Influence Mechanism Of Thinning And Pruning On Water Use Of Caragana Korshinskii On The Longzhong Loess Plateau

Caragana korshinskii has been introduced into the hilly and gully area of the Longzhong Loess Plateau for vegetation restoration and establishment since the 1980s.However,the over-dense C.korshinskii increased the excessive consumption of soil water due to unreasonable afforestation in some areas,causing soil desiccation and deterioration of the ecological service in the C.korshinskii stand.Thus,the typical C.korshinskii in Dingjiashan watershed,located in Chankou town of the Dingxi city in Gausu Province was chosen.To optimize structure adjustment and utilization of the soil water,the field experiments were conducted from 2018 to 2020.With the grid distribution of points for soil water monitoring,the temporal and spatial distribution characteristics and key driver factors of soil water under different site conditions(slope aspect and slope position,etc.)were clarified.Then,the effect of the thinning and pruning on soil water restoration,vegetation growth,and transpiration of different scales in C.korshinskii stand were quantified.The experimental treatments included CK(maintained in its original state),T1(thinned once at a one-shrub interval),T2(thinned once at two-shrub intervals),T3(pruned once at one-shrub intervals),P25(25%of the branches pruned),P50(50%of the branches pruned),P75(75%of the branches pruned).In addition,the rainout shelter was used to simulate the effect of precipitation regimes on shrub growth and water use,including P-30%(30%decreased precipitation),PCK(normal precipitation treatment)and P+30%treatments(30%increased precipitation).The main results of this study are as follow:(1)Exploring spatio-temporal variability of soil water in the 0-180 cm soil layers for the C.korshinskii under different positions on sunny and shady hillslopes.Soil water content(SWC)in the 0-180 cm soil layers for the C.korshinskii presented different horizontal but similar vertical change trends.The mean SWC of the sunny hillslope in the 0-180 cm soil layers was lower than that of the shady hillslope.The soil desiccation indexes of sunny and shady hillslopes were 54.3%and 59.8%,respectively,which was the moderate desiccation grade.The SWCs on both hillslopes were not enough to maintain the good growth of the C.korshinskii stand(7000-9000shrub·hm^-2).The temporal stability characteristics of each soil layer on sunny and shady hillslopes varied in temporal persistence and representative locations.With the increasing soil depth,the average value of soil water stability index was 0.13 at the 0-20 cm soil layer and decreased to 0.11 at the 0-180 cm soil layer.(2)Revealing the effects of thinning and pruning on soil water in the 0-600 cm soil layers for the C.korshinskii and vegetation growth.Thinning C.korshinskii was beneficial to the recharge of SWC at the 0-180 cm soil layer,but it had no significant effect on the recovery of SWC in deep soil layers(200-600 cm),while the effect of pruning on SWC was not significant.Thinning and pruning promoted the regeneration of the C.korshinskii,which was beneficial to the increase of the stem length,basal diameter and biomass.However,there was no significance on the diversity index of understory herbaceous vegetation.On the sunny hillslope,the SWC in the T3 treatment at the 0-180 cm soil layer was 21.9%-55.0%significantly higher than that in the CK treatment(P<0.05).The SWC in T1 and T2treatments was significantly increased by 4.0%-32.5%and 5.6%-26.4%compared with those in CK treatment in 2020,respectively(P<0.05).On the shady hillslope,SWC gradually increased with the increase of thinning intensity.T1 treatment had the highest SWC,followed by T2 and T3 treatments,and CK treatment was the lowest.(3)Clarifying the effects of thinning and pruning on transpiration of C.korshinskii at the stem-scale.The stem-scale SF of C.korshinskii increased firstly,and then decreased during the growing seasons under different thinning and pruning treatments.Thinning and pruning treatments increased the sensitivity of the SF at the stem scale to meteorological factors.In 2019,the average SF in T1 and T2 treatments was decreased by 34.2%and57.9%than that in the CK treatment,respectively.In 2020,the SF in T1 and T2treatments was decreased by 26.9%and 39.0%than that in the CK treatment,respectively(P<0.05).The air temperature(T_a),photosynthetically active radiation(PAR),vapour pressure deficit,the variable of transpiration(VT),and reference evapotranspiration significantly affected the SF.Compared with CK treatment,the lag time between SF and meteorological factors of the PAR,T_a,and VT was shorter,but the sensitivity was higher.(4)Determining the effects of thinning and pruning on transpiration of C.korshinskii at the individual and stand scales.The individual-scale water consumption(T)of the C.korshinskii decreased significantly with the increase of thinning intensity,but the effect of pruning intensity on individual-scale T was not significant difference.At the stand scale,transpiration water consumption(T_stand)decreased with increasing thinning and pruning intensity.At the individual scale,the T in T1 and T2 treatments was significantly reduced by 38.2%and 43.5%compared with that in CK treatment,respectively;the T in T1 and T2treatments was significantly lower by 17.9%and 37.6%than that of CK treatment,respectively(P<0.05).The water use efficiency(WUE)was not significant difference among the pruning treatments in 2019.In 2020,the WUE in the T2 treatment was significantly increased by 13.8%than that in CK treatment,and it in P25,P50,and P75treatments was significantly lower by 53.2%,44.5%,and 35.3%than that of the CK treatment(P<0.05).In 2019,the T_stand in T1 and T2 treatments were significantly lower by 69.1%and 81.2%than that of CK treatment(P<0.05).In 2020,compared with CK treatment,the T_stand in T1 and T2 treatments was significantly reduced by 59.0%and79.2%,respectively(P<0.05).As for the pruning treatments,the T_stand in the P75treatment was the lowest,and were significantly lower by 59.1%in 2019 and 40.1%in2020 than those of the CK treatment,respectively(P<0.05).(5)Simulating and predicting the water consumption characteristics of C.korshinskii under different precipitation regimes.It increased soil water content,canopy transpiration(T_c)and water use efficiency(WUE)after increasing 30%precipitation.There were significant differences on SWC of the 0-180 cm soil layer under different precipitation regimes,and the SWC in the 0-80 cm soil layer had a more significant change.In 2019 and 2020,the mean SWC at the 0-80 cm soil layer in the P+30%treatment was significantly increased by 13.6%-57.9%than that in PCK and P-30%treatments,respectively(P<0.05).In 2019,there was no significant difference in T_c and WUE of the C.korshinskii under different precipitation regimes.In 2020,the T_c in the P+30%treatment was significantly increased by 21.1%and 187.5%compared with that in PCK and P-30%treatments,respectively(P<0.05).Compared with PCK and P-30%treatments,the WUE in the P+30%treatment was significantly increased by 19.4%and 60.9%,respectively(P<0.05).In summary,thinning C.korshinskii was beneficial to promote the recharge of SWC at shallow soil layers,and recovery at deep soil layers.Thus,the treatment of thinned two plants at two-shrub intervals(T2)was recommended.The results of this study can theoretically help to reveal the ecological and hydrological processes and impact mechanisms in the C.korshinskii after the structure adjustment in semiarid region,and provide guidance for the rational structure of vegetation and high-efficient utilization of water resources in the semi-arid area of the Longzhong Loess Plateau.