| Poplar is one of the fast-growing timber species with the largest cultivated area and the highest wood yield in the mid-latitude plains of the world.It is also the main tree species of fast-growing timber forests in northern China.Populus tomentosa is widely planted in the North China Plain due to its fast-growing and high-yield advantages.Achieving the great growth potential and improving the stand productivity of P.tomentosa need to depend on the high-effective root water uptake.In order to maximize the root uptake function,it is necessary to clarify the water uptake potential of root system and the corresponding water uptake and utilization strategies.In this study,root system characteristics and their spatiotemporal variations,root water uptake strategies,canopy transpiration behaviors and stand water consumption prediction models of the P.tomentosa planted in the North China Plain were investigated.Based on the above research,the systematic process of "root distribution-root water uptake-canopy water consumption" in P.tomentosa plantation was comprehensively analyzed and the water uptake,transport and utilization strategies in SPAC system of P.tomentosa was clarified.Meanwhile,the accurate prediction of canopy transpiration and soil evaporation of P.tomentosa plantation under the condition of sufficient moisture in shallow soil was also achieved.The results of this study will provide references for precise water management of P.tomentosa plantation.The main results and conclusions of this study are as follows:(1)Fine roots(?2 mm)were continuously sampled in P.tomentosa plantations with different ages and planting densities.Six root factors including root length density,root area density,root biomass density,root diameter,root tissue density and specific root length were measured and their differences among stands were compared.Results showed that fine roots of P.tomentosa were mainly distributed within 20 cm soil layers(25%-44%root length density),and relatively enriched in the soil at the depth of 70-140 cm.The variation of stand age and planting density would not change the vertical distribution pattern of fine roots but lead to a gradual increase of fine roots enrichment in deep soil layers with stand age.The increase of stand age and planting density caused the horizontal distribution of fine roots to change from decreasing to uniform.The spatial variation of fine root morphology in P.tomentosa plantation was relatively small,but it showed temporal variation with stand age.As increase of stand age,the proportion of absorbing roots within the diameter range of 0-0.1 mm decreased and the proportion of transport roots within the diameter range of 0.2-0.7 mm increased,which indicated that the transport function of root system enhanced.The dso(depth corresponding to 50%of the cumulative fine root)and d90(depth corresponding to 90%of the cumulative fine root)of P.tomentosa fine roots varied in the range of 32.3-62.0 cm and 118.0-137.0 cm,respectively.The variations of stand age and planting density did not apparently change the distribution depth of fine roots but affected the distribution proportion at different depths.(2)Under the environmental condition of an average fluctuation of 363 cm in groundwater table,three coordinate rooting adaption strategies were observed in P.tomentosa plantations experiencing a long-term large groundwater fluctuation:a deep root system(maximum depth 270 cm),wide-spreading and dense lateral roots in shallow soil layers,and structural roots exploring both deep and shallow soil resources.(3)In the 7-year-old P.tomentosa plantation under natural growing condition,soil water content was consecutively measured using time-domain reflectometry(TDR)technique.The soil water balance method was used to deduce the root water uptake amount at each spatial location.Then,root water uptake efficiency was calculated combined with fine root spatial distribution characteristics.Meanwhile,stable isotope ratios of branch,soil water and groundwater were also measured at different periods of the growing season.Results indicated that root water uptake rate showed a strong spatial and temporal variation.During the period when the soil moisture in shallow layers was high,the root water uptake rate increased correspondingly.Although there was more sufficient water resource in deep soil,P.tomentosa roots still preferred to use the soil water in the shallow layers.The root water uptake amount within 140 cm soil depth accounted for 69%of the total water uptake amount throughout the soil profile.Root water uptake amount in shallow soil was significantly higher than that in deep soil and was relatively dominant within 50 cm away from the trees and in the center of wide row.However,root water uptake amount from the soil layers at the depth of 220-260 cm was relatively low,with the average of 0.0835 cm3 cm-3.Root length density showed a similar spatial distribution characteristic as root water uptake amount,vertically,they both concentrated in 0-10 cm and 80-140 cm soil layers.There was only 3%fine root distributed in 220-260 cm soil layers,but their water uptake amount accounted for 12%of the total water uptake amount throughout the soil profile.Root water uptake efficiency in deep soil layers(220-260 cm)was three times higher than that at the depth of 0-140 cm.Therefore,P.tomentosa adopted the strategy of increasing the root water uptake efficiency to utilize the deep water resources.Additionally,P.tomentosa is a kind of phreatophyte,as it is capable to directly absorb and use groundwater resource.The contribution proportion of groundwater to root water uptake increased with the rising groundwater table,and the maximum during the experimental period reached 81%.(4)With the help of thermal dissipation probes,the sap flux dynamic of P.tomentomsa was monitored continuously.In addition,the seasonal variation in the diurnal(Qday)and nocturnal stand water use(Qnight)of P.tomentosa plantation were investigated,and their responses to groundwater table(GT)fluctuations and the meteorological factors were also explored.Results indicated that the two-year average of Qday and Qnight were 1.90 and 0.12 mm d-1,respectively.Qnight occurred with a different seasonal dynamic from Qday,and an inter-annual variation was also observed in both Qday and Qnight.During the experimental period,stem refilling accounted for 61%of Qnight,and a distinct increase in the ratio of nocturnal to daily water use appeared at the beginning and the end of the growing season,which might be helpful to the timely recovery of the xylem hydraulic conductivity of P.tomentosa.Groundwater had a compensatory effect on Qnight of P.tomentosa.When the GT varied within 170 cm,the compensation effect of groundwater on the Qnight could reduce the consumption of water stored in the stem.When the GT was deeper than 210 cm,the compensatory effect of groundwater decreased,so water supply during this period was essential for P.tomentosa plantation.Positive(P<0.001)and negative(P<0.01)correlations were found in meteorological factors with Qday and Qnight,respectively.As the stand age of P.tomentosa plantation varied,the key meteorological drivers of both Qday and Qnight also changed.Among the single meteorological factors except for ET0(reference crop potential evapotranspiration),vapor pressure deficit(VPD,53.0%)and photosynthetically active radiation(PAR,63.8%)explained the highest variation in Qday of P.tomentosa plantation in 2010 and 201 1,respectively.In contrast,Qnight was dominantly affected by VPD(27.0%)in 201 1.(5)Using the canopy transpiration and soil evaporation data measured in the field,the quantitative relationships of the basal crop coefficient(Kcb)and soil evaporation coefficient with leaf area index(LAI)and soil temperature were established according to the FAO crop coefficient methods,i.e.the empirical prediction models of canopy transpiration and soil evaporation.Results indicated that the Kcb model constructed based on LAI data can be used to accurately predict the canopy transpiration of P.tomentosa plantation under well-watered condition with the RMAE of 16%.But the relationship between Kcb and LAI changed with stand age and shallow soil water availability,and this variation was mainly reflected in the Kcb plateau value and the LAI critical value.Under well-watered drip irrigation condition,distinct spatial heterogeneity in Es appeared only before the rainy season(from April to June),during which the Es from the wet soil zone accounted for 66%of the total Es on average.Among the four empirical models,the L-ww(constructed model based on LAI data during the "well-watered" period)and LT-ww(constructed model based on data of LAI multiplied by soil temperature at 20 cm depth during the "well-watered" period)models can be used for Es prediction during the whole growing season and the "well-watered" period,respectively,with RMAE of 19%and 18%,respectively.(6)In summary,in the North China Plain,the following strategies should be adopted when applying water management in P.tomentosa plantation under groundwater fluctuation environment.Firstly,for P.tomentosa plantations with small age and planting density,irrigation should be mainly applied to the surface 20 cm soil with small amount but multiple times.In the horizontal direction,irrigation should be conducted to the area around the tree.However,for plantations with large age and planting density,irrigation should be applied to the surface soil in the whole growing space of each tree.And in order to promote the water migration to deep soil,the duration of each irrigation should be increased.Secondly,when the GT was deeper than 210 cm,both the irrigation frequency and the irrigation amount should be increased.But as the GT becoming shallower,irrigation measures could be reduced correspondingly.Thirdly,in order to avoid the adverse impacts of water consumption of plantations on groundwater resource and prevent plantations from being ecologically vulnerable due to the excessive reliance on groundwater resource,appropriate water supply should be applied to the shallow soil of P.tomentosa plantation,but the irrigation amount should be controlled.Fourth,when the ground was not completely covered by canopy,water conservation methods should be taken for wet soil area to reduce the unnecessary water consumption.Finally,when determining the irrigation amount of P.tomentosa plantation,the empirical models based on LAI and soil temperature data constructed in this study can be used to accurately predict canopy transpiration and soil evaporation. |
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