationships with net radiation (P<0.01) and dominated both by about 70% (Fig.4a) and the rest had positive relationships with% RH, SWC, air temperature and precipitation (Fig.4b, 4d, 4f) respectively. However the reduction in GPP and ET was obvious when higher temperature occurs (Fig.4e). Furthermore, under high VPD condition, the GPP and ET declined (Fig.4c), because the maximum photosynthesis in dry dipterocarp forest flux site at the Ratchaburi site Thailand (DFR site) occurred under with VPD. The exponential relationship between GPP, ET and VPD in such periods coincided with less rainfall and SWC (<10%VWC) (Fig.1) and are in phase with the results reported by Tong, et al. (2014).Moreover in the dry season, WUE in dry dipterocarp forest decreased significantly with an increase of VPD (P<0.01) while in wet season this was not important (Fig.5). Typically, the VPD involved photosynthesis and respiration processes; if VPD was very high (lower air humidity) that means we have extensive dry conditions and thus a response by the plant stomata, which were closed and less transpiration and soil evaporation occurs. With lower stomatal conductance the net photosynthesis rate drops steeply but the on-going needs of evapotranspiration ensures that water loss continues. This leads to a reduction in WUE (Fig.5a). During the study period (2013 to 2015) the seasonal average WUE in DDF was negatively related to lower soil water content in the dry season. This returned to a positive relationship in the wet season (Fig.5b). D. Conclusions The major aim of the study was to apply eddy covariance technique to estimate the ecosystem WUE in dry dipterocarp forest in northern Thailand. It was found that WUE at above canopy ranged from 1.39 to 3.39 g C kg-1 H2O. The technique here can be useful. The monthly and seasonal variability of GPP and ET had similar relationships to net radiation (70%). VPD was driver of WUE in dry season, while soil water content was the main factor to control WUE throughout the year. It is hoped that these findings will contribute to improving forest management practices. Proceedings of the International Conference on Climate Change, Biodiversity and Ecosystem Services for the 271 Sustainable Development Goals (SDGs): Policy and Practice 27-29 June 2016, Cha-am, Phetchaburi, Thailand
Proceedings of International Conference on Climate Change, Biodiversity and Ecosystem Services for the Sustainable Development Goals : Policy and Practice 27-29 June 2016 at the Sirindhorn International Environmental Park, Cha-am, Phetchaburi, Thailand
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