Water Vapour,heat Fluxes And Water Use Efficiency Over A Young Pinus Tabuliformis Plantation In Beijing

Northern China's plantations have long played a role in providing services to the people of China.Climate models predict large climatic variability and more frequent,prolonged droughts in northern China,which will have a significant impact on the structure and function of regional plantation ecosystem.Reliable detection and understanding of ecosystem energy partitioning,evapotranspiration and water use efficiency and their relations to drought is a precondition to the development of strategies for the sustainable management of these plantations.Here,the seasonal and interannual variability in surface energy exchange,evapotranspiration and water use efficiency over a young Pinus tabuliformis plantation in Beijing was investigated using eddy-covariance measurements collected over a six-year period(2012-2017).We evaluated the applicability of eddy-covariance technology in norethern plantations ecosystem,and advanced the air filtering system of eddy-covariance systems.We focused on understanding the regulation mechanism of energy partitioning,evapotranspiration and water use efficiency at different time scales and their relationship with drought.Main results and conclusions were as follows(1)Seasonal patterns in net radiation(Rn),sensible(H),latent(LE)and soil heat fluxes(G)were similar over the six years.H was the dominant energy component,with LE exceeding H only during the mid-growing season.A significant share of Rn was allocted to H during the summer and autumn drought.G contributed significantly to the energy balance on both daily and seasonal timescales,but had negligible impact at annual timescales(2)Energy exchange for the young plantation was characterized by high Bowen ratios(?=H/LE;6.28,6.42,5.65,5.34,6.40 and 8.49 for the six years),high H/Rn(0.37,0.36,0.36,0.30,0.34 and 0.39),and low LE/Rn(0.15,0.17,0.17,0.16,0.15 and 0.16),with mean annual values of 6.43,0.35,and 0.16 for ?,H/Rn,and LE/Rn,respectively.(3)Evaportranspiration(ET)of the young plantation had obvious daily,seasonal and interannual variations.Daily maximum and annual ET were 4.8,4.5,4.1,3.3,4.1,and 5.1 mm day-1 and 328,371,290,326,334 and 365 mm for the six years,with a mean annual value of 336 ± 29 mm.Seasonal variation in ET was strongly controlled by biological factors(i.e.,bulk surface conduction and vegetation greenness and density,as characterized by the normalized difference vegetation index)regulated by soil water availability and water vapor pressure deficit(VPD).Evaportranspiration varied interannually and the evapotranspiration-to-precipitation ratio(ET/P)ranged from 0.66 to 1.62.Soil water replenishment through precipitation during the non-growing period of the previous year and mid-growing season of the current year was responsible for the interannual variation in ET.These results clearly indicate the importance of precipitation timing and soil moisture carry-over from previous years in controlling ecosystem ET.(4)The annual mean water use efficiency(WUE)for this young plantation was 1.39-1.93 g C per kg H2O for years 2012-2017.The time and duration of drought can significantly affect the seasonal and intreannual variation of ecosystem WUE.Drought reduced WUE in summer,but increased it in autumn,with no apparent effect in spring.Long-term droughts usually had a greater effect on the annual GEP,ET and WUE than short-term droughts.Intense drought was observed to have a long-term lagged effect on ecosystem GEP,ET and WUE of about 3 years(5)Monthly WUE had a strong positive linear relationship with normalized difference vegetation index(NDVI)and soil volumetric water content(VWC)during the growing season and a quadratic relationship with precipitation(P).Seasonal variation in WUE was mainly regulated by the biophysical factors controlling GEP.In contrast,interannual variation in WUE was controlled to a greater extent by vegetation cover(i.e.,NDVI),which in turn responded to variation in VWC.Collectively,WUE was more strongly affected by factors affecting carbon uptake than water consumption.In summary,due to intensive afforestation programs and increasing growth of existing stands in northern China,it is expected that the water shortage caused by drought will continue in the future,thus having a certain impact on the development of regional plantations and water resources management.We found that spring was a critical period for plant growth,with high soil water storage during the spring being more important than any other period of the year.In addition,we also found that long-term drought did more harm to plantation than short-term drought.Therefore,in order to optimize plantation functioning,while diminishing the need for water,irrigation during spring and the long-term drought is more favorable to plantation development than that in other time of a year.