Abstract:

Objective: The balance relationship between carbon absorption and water loss of plantation ecosystems under the arid and semi-arid environment conditions is poorly understood. The understanding of response mechanism of forest ecosystem water use efficiency (WUE) to environmental factors is useful for quantifying carbon and water balance of urban artificial forests. Method: Eddy covariance technology combined with micrometeorological sensors were used to monitor ecosystem carbon and water fluxes and environmental factors of Pinus tabuliformis plantation of Beijing Olympic Forest Park for years 2012—2014, and statistical and simulation methods were used to analyze balance relationship between carbon and water fluxes in plantation ecosystems at the seasonal scale. Result: All 3 years were dry years with little precipitation during the study period, resulting in seasonal soil drought. Annual precipitation values were all lower than the average of the past 60 years and decreased year by year, with total precipitation in year 2014 being only 271 mm. Annual WUE value were 2.2?2.9 g?kg^?1, with the lowest value being 2.2 g?kg^?1 in 2012 and the highest value being 2.9 g?kg^?1 in 2013. During the growing season, WUE was mainly influenced by vegetation index (NDVI), net radiation (R_n), temperature (T_a), soil moisture content (SWC), and saturated vapor pressure difference (VPD). Main environmental factors of seasonal WUE were changing as the following patterns. In spring, NDVI had the greatest impact on seasonal WUE, which explained 48% of spring variations in WUE (P<0.05); in summer, the influence of R_n and T_a had the greatest impacts on seasonal WUE, which explained 52% and 57% of summer variations in WUE, respectively (P<0.05); and in autumn, the influence of SWC and VPD had the greatest impacts on seasonal WUE, which explained 44% and 54% of autumn variations in WUE, respectively (P<0.05). During the growing season, WUE and T_a showed a linear negative correlation (P<0.01); WUE showed a non-linear negative correlation (i.e., logarithmic curve relationship) with VPD and R_n; and WUE showed a linear positive correlation with SWC (P<0.01). However, in extremely dry months, such as May, June, and August of the year 2014, WUE was negatively correlated with SWC (P<0.01), and WUE dropped to its lowest point. In July of years 2012 and 2013, there were consecutive dry periods without rain, and WUE values reached its lowest value of 3.7 g?kg^?1; while WUE values reached its maximum value of 10.3 g?kg^?1, as SWC being increasing, during the continuous precipitation period of the growing season. Conclusion: Carbon and water fluxes of plantation ecosystems under the arid and semi-arid environment conditions have coupled; and seasonal drought and heat have reduced carbon absorption and water loss, and thus, leading to a decrease in WUE. During the dry season of the growing season, in environmental conditions with high T_a, as SWC being decreasing, WUE value has dropped to its lowest level. The combination of available soil water and drought characteristics (i.e., drought intensity and duration) of plantation has affected the seasonal variations of ecosystem carbon and water fluxes, thereby affected the interannual response differences of plantation WUE to drought events, under different temperature conditions during the growing season.

Key words: ecosystem water use efficiency, carbon and water fluxes, drought, plantation