Transpiration Characteristics Of Two Typical Forests And Main Constructive Species In The Loess Hilly Region Of China

The Loess Plateau,with fragile ecosystems and serious soil erosion,is a key area for ecological improvement and protection in China.With the implement of ecological restoration projects,the vegetation coverage in this region has greatly increased,resulting in improvement of the ecological environment.However,the contradiction between vegetation and water and soil resources potentially restricts the sustainability of ecosystems.Under the background of vegetation restoration and global climate change,water use characteristics and the adaptation to drought in typical forest communities and main tree species in this region need intensive investigation.In this study,two typical forest communities(a secondary natural Quercus liaotungensis forest and a pure Robinia pseudoacacia plantation)and main tree species(Q.liaotungensis,Platycladus orientalis,Armeniaca sibirica,Acer stenolobum,R.pseudoacacia and Pyrus betulaefolia)were investigated.Besides measurements of tree growth in the plots,Granier-type thermal dissipation probes were applied to continuously monitor xylem sap flow in trees of the two forests from2011 to 2019.Solar radiation,air temperature,relative air humidity,precipitation,and soil moisture were monitored throughout the study period.Based on large-sample measurements of sap flow,characteristics of water use in the two forests and main tree species were explored,including sap flow dynamics in different timescales and relationships between transpiration and environmental factors.With the measurement of several leaf morphological and physiological indices(leaf water retention capability,specific leaf area,leaf water potential at predawn and midday,and leaf stable carbon isotope abundance),eco-physiological and water use characteristics of different tree species were clarified.The main results and conclusions are as follows:(1)The diurnal variation of sap flux density for main tree species showed a similar trend to those of solar radiation and vapor pressure deficit.With phenological dynamics for trees in growing season,sap flow of all species showed a trend of gradually increasing in the early growing season(May-June),reaching the highest value in the middle growing season(July-August),and slowly decreasing to a lower level in the late growing season(September-October).During the growing season from June to September,daily transpiration was positively correlated with solar radiation,vapor pressure deficit and air temperature(P<0.05),and negatively correlated with relative air humidity(P<0.05).Compared to the period with higher soil water content,all species had significantly smaller sap flux density during the period with lower soil water content(P<0.05).Sap flux density of R.pseudoacacia and P.betulaefolia reached the peak value obviously later in higher soil water content,whereas other species showed no significant change.These results suggested that the improvement of soil water condition could significantly promote the transpiration level.R.pseudoacacia and P.betulaefolia had stronger sensitivity to soil water.(2)In terms of leaf morphological characteristics,specific leaf area of R.pseudoacacia was the largest,with an average value of 18.83 m²/kg during 2017-2019.The specific leaf area of P.orientalis was the smallest,with an average value of5.15 m²/kg.The mean values of specific leaf area of Q.liaotungensis,A.sibirica,A.stenolobum and P.betulaefolia were 13.69,13.40,12.60 and 12.68 m²/kg,respectively.In terms of leaf physiological characteristics,the leaf water loss rate showed a descending order as:R.pseudoacacia(2.91%/h)>A.sibirica(1.25%/h)>Q.liaotungensis(1.22%/h)>A.stenolobum(1.06%/h)>P.betulaefolia(0.91%/h)>P.orientalis(0.42%/h),implying the best leaf water retention capability in P.orientalis,the worst in R.pseudoacacia and similar in other species.There were significant differences in leaf water potential at predawn or midday and sapflow driving force among tree species and among months(P<0.01),which may be related to species-specific characteristics and regulation in response to soil water changes.Sapflow driving force of Q.liaotungensis and P.orientalis(0.98-1.65 MPa and 0.55-1.30 MPa,respectively)were lower than those of other species,indicating that Q.liaotungensis and P.orientalis were less sensitive to changes in environmental factors and had stronger drought resistance.Leaf?¹³C decreased with increasing soil water content.The differences of leaf?¹³C among species,months and years were extremely significant(P<0.01),reflecting species-specific water use efficiency.By comparing and analyzing the diurnal patterns of sap flow and leaf morphology and physiology characteristics,we found that the stomatal sensitivity of P.orientalis,A.stenolobum,Q.liaotungensis and A.sibirica were stronger to ambient air humidity,but insensitive to soil moisture,showing conservative water use strategy.However,P.betulaefolia and R.pseudoacacia had weaker stomatal sensitivity to ambient air humidity and more sensitive to soil water,tending to use water more actively.(3)The seasonal variation of transpiration driving factors(solar radiation,air temperature,vapor pressure deficit,etc.)was basically stable.The annual rainfall ranged from 418.2-891.0 mm,and the annual potential evapotranspiration were932.5-1082.6 mm during the study period.Changes of daily average soil water content in the two plots were identical.The annual mean soil water content was positively correlated with annual rainfall.During the experiment,the mean diameter at breast height(DBH)and total sapwood area of the trees in Q.liaotungensis forest increased by 4.5 cm and 1.10 m²/ha,respectively.For the R.pseudoacacia plantation,the average DBH and the total sapwood area increased by 1.0 cm and 0.22 m²/ha,respectively.(4)Stand transpiration in Q.liaotungensis forest ranged from 51.3 mm to 105.2mm during the study period..The proportion of water consumption of species Q.liaotungensis in the stand water use was basically stable,indicating that Q.liaotungensis forest was in a stable growth stage.The total stand transpiration of R.pseudoacacia forest was 57.2-72.0 mm during the study period.Monthly stand transpiration of the two forests was mainly affected by phenological and meteorological factors.It had a significant positive correlation with leaf area index,monthly average solar radiation,monthly average air temperature and monthly total potential evapotranspiration(P<0.05),but was not correlated to monthly total rainfall and monthly average soil moisture(P>0.05).At the annual scale,for Q.liaotungensis forest,there was a significant negative correlation between annual stand transpiration and annual potential evapotranspiration(P<0.05),and a significant positive correlation between annual stand transpiration and rainfall of previous year(P<0.05),but no significant correlation with annual rainfall and annual average soil water content(P>0.05).For R.pseudoacacia forest,annual stand transpiration showed significant linear relationship with annual average soil water content(P<0.05),and non-significant correlation with annual potential evapotranspiration,annual rainfall and the previous year's rainfall(P>0.05).Stand transpiration of the two typical forests showed different responses to meteorological factors in dry and wet years.Daily stand transpiration could be fitted with solar radiation and air temperature using linear equation.The relationship between daily stand transpiration and vapor pressure deficit was fitted by exponential saturation function.For R.pseudoacacia forest,the slope of both the two linear equations and parameter a of the exponential saturation function was not significantly different(P>0.05),whereas parameter b showed significantly difference between dry and wet years(P<0.05).For Q.liaotungensis forest,the correlation coefficient(R²)of these relationships in wet year was higher than that in dry year.Meanwhile,the correlation parameters were all significantly different between dry and wet years(P<0.05).The results suggested that main tree species in the Loess Hilly region varied in water utilization.P.orientalis,Q.liaotungensis,A.sibirica and A.stenolobum tended to use water conservatively,whereas R.pseudoacacia and P.betulaefolia showed risky water use strategy.The growth condition and stomatal regulation were weakened in R.pseudoacacia forest under long-term drought,as well as lower sensitivity to environmental factors.R.pseudoacacia forest showed obvious impact of the contradiction between water supply and demand.Therefore,appropriate management should be carried out to improve the current situation.However,Q.liaotungensis forest could reduce stand transpiration by regulating stomatal efficiently to deal with drought.It would better maintain the water balance and stable growth to a certain extent.