A Study On Water Use Efficiency And Xylem Embolism Resistance Of Main Afforestation Tree Species In Loess Hilly Region

Affected by climate and human activities,the loess hilly region was a typical ecologically fragile region.Through the project of returning farmland to forests,soil erosion and plant diversity had been effectively improved,but the water consumption of vegetation and insufficient water resources had became bottlenecks restricting the growth and restoration of artificial forests.This study aims to investigate the water use efficiency and its underlying mechanisms in different artificial forests.Specifically,the focus is on the main tree species found in the Loess Hilly Region,namely Robinia pseudoacacia,Hippophae rhamnoides,Pinus tabulaeformis,and Populus microphylla.Measurements were taken for photosynthetic parameters and water potential of these four tree species.The diurnal variations and seasonal dynamics of these parameters were analyzed.Furthermore,the anatomical structure of leaves and xylem were observed,providing insights into the characteristics of leaf stomata and xylem vessels among the different tree species.Through correlation analysis,the relationship between water use efficiency and leaf photosynthetic parameters,leaf traits,xylem anatomical structure,and function of the four tree species was established.The aim was to offer a theoretical foundation for the efficient utilization,sustainability,and stable restoration of water resources in artificial forests within the Loess Hilly Region.The key research findings are as follows:1.The four sample plots have low soil moisture periods in May and June,and high soil moisture periods from July to September.The diurnal variation of leaf water potential of four tree species is U-shaped,and the seasonal variation of leaf water potential at dawn and noon increases with the increase of soil water content.By fitting the dawn leaf water potential and noon leaf water potential of four tree species,it is found that R.pseudoacacia and P.microphylla adopt equal water regulation strategy,while H.rhamnoides and P.tabulaeformis prefer unequal water regulation strategy.The daily variation trends of instantaneous water use efficiency（WUE_i）and intrinsic water use efficiency(WUE_inst)of four tree species are:gradually decreasing with time,first decreasing and then increasing with time.The WUE_inst of the four tree species during the high soil moisture period was higher than that during the low soil moisture period,while the WUE_inst during the high soil moisture period was lower than that during the low soil moisture period.2.The stomata of H.rhamnoides leaves are covered with peltate hairs,and the stomatal characteristics of the other three tree species are as follows:the stomatal length,width and opening of P.tabulaeformis are significantly higher than those of P.microphylla and R.pseudoacacia（P<0.05）,while the stomatal density is significantly lower than that of P.microphylla and R.pseudoacacia（P<0.05）.It shows that non-water species have high stomatal opening,and xylem is prone to embolism when water is deficient.The wood embolism vulnerability curves of R.pseudoacacia and H.rhamnoides show an"r"shape,while the xylem embolism curves of P.tabulaeformis and P.microphylla show an"s"shape.It shows that R.pseudoacacia and H.rhamnoides are prone to embolism under water stress,while P.tabulaeformis and P.microphylla are prone to embolism under higher water stress.The magnitude of embolism resistance of four tree species is P.microphylla>P.tabulaeformis>H.rhamnoides>R.pseudoacacia.4.Photosynthetic characteristics of leaves and soil water content are the main influencing factors of water use efficiency.The water use efficiency of four tree species was positively correlated with soil water content,net photosynthetic rate and specific leaf area（P<0.05）,and negatively correlated with saturated vapor pressure deficit（P<0.05）.The water use efficiency of plants is also affected by the anatomical structure and hydraulic function of xylem.The results of correlation analysis showed that the water use efficiency of four tree species was positively correlated with xylem vessel diameter and embolism vulnerability（P<0.05）,and negatively correlated with xylem density and xylem vessel wall reinforcement coefficient（P<0.05）.The water use strategies of R.pseudoacacia and P.microphylla belong to isohydry.When the soil moisture is low,the leaves of R.pseudoacacia and P.microphylla will reduce their stomatal opening,which will reduce the risk of plant death due to water loss,but also affect their photosynthetic rate and water use efficiency,which is why P.microphylla has strong resistance to embolism and low water use efficiency.R.pseudoacacia’s resistance to embolism is weak,which may be related to its low vessel wall reinforcement coefficient and xylem density,and its water use efficiency is high,which may be due to its low transpiration rate.The water use strategies of H.rhamnoides and P.tabulaeformis tend to be non-equal,that is,in the period of low soil moisture,the stomatal opening of H.rhamnoides and P.tabulaeformis leaves decreases slightly,which can ensure their photosynthesis and water use,but it will increase the risk of xylem vessel embolism,which is why H.rhamnoides and P.tabulaeformis have low resistance to embolism but high water use efficiency.