Environmental Responses And Physiological Controls Of Transpiration Of Three Common Coniferous Tree Species In North China

Since the 20th century,the increasing temperature caused by global warming has gradually increased the frequency and intensity of drought,which has reduced the biodiversity of global forest ecosystems and increased the risk of tree mortality.In the context of climate change,it is critical to systematic study tree transpiration and analyze its response mechanism to drought for evaluating global change impacts,properly using water resources,and improving forest management strategies.In order to reveal the characteristics of tree transpiration and its environmental responses and physiological controls in semi-arid areas,three pure stands composed of common coniferous tree species（Pinus tabulaeformis,Picea asperata,and Pinus sylvestris）were selected in Hohhot arboretum to measure sap flow by using the thermal diffusion method（TDP thermal probe）,environmental factors,and soil water content（VWC）,as well as the leaf water potential of P.tabulaeformis through discrete sampling,and stand transpiration（E）,stand nocturnal sap flow（E_n）,canopy conductance（Gc）,vapor pressure deficit（VPD）,and relative extractable water（REW）were calculated accordingly.Our findings showed:（1）During the growing season,the average daily stand transpiration of P.tabulaeformis,P.asperata and P.sylvestris were 0.83±0.46 mm·d^-1,0.18±0.10 mm·d^-1,and 0.62±0.26 mm·d^-1.VPD,global radiation（GR）,and wind speed（Ws）significantly affected and explained the major variation of stand transpiration velocity（Js）of each tree species.The hysteresis between GR and Js of each tree species were 90-120minutes.No hysteresis existed between VPD and Js of P.asperata,Js for P.tabulaeformis and P.sylvestris had a 30-min hysteresis with VPD.E for P.tabulaeformis and P.sylvestris were inhibited by soil water stress,while P.asperata had higher stand transpiration under soil water stress（REW≤0.4）.（2）During the growing season,the average daily stand nocturnal sap flow of P.tabulaeformis,P.asperata,and P.sylvestris were 0.048±0.024 mm·d^-1,0.022±0.019 mm·d^-1,and 0.034±0.017 mm·d^-1.E_nof each tree species was significantly affected by VPD,and Ws was only significantly correlated with E_nof P.sylvestris in a negative way,rather than with E_n of P.tabulaeformis and P.asperata.Under soil water stress,E_n of P.sylvestris and P.asperata increased as decreasing REW.Compared with the period when soil water stress was absent（REW>0.4）,E_n and E_n/E of each tree species was significantly higher,which indicates that nocturnal sap flow might be crucial for trees in coping with drought.（3）During the growing season,the average daily canopy conductance of P.tabulaeformis,P.asperata and P.sylvestris were 1.47±0.83 mm·s^-1,0.27±0.11 mm·s^-1,and 1.01±0.49 mm·s^-1.The response and sensitivity of Gc to environment factors of different tree species were different.During the period of soil water stress,both Gc of P.tabulaeformis and P.sylvestris were significantly supressed,but for P.asperata,it had no significant change.The response ofφof P.tabulaeformis to sap flow was variable between different time in one day.At noon,Js had a significant negative correlation withφof P.tabulaeformis and Gc.This may because high Js led to leaf water stress,and the decreasing leaf surface swelling pressure made guard cells receive higher pressure from epidermal cells and accordingly close stomata.At night,the significant positive correlation betweenφof P.tabulaeformis and E_n implying E_nwas helpful to imporve leaf water status.（4）During the period of soil water stress,different tree species in the same area showed different water use strategies.P.tabulaeformis and P.sylvestris tended to be isohydric behavior,while P.asperata tended to be anisohydric behavior.P.tabulaeformis and P.sylvestris could close stomata partly,reduce Gc,and inhibit the sensitivity of E to environmental factors as the result of soil water stress and high VPD,which led to the decrease of E.Due to low transpiration,high proportion of nocturnal sap flow to the daily total,and strong water absorption capacity of deep roots,P.asperata could refill capacitance efficiently.Compared with the period of non-soil water stress,its Gc,E,and their responses to environments did not change significantly.Therefore,there are differences in transpiration,environmental responses,and physiological controls between different tree species in the same area.For evaluating global change impacts,properly using water resources,and improving forest management strategies,specific local conditions and water use characteristics of tree species,should be taken into consideration to meet the requirements of tree growth and forest ecosystem development.