| Energy exchange and evapotranspiration (ET) between land surface and the atmosphere is among the most important processes in any ecosystem since it can significantly affect temperature, water transport, plant growth and many other ecosystem processes. Understanding the magnitude and changes of various energy fluxes as well as related regulatory mechanisms is critical for quantifying ecosystem functions and their responses to climate change. Located between the interface of land and sea (intertidal zone), estuarine wetland ecosystems is affected by the periodic tidal flooding, where tidal activities and upstream hydrology can play vital roles in regulating the magnitude and dynamics of the energy budget through horizontal transportation of mass and energy, and then affect the ecological processes and functions of estuarine wetlands,. Despite of the importance roles mangrove forests playing in regulating matter exchange in the coastal zones, there are still limited studies on the energy balance and ET in mangrove ecosystems. In this dissertation study, energy balance, ET variations and water use of mangrove trees of subtropical mangrove ecosystems in Yunxiao (23°55’N,117°23’E) of Fujian province, China were investigated by combining eddy covariance technique and tree sap flow methods. The characteristics of energy distribution of energy balance components, energy closure level, ET variation, water use of mangrove trees as well as their response to environmental factors were all discussed, including tidal influence on energy balance, ET and water use of mangroves. The purpose of this research is to understand the characteristics of energy flux, water vapor flux and water consumption by transpiration of the mangrove forests and lay the foundation for further researches on environmental mechanism, especial the tidal regulation in coastal zone. Main results and conclusions are listed below.1. The results of energy closure in the past three years (2009-2011) as indicated as the correlation index of liner regression between Rn-G (Rn:net radiation, G:soil heat flux) and LE+HS(LE:latent heat, Hs:sensible heat) were0.63-0.69and0.67-0.79, indicating that the flux data quality was reliable and suitable for the ecosystem energy partitioning studies. Daily flux of net radiation (Rn), latent heat flux (LE), sensible heat flux (Hs), and soil heat flux (G) had remarkable seasonal variation. Seasonal energy flux was controlled by the seasonal change. The sensible heat was greater than latent heat and the bowen ratio was larger than1during winter season. The latent heat dominated the energy flux and had greater energy flux than sensible heat and the bowen ratio reduced to0.4-0.6from spring to autumn. Overall, the latent heat showed the higher ratio (38.15%) in net radiation than that of the sensible heat (28.56%), whilst the ratio of soil heat was neglectable (<0.1%). The latent heat was almost equal to sensible heat under flooding and high salinity stress.2. When comparing the energy partitioning in different ecosystem, the LE/Rn was closed to that of wetland ecosystems, but was much lower than that of terrestrial ecosystems including agriculture ecosystem, broad-leave forest ecosystem and so on. This indicated that mangrove forest in subtropical zone was water conservative. In addition, tidal activity was also the important factor influencing energy closure and energy partitioning. Tidal activity can significantly reduced the bowen ratio and energy closure in winter, whereas it would dramatically increased the bowen ratio, and at the same time enhanced the energy closure to some extent in summer. The low value of LE/LEeq all year and low Ω value in winter and summer after indicated the stomata and VPD mostly controlled ET in any hydraulic or salinity stress condition specifically during dry season. And the low canopy surface conductivity (gc-e) showed that transpiration might be reduced though stomatal closure under sever water stress, and the reduced stomatal conductivity can limited energy partitioning to latent heat.3. ET variation had regular diurnal process, and presented obvious seasonal dynamic with the seasonal change. Compared with the other terrestrial ecosystems using eddy covariance system, it had more water consumption in summer with the mean daily value of3.4mm d-1and the whole year ET value of838-1062mm during2009-2011. Both the Priestly-Taylor and Penman-Monteith models could perform well to ET for mangrove ecosystems. It was also seen that the environment factor could markedly effected ET. Net radiation and VPD were key factors to affect ET. In addition, tidal activity also adjusted the ET, which was reduced in summer and autumn, while was enhanced in winter and spring.4. Variations of sap flow density (SFD) following closely the variation in PAR and VPD showed regular single-peak curve. The peak value of SFD of Kandelia obovata and Avicennia marina of different diameter was15.37-38.21g m-2s-1and21.26-73.15g m-2s-1, respectively. Compare with the other species, sap flowwas fairly high in the outer xylem of mangroves in our sites. Maximum daily water use for Kandelia obovata and Avicennia marina was ranged from0.31-5.43kg d-1,2.01-10.61kg d-1. Further extrapolated to the whole stand transpiration (Es), Es was85.56mm for Kandelia obovata, and129.51mm for Avicennia marina during observation period (201006-201105), respectively. The percentage of Es/ET in four seasons in our sites was at the range of16-26%with mean value of23%, clearly indicating that the individual mangrove tree water use followed leaf-level mechanisms in being conservative.5. Different environmental factors had different influences on sap flow (SFD), daily water use (F) and ET in different season. Photosynthetically active radiation (PAR), vapor pressure deficit (VPD) and windspeed (v) were key factors to affect SFD in summer. While PAR, VPD and Tα were key factors to affect SFD in winter. For daily water use, relative humility was also the key factor beside of PAR and VPD. Among those factors, PAR and VPD were major factors affecting mangrove transpiration. However, except for PAR, the dependence of ET on environmental factors was weaker.6. The gc-t decreased with VPD increasing, increased linearly with PAR increasing. And with the increased level of VPD, the sensitivity of gc-t to PAR decreased gradually. Like gc-t, the gc-e also decreased with VPD increasing, and increased linearly with PAR increasing. Compared with gc-t, its correlation coefficient was lower, indicating the lower sensitivity of gc-e to environmental factors.7. Periodic flooding reduced ET and stand transpiration (Es) in mature trees of K. obovata and A. marina in our study. The mean daily water use was reduced by16.83%for A. marina trees,12.35%for K. obovata and10.26%for daily ET, respectively. Meanwhile, ecosystem GEP, NEE and ER were reduced by6.56%,6.84%, and12.08%under tidal flooding.The above results suggest that, due to the unique geographical location, the mangrove plants are water conservative under tidal flooding and salinity stress in subtropical zone, so their transpiration are mainly regulated by plant physiological processes. Moreover, tidal activities exerte significant impact on the water heat balance of mangrove ecosystems and the water use patterns of mangrove forests. This study lays the foundation for further study of mechanism of high water efficiency and high productivity about mangrove ecosystem. Furthermore, this will be of great importance in studing of the relationship of ecosystem CO2exchange and water vapor flux, understanding of the characteristics of material cycle and energy exchange, grasping the carbon cycle and its driving mechanism in wetland. |
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