| Water availability is the key restricting factor for vegetation construction and ecological restoration in arid and semi-arid area.Accurately quantifying the revegetated plant transpiration characteristics and understanding their water use strategies is essential to evaluate their ecohydrological effects,guide revegetated plant species selection and optimize revegetated plant composition configuration.This dissertation(1)designed an external heat-ratio sap flow(EHR)gauge suitable for thin stem plants as shrubs accurate sap flow measurements,and numerically simulated the effects of isotropy in sapwood thermal properties on sapwood sap-flux density and thermal properties sensing using a smart penta-needle heat-pulse probe(PHPP),developed a calibration model and validated in field tests;(2)investigated plant transpiration characteristics using sap flow methodologies and its responses to meteorology,soil water conditions,soil texture and topography,to typical revegetated shrub species Caragana korshinskii(C.k.)and Salix psammophila(S.p.)and tree species Salix matsudana(S.m.)and Populus simonii(P.s.)in the water-wind erosion crisscross region on the Northern Loess Plateau,under the background of Grain-for-Green project.This thesis mainly summarized and concluded as follows:(1)We developed a new EHR sap flow gauge,validated suitable for accurate sap-flux density(Vs)measurement of thin stem with diameter<15 mm and with Vs<50 cm3 cm-2 h-1.Results of artificial induced flow-gravimetric method test indicated that,the output heat pulse velocity(Vh)signal by EHR gauge was significantly linear correlated with actual Vs(R2 = 0.95,P<0.001),but the coefficients,msap,transferring Vh into Vs were apparently different,and the potential cause was the difference in stem physical structure between C.k.and S.p.,we strongly recommended separate gauge calibration experiment for different plant species.Field tests indicated that,EHR gauge is capable for accurate measurement of low rate and reverse flow in plant stems.We numerically simulated the effects of isotropy in sapwood thermal properties on sapwood Vs and thermal properties sensing using a smart PHPP coupled with INV-WATFLX algorithm,results indicated that,under different probe installation angle ?,errors in Vs measurement induced by isotropicy were ranged from-30%?+40%,and errors in thermal properties(thermal diffusivity,?,thermal conductivity,?,and heat capacity,C)under zero-flow conditions were within 12%;we developed a calibration model.Field tests indicated that,when PHPPs installed at ? = 15° and 30°,Vs and thermal properties were accurately obtained;but at ? = 0°,obtained Vs were underestimated by 30%,and the potential cause is the flow obstruction by insertion of three needles in a plane parallel to stem axial direction.But under zero-flow conditions,PHPP installed at ?=0° would obtain more steady thermal properties.(2)In study plots at the water-wind erosion crisscross region on the Northern Loess Plateau,results indicated that,mean Vs and whole-pant transpiration(T)of the 7-year-old S.p.stems were significant higher than that of C.k.During July to September,2017,mean Vs of S.p.was 8.26 cm3 cm-2 h-1,and mean whole-plant T was 2.6 kg d-1(or 2.22 mm d-1);mean Vs of C.k.was 5.43 cm3 cm-2 h-1,and mean whole-plant T was 1.92 kg d-1(or 0.77 mm d-1).Meteorological factors(reference evapotranspiration,ETo,solar radiation,Rn,and vapor-deficit pressure,VPD)is the most important driving factor of C.k.and S.p.,and they were significantly positively linear correlated with T(P<0.01).For C.k.,univariate ETo interpreted 91%of the daily whole-plant T variation,univariate Rn interpreted 83%,and univariate Rn interpreted 83%;for S.p.,univariate ETo interpreted 77%of the daily whole-plant T variation,univariate Rn interpreted 79%,and univariate Rn interpreted 61%.Partial correlation analysis indicated that,there were not significant correlationship between T of C.k.and soil water contents(SWC)in soil depth 0-100 cm;but T of S.p.were significantly correlated with SWC in soil depth 0-50 cm.Root investigation indicated that,S.p.had well-developed lateral root,and lateral root gathered in shallow soil layers to rapidly and effectively absorb shallow soil water transferred from rainfall;C.k.had fewer lateral root in shallow soil layers,mean fine-root-length-density(RLD)was only half that of S.p.,but it had well-developed vertical deep-root system,which could steadily absorb deep soil water,to avoid being affected by suffering drought stress in shallow soil layers.C.k uses water-saving and drought tolerance water-use strategy,and it uses deep soil water as a substantial water sources;S.p.uses water-consumption water-use strategy,and it uses shallow soil water as main water resources.Given the fact that C.k.consumes less soil water,has higher drought tolerance and soil reinforcement intermedia root and soil,we recommended C.k.prefer to S.p.to use in revegetation practices in wind-water erosion crisscross region.(3)By long-term(at most up to seven years)and simultaneous monitoring of sap flow,meteorological factors,soil water conditions and et al.for>30-year-old S.m.and P.s.in an aeolian sandy soil sloping field and a loessial soil dam field in the water-wind erosion crisscross region,results indicated that,mean whole-tree T of S.m.and P.s.in dam field in growing season were 46.7 and 175.2 kg d-1,respectively,and they were 5.6 and 4.2-fold of the same species in sloping field,respectively;overall,whole-tree T of P.s.was about 4-fold of S.m.Meteorological factor(ET0,Rn and VPD)were the controlling factors driving tree T of S.m.and P.s.,and they were significantly correlated with tree T(P<0.01).Univariate ETo interpreted 37%of variation in S.m.T in sloping field,77%for S.m.in dam field,and 80%for P.s.in both sloping field and dam field.T of S.m.in sloping field were significantly correlated with shallow layer SWCs in 0-200 cm depth(P<0.05);but for P.s.in sloping field and S.m.and P.s.in dam field,there had no consistent correlationship between T and SWCs in shallow soil layers.Annual rainfall soil-water recharge depth reached up to 600 cm in wet years,but only 120 cm in dry years on sloping field with aeolian sandy soil;soil water deficit formed in the 0-600 cm soil layer on the aeolian sandy soil field in normal and dry years,and soil water was replenished during wet years;both shallow and deep soil water are primary water sources for trees in aeolian sandy soil sloping field.For loessial soil,recharge depth was only 200 cm even in wet years in dam field,and trees on the dam field absorbed water mainly from shallow groundwater.Tree fine roots gathered in the subsurface soil on the sloping field but not in the dam field indicated a root distribution adaption to precipitation patterns and water-use strategy.Neither species is suitable for extensive revegetation implementation in this area due to their negative impacts on water resource sustainability. |
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