| Shortage of water resources is the main limiting factor for the functional stability of the forest ecosystem in the loess hilly region,and the transpiration and evapotranspiration of trees as the main body of the forest account for a large proportion in the forest water cycle.In this study,the Granier-type thermal diffusion probes(TDP)and Circumference Dendrometer 3(DC 3)were applied to measure the xylem sap flow and trunk diameter variation of typical afforestation tree species of Quercus liaotungensis,Robinia pseudoacacia,Platycladus orientalis and Acer stenolobum in loess hilly region,and simultaneously monitoring of environmental factors related to tree transpiration and water consumption,including photosynthetically active radiation(PAR),air temperature(T),air relative humidity(RH)and soil water content(SWC).We studied the changes of the trunk sap flow and diameter changes on the daily and seasonal scales,as well as the response characteristics to environmental factors.Differences in the time lags between Q.liaotungensis and R.pseudoacacia Fd and environmental factors and possible influence by different diameter classes and soil moisture conditions were analyzed.The main conclusions are as follows:(1)The characteristics of the diurnal variation of the sap flow of the sample tree basically follow the diurnal variation process of transpiration driving factors(PAR,T and RH),but there are differences in the specific start-up time and peak occurrence time.At the early and middle of the growing season,the sap flow start time of Q.liaotungensis was earlier than that of the other three tree species,Q.liaotungensis,P.orientalis and A.stenolobum reached the peak earlier than R.pseudoacacia,and the time of the peak of R.pseudoacacia was close to the peak of the daily variation of VPD.On the seasonal scale,the Fd of the four tree species is higher in the middle of the growing season,but lower in the early and late stages.Fd is significantly positively correlated with PAR and VPD and T,and significantly negatively correlated with RH.The exponential saturation curve function can be used to fit the relationship between Fd and VPD.(2)The diurnal courses of sap flux density were similar to those of meteorological factors,with daily peaks earlier than VPD.The peak of VPD lagged behind the sap flux densities of Q.liaotungensis and R.pseudoacacia 118.2 min and 39.5 min,respectively.The peak of PAR lagged behind the sap flux density of Q.liaotungensis 12.4 min,but was 68.5 min ahead of that for R.pseudoacacia.Time lags between sap flux density and VPD were significantly varied between tree species and was affected by soil water content.There was no correlation between time lag and tree diameter classes,but the time lag between VPD and sap flux density for R.pseudoacacia were longer in smaller diameter trees than in larger trees,which was significantly different under the lower soil water content.Those during higher soil water content period were about 32.2 min and 68.2 min longer than those during the period with lower soil water content for the Q.liaotungensis and R.pseudoacacia,respectively.Our results suggested that the time lag difference between the two tree species shows that the native tree species Q.liaotungensis implemented stomatal adjustment function earlier than the exotic tree species R.pseudoacacia.(3)The slight changes in the trunk diameter of the four tree species on the daily scale show a contraction-expansion periodical change rule.During the whole growing season,which on the seasonal scale,it shows a diameter growth trend.Throughout the non-growth season,the overall trend of the radial change of trees showed a slight decline and plateau.The diurnal variation trend of the trunk diameter of the four tree species in the non-growing season is different from that in the growing season.In the non-growth season,the daily change in trunk diameter has a similar diurnal variation trend as VPD and T,but the peak time appears later.The non-growth season has a high correlation with the air temperature,which may be related to the freezing-thawing dynamics of winter trunk moisture related.During the growing season,the daily changes in trunk diameters of the four tree species are opposite to the changes in PAR,VPD,and T.However,the peak time of each tree species varies,which may be related to the regulation mechanism of trunk sap flow and stomatal transpiration.The correlation between diameter maximum daily shrinkage(MDS)and soil water content is significant,which may be related to the higher soil water content is beneficial to the tree trunk to obtain water supplement and the daily transpiration water consumption is relatively large.(4)The Fd of the four tree species is generally higher than the period of low soil water content in the period of high soil water content.The trunk sap flow of the four tree species and the daily change of trunk diameter showed an opposite trend,and there was a time difference of up to 5 hours between the peaks of the two species.Under the two soil water conditions,the peak time of Fd of the four tree species in the period of high soil water content was slightly earlier than the period of low soil water content.On an hourly scale,there is a significant negative correlation between the trunk flow of the four tree species and the trunk diameter.On the multi-day scale,there is a very significant positive correlation between the sap flow and MDS of the four tree species,and there is a positive correlation between the sap flow,MDS and soil moisture.The maximum daily shrinkage of the trunk is affected by the photosynthetically active radiation and soil water content.When the transpiration of the tree is relatively strong and the Fd is large,the trunk shrinks radially,and the change in the daily maximum shrinkage of the trunk diameter increases.The daily growth of the trunk diameter of the four tree species gradually decreases with the growth season,which is mainly related to the characteristics of different tree species.Regarding the coupling relationship between small changes in trunk diameter and physiological indicators such as trunk sap flow,leaf water potential,and stomatal conductance,whether MDS can be used as an indicator to indicate tree water physiological ecology needs further study and analysis.Explored the linkage mechanism between xylem sap flow dynamics and trunk diameter,which provided basic data and theoretical basis for ascertaining the water physiology and ecological characteristics of various tree species and the environmental adaptability and adaptation mechanism. |
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