Photosynthetic Physiological And Ecological Characteristics Of Photosynthetic And Transpiration Of Artificial Forest & Shrub Stands In Loess Hilly Region

In recent years,climate warming had led to uneven distribution of water on the earth’s space and time,which had exacerbated the polarization of dry and wet regions in China.Since the“returning farmland to forest and grassland”project was launched in the Loess Hilly Area,ecological vegetation coverage had increased year by year,but the selection of tree species was unreasonable and single forest stand has deepened the water shortage in the area.It was of practical significance for the sustainable utilization and promotion of regional water resources to give full play to the benefits of water and soil conservation through a reasonable combination of artificial tree and shrub tree species.This research was based on pure artificial tree and shrub in Loess Hilly Area:Pinus tabulaeformis,Platycladus orientalis,Forsythia suspensa,Syzygium aromaticum,Amorpha fruticosa and mixed plantation（mixed Pt.&Fs.）,LI-6400XT photosynthetic physiological tester was used to clarify the physiological characteristics and influencing factors of photosynthesis and transpiration,as well as the law of transpiration water consumption,provided a theoretical basis for future artificial tree species selection and ecological stability evaluation in this area.The main conclusions were as follows:（1）The net photosynthetic rate（Pn）and transpiration rate（Tr）of different tree and shrub species showed two kinds of diurnal changes in each month.Pn of Pt.and Fs.showed a bimodal curve in Jul.,the second peak appeared 2 h earlier than Pt.,the daily change of Pn of Po.showed a single-peak pattern,and the daily change of Pn of Af.showed an inverted“V”pattern.Tr had a small intra-day fluctuation range among tree species,basically showing a"single peak"or"double peak"trend.Tr of Fs.and Af.both showed a single peak change curve from Jun.to Sept..Tr of Pt.showed a bimodal change pattern only in Sept..Sa.was prone to transpiration at the beginning and end of the growth season.For the Po.effect caused by the low value,Tr peak appears 2 h later than other tree species.The daily progress of water use efficiency（WUE）of Pt.and Po.showed a"single-peak"trend in Aug.,and the other periods were all"M"-shaped curves.The daily changes of WUE of Fs.,Sa.and Af.were in May and Jun.showed a"V"-shaped curve.（2）Seasonal changes of Pn,Tr and WUE of different tree species had strong regularity,and the seasonal differences were different.Seasonal variation of Pn of each tree species was the highest in the peak growing season.Difference in Pn between species was shrub species>tree species.The average Pn(μmol·m^-2·s^-1)order in the growth season of tree and shrub pure forest was:Sa.（13.25）>Fs.（12.61）>Af.（12.32）>Pt.（9.92）>Po.（6.93）.At the beginning and end of the growing season,transpiration of shrub species was weaker than tree species,while the opposite was true in the peak growth period.Tr values of Pt.in May and Aug.were 83.59%and 64.90%higher than those in the weaker transpiration period（Sept.）,and the other tree species reached their Tr peaks in Jun.and Aug..The average sequence of Tr(mmol·m^-2·s^-1)for tree and shrub species from May to Sept.was:Af.（4.09）>Pt.（3.40）>Sa.（3.32）>Fs.（3.16）>Po.（3.06）.WUE of Po.was only lower than Sa.by 0.35μmol·mmol^-1 in Aug.,and water use capacity of Sa.and Fs.was significantly higher than the other three tree species in rest of the period.Seasonal variation of WUE of all tree species showed a"W-shaped curve".The average WUE(μmol·mmol^-1)of tree and shrub species from May to Sept.was as follows:Fs.（4.94）>Sa.（4.70）>Af.（3.63）>Pt.（3.16）>Po.（2.78）.（3）Through photosynthesis-light response process,it was found that Pt.and Po.were typical high-light-positive tree species,and three shrub species were low-light-positive tree species.The shrub species had a stronger ability to use low light than tree species.Fs.and Sa.had the highest photosynthetic potential;Tr of Po.,Sa.,and Af.always showed an upward trend with the increase of light intensity（PAR）.The transpiration-light response curves of Pt.and Af.were overall higher;WUE of Af.showed a"parabolic"curve with the increase of PAR.In different PAR intervals,both Sa.and Fs.WUE maintained a high trend.（4）Stomatal conductance（Cond）,stomatal limit value（Ls）and PAR factor had a positive feedback correlation to Pn of different tree species;Cond,air relative humidity（RH）and PAR were the dominant factors that cause the Tr of different tree species to change,and Tr and RH show negative correlation.Pn and Tr of different tree species were inversely proportional to soil volumetric water content（VWC）,while Pn of Po.increased with the increase of VWC1（20 cm）,and Tr of Fs.was proportional to VWC1-2（20 cm-50 cm）.（5）At the single tree level,water consumption of Af.was much higher than other tree species,and transpiration water consumption of Pt.was inferior to Af.;water consumption of Pt.and Fs.in May and Sep.was higher than mixed Pt.and Fs.,performance was opposite from Jun.to Aug.;seasonal variation characteristics of transpiration water consumption of all tree species were inverted"V"-shaped curves,and water consumption peaks appeared in the peak growth season;the order of the cumulative amount of transpiration water consumption of each tree species from May to Sept.was:Af.（53.69 kg）>Pt.（13.43 kg）>Fs.（7.71 kg）>Sa.（4.98 kg）>Po.（3 kg）;performance of pure and mixed tree species was as follows:Pt.mixed（18.65 kg）>Pt.（13.43 kg）,Fs.（7.71 kg）>Fs.mixed（7.51 kg）.The order of photosynthetic efficiency of each tree species was:Sa.>Fs.>Af.>Pt.>Po.;the order of transpiration water consumption was:Af.>Pt.>Sa.>Fs.>Po.;the order of water use efficiency was:Fs.>Sa.>Af.>Pt.>Po..In the future afforestation process in the Loess Hilly area,Sa.and Fs.should be selected as the constructive species for planting in this area,and Af.was suitable for planting as mixed tree species in this area due to its high transpiration water consumption.