| Loess hilly region has low rainfall and large evaporation.Rainfall is the main limiting factor for vegetation restoration in this area.The survival and growth of plants are closely related to the availability of rainfall.Since the relationship between water supply and plant water consumption was not fully considered in the previous afforestation process,with the increase of water consumption in a plantation,the survival rate,the preservation rate and the growth rate of which were generally low,and a large area of small old trees were formed.In order to evaluate the stability of plantation for water use in this area,it is necessary to study the impact of rainfall on soil volumetric water content?SWC?,water source of plantation and the response of plantation's water consumption to environmental factors?especially rainfall?for vegetation restoration.Due to the highly soil and water conservation values,mixed plantation were widely used during grain for green.However,there were less report about whether or not a mixed plantation have changed the plants water strategy.This study was based on Pinus tabulaeformis and Hippophae rhamnoides in a pure and mixed plantation,from June to October 2015&2016,using automatic weather station and thermal dissipation probe?TDP?and soil water system?EMH2O-EM50?to continuously measure the meteorological factors?rainfall,photosynthetically active radiation?PAR?and water vapor pressure deficit?VPD??of sample area,the sap flux density?Fd?of sample tree and the SWC data of woodland,respectively.In addition,ultra-vacuum extraction and isotope mass spectrometry device were used to analyze the ratios of hydrogen and oxygen isotope(?D/?18O)in rainfall,xylem and soil water.Using the methods of variance analysis,Threshold-Delay?T-D?model,stepwise multiple regression analyses,Isosource model and partial correlation analyses,first we studied the SWC in response to rainfall in different woodland,then we have detected the water resource from two trees using?D and?18O,and analyzed the process of Fd of two species response to the rainfall pulses and we also determinedthe effects of environmental factors on Fd.The results were as follows:?1?The SWC in 0-20 cm?SWC0-20?was increased after continuous small rainfall?<5mm?,but did not change after individual rainfall,indicated the accumulation of the process.When the rainfall beyond 5 mm,the SWC was attached to pick value and then decreased in all layers.However,another rainfall happened after which usually not increased the SWC,illustrated the relationship between SWC and rainfall amount.Rainfall above 5 mm could caused SWC0-20 in all plantation and SWC20-50 in a pure plantation?Hippophae rhamnoides?,rainfall above 10 mm could caused SWC20-50 in a mixed plantation and SWC50-100 in a pure plantation?Hippophae rhamnoides?,and rainfall above 15 mm could caused SWC between 20and 150 cm from Pinus tabulaeformis in pure plantation and SWC50-100 and SWC100-150000-1500 in a mixed plantation increased significantly.There was significant correlation between rainfall and SWC in 0-50 cm for Pinus tabulaeformis in a pure plantation,and 20-100 cm for Hippophae rhamnoides in a pure and mixed plantation,but SWC in 0-20 cm was loss to vaporation largely,so that rainfall has good infiltration in 20-50 cm,20-100 cm and 20-100cm in three woodland.The relationship between SWC and rainfall were as follows,mixed plantation with Pinus tabulaeformis and Hippophae rhamnoides>Hippophae rhamnoides in a pure plantation>Pinus tabulaeformis in a pure plantation.?2?The?D/?18O from xylem water of Pinus tabulaeformis in a pure plantation was between-65.77‰and-49.50‰,and between-9.28‰and-6.60‰,which was similar to the SW in middle?-68.57‰and-51.24‰,-7.04 and-8.96‰?.The xylem water of Hippophae rhamnoides in pure a plantation was ranged from-66.33‰to-50.33‰and from-8.85‰to 5.98‰,which was similar to the SW in shallow?-58.42‰and-46.02‰,-7.34‰and-4.86‰?and middle?-60.77 and-54.14‰,-8.79‰and-7.06‰?.The?D/?18O from xylem water of Pinus tabulaeformis in a mixed plantation was ranges from-67.51‰to-49.44‰and from-9.68‰to-5.80‰,which was similar to the shallow?-57.44‰and-37.33‰,-9.22 and-3.51‰?and middle?-64.79‰and-34.07‰,-10.89‰and-6.76‰?SW.The xylem water of Hippophae rhamnoides in a mixed plantation was ranged from-66.90‰to-37.97‰and from-10.89‰to-6.76‰,which was similar to the shallow?-74.41‰and-50.04‰,-10.89‰and-6.76‰?SW.The relationship between?D and?18O from xylem water of Pinus tabulaeformis in a pure plantation?R2=0.90?was similar to the local precipitation line?R2=0.94?,while the Hippophae rhamnoides in a pure plantation?R2=0.54?was closed to shallow?R2=0.48?.),middle?R2=0.42?and deep?R2=0.50?SW.Both Pinus tabulaeformis?R2=0.65?and Hippophae rhamnoides?R2=0.70?in a mixed plantation was closed to the deep?R2=0.64?SW.Two species in different woodland were mainly used deep SW after small rainfall?0.1-9.9 mm?,but it were used shallow SW when the rainfall beyond 10 mm.?3?With thean rainfall class increase,the response percentages of Fd for Pinus tabulaeformis was sustained increased in a pure plantation,while the Fd of Hippophae rhamnoides in a pure and mixed plantation with Pinus tabulaeformis and Hippophae rhamnoides were increased at first but then decreased.In the range of 1–5 mm rainfall,the Fd of Pinus tabulaeformis in a pure plantation and Hippophae rhamnoides in a mixed plantation were increased significantly,whereas both Hippophae rhamnoides in a pure plantation and Pinus tabulaeformis in a mixed plantation were decreased.The lower rainfall threshold?RL?of Fd for Pinus tabulaeformis and Hippophae rhamnoides in a pure and mixed plantation were4.1 mm,3.3 mm,6.4 mm and 1.9 mm,respectively,with a corresponding time-lag??of 1.43days,1.06 days,1.96 days and 1.67 days.Compared with Pinus tabulaeformis and Hippophae rhamnoides in a pure plantation,the RL of Pinus tabulaeformis and Hippophae rhamnoides in a mixed plantation were increased 56.10%and decreased 42.42%,and the of which were increased 37.06%and 57.55%.In the pre-and post-rainfall period,the peak time of Fd for Pinus tabulaeformis in a pure and mixed plantation converged upon 10:30—13:00?58.5%?,while the Fd of Hippophae rhamnoides in a pure plantation peaked twice,between 10:00 and11:30?41.5%?and again between 12:30 and 14:30?18.5%?,Hippophae rhamnoides in a mixed plantation peaked between 10:30 and 13:00?50%?and again between 16:00 and 17:00?27%?.When the SWC was enough in earlier,the changes Fd of Hippophae rhamnoides was effected by SWC,but it was effected by rainfall when the SWC was short.However,the Fd of Pinus tabulaeformis was just effected by rainfall.The effects of PAR to the Fd of two species in a mixed plantation was decreased after rainfall,and it was decreased for the effects of VPD to the Fd of Pinus tabulaeformis,while of which was strengthened for Hippophae rhamnoides in a mixed plantation.Both species were effected by SWC after rainfall,and SWC0-20 has less effects to the Fd of Pinus tabulaeformis,but it was become the dominant factor for the Fd of Hippophae rhamnoides in a mixed plantation.As it was shown above,>5 mm rainfall has caused SWC increased significantly between 0 and 150 cm layers in a mixed plantation,indicated that of which has maximum infiltration.When the rainfall range between 0.1 mm and 9.9 mm,both Pinus tabulaeformis and Hippophae rhamnoides were used deep SW largely,but it were used shallow SW,when the rainfall exceed 10 mm.Compared with Pinus tabulaeformis and Hippophae rhamnoides in a pure plantationn,the RL of which increased 56.10%and decreased 42.42%,respectively,in a mixed plantation.In summary,two tree species have adapted to the local climatic environment,and formed different water use strategies. |
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