Water Isotope Eco-hydrological Process In A Pinus Yunnanensis Forest,in Lijiang Southwest China

Terrestrial evapotranspiration is one of the key processes in ecological processes and hydrologic cycles,including soil evaporation and plant transpiration.The study on evapotranspiration processes is crucial for understanding the terrestrial water balance and global water cycle,and for better understanding its dynamics and controlling factors to predict ecosystem responses to climate change.Yet,partitioning of evapotranspiration is still a big challenge,and its mechanism for a regional hydrologic cycle is argument.Stable isotope technique is an effective and efficient approach to provide insights into hydrological cycle,and it has been widely applied in the global water cycle.Forest ecosystems cover approximately 30%of the Earth’s land surface and play an essential ecological role in the earth-atmosphere interaction,the earth’s surface energy balance and hydrological cycle.In particular alpine forest exerts a strong regulating function on preventing soil erosion and regulating climate system.Here,we performed an in-situ measurements of water isotopes(δ¹⁸O andδ²H)in precipitation,atmospheric vapor,soil pools and plant tissues in a Pinus yunnanensis forest ecosystem in southwest China,by employing a high-frequency laser spectrometry and chamber method throughout an entire raining season.The aim is to diagnose the processes in association with isotope fractionation from precipitation to soil water,xylem water and to leaf water,to revel soil water infiltration and plant water uptake strategies.Another purpose of the paper is to partition ET and to find its controls by estimating the stable isotopic compositions of ET and that of its two components,i.e.,plant transpiration and soil water evaporation.We used combined high-frequency laser spectroscopy and chamber methods,to constrain the estimates of T/ET.Our results here provide insight into the regional hydrological cycle in alpine forest ecosystem and potentially benefits many applications from forest ecosystem protection to paleo isotope archives.The main findings are as follows:（1）We presented the seasonal variation ofδ¹⁸O andδ²H in different water bodies,e.g.,precipitation,atmospheric vapor,soil water and plant water in a Pinus yunnanensis forest ecosystem.The fluctuation of atmospheric vaporδ¹⁸O and vapor mixing ratio are smaller in premonsoon season,but with large fluctuations in summer monsoon season owning to the local precipitation events.Both atmospheric vaporδ¹⁸O and vapor mixing ratio show consistent vertical variation from 0.3m to20m above ground,and the overall diurnal change of atmospheric vaporδ¹⁸O is small,within 2‰in a day.The seasonal variation of precipitationδ¹⁸O during the observation period was significant,with enrichment in premonsoon season and depletion in the monsoon season.Plant xylem and leaf water isotopes showed consistent variations as the same precipitation,but became leaf water isotopes enriched,of 4.9‰inδ¹⁸O and 21.5‰inδ²H as compared with plant xylem water.We found an abnormal phenomenon than the leaf water isotope is lower than the source water with a specific leaf water line.The average and magnitudes of water isotope ratios varied considerably among precipitation,soil,xylem and leaf samples.Xylem water isotopes varied less than either soil water or bulk leaf water.In the process of water transport from precipitation to soil water to plant water,δ¹⁸O andδ²H become enriched gradually.The local meteoric water line（LMWL）is very close to the global meteoric water line（GMWL）,but the slope and intercept are slightly lower than the global meteoric water line,likely owing to the evaporative enrichment of heavy isotopes when the condensate falls in the unsaturated atmosphere.Soil water isotope signatures are mainly scattered in the lower right of the local meteoric water line as soil water.The soil water line with lower slope and intercept than the local meteoric water line is resulted from the slight evaporation of soil water in storage after rainfall.The plant xylem water line is roughly parallel with the soil water line but with a smaller intercept.The leaf water line is significantly lower in slope than in xylem water in association with the fractionation in leaf transpiration.（2）We presented the relationship between precipitation and soil water isotope transformation,and plant root selective water usage strategies were clarified.During the monsoon season,soil waterδ¹⁸O is most enriched in the surface layer（5cm）and most depleted in the deep layer（80 cm）in Lijiang area,and soil waterδ¹⁸O tends to stabilize with the increase of soil depth.Evaporation in surface soil layers（5cm）enriched heavy isotopes and most depleted in the deep soil layer（80 cm）in Lijiang area,particularly in the premonsoon season.And soil waterδ¹⁸O tends to stabilize with the increase of soil depth.The root preferentially selected water from5-10 cm of surface soil layers in premonsoon season but an unidentified moisture source in the rainy monsoon season as soil water isotopes overlapped in difference depth.Leaf water d-excess inherited the xylem water isotope signal,and therefore,d-excess in leaf water changed in phase with that in xylem water,and the relative humidity was found to exert a significant influence on leaf transpiration and isotope fractionation.The modeled enrichment of water is roughly in the range of the observed leaf water,and the two show consistent fluctuations in the observation period.The predicted leaf waterδ²H is more consistent with the observations thanδ¹⁸O.In addition,stable isotopic proof is the in-phase variations between the observedΔ¹⁸O_Land precipitationδ¹⁸O,confirming that the Pinus yunnanensis needle absorbs rainfall directly in rain events and therefore leads to coherent variations between the two.（3）The seasonal variation of transpiration to evapotranspiration（T/ET）and it main control at Lijiang station was evaluated..The estimated daily T/ET ratio ranges from 0.59 to 0.81,with an apparent increasing shift in the early growing season and maintaining a plateau level of over 0.75 during the peak growing season.This higher averaged T/ET of 0.73±0.06 indicates that plant transpiration is the main component of evapotranspiration.The estimated soil evaporation and plant transpiration are in agreement with result from customized chamber method and vary with precipitation isotopes.In monsoon season（in June-September）,soil water content（Swc）is the main control of T/ET variations,while leaf area index（LAI）playing only a partial role.Our study confirms the critical impact of soil water on the seasonal change of T/ET in Pinus yunnanensis ecosystem as in the SW China.However,we are also aware the sensitivity of controls on estimated T/ET at different time scale interested.