| The ecological environment of the Loess Plateau has gradually improved since the early1970s.However,unsuited afforestation has lead to excessive water consumption,sustained loss of soil reservoir,negative feedback on vegetation growth and soil water supply capacity,resulting in the limitation of vegetation growth and even death.Therefore,it is important to study the response of plantation ecosystem to drought for understanding the forest-water interaction process.In this paper,we carried out a case study in the semi-humid deep loess artificial Robinia pseudoacacia forest ecosystem in the south of the loess plateau.The physiological characteristics of Robinia pseudoacacia and the characteristics of soil aggregates were studied by throughfall-exclusion and the removal of understory vegetation experiment at the slope scale on the Loess Plateau.The distribution and seasonal variation of non-structural carbohydrate in different stands were also revealed.The main conclusions are as follows:1)At the 0-20 cm soil depth,drought stress and the removal of understory vegetation treatment significantly reduced the content of macro-aggregates?>0.25 mm?and increased the content of micro-aggregates?0.25-0.053 mm?.Drought stress and the removal of understory vegetation significantly reduced the concentration of soil aggregate-associated organic carbon?>0.053 mm?,indicated that sufficient rainfall and well developed vegetation played an important role in the formation and stabilization of soil aggregate-associated organic carbon.Drought stress and the removal of understory vegetation reduced the stocks of soil macro-aggregate-associated organic carbon?>0.25 mm?and increased the stocks of micro-aggregate-associated organic carbon?0.25-0.053 mm?.Drought stress and removal of understory vegetation significantly reduced the total soil organic carbon concentration?p<0.05?at the 0-10 cm soil depth,but not the 10-20 cm soil depth.For the total soil organic carbon stocks,there was no significant change under drought stress and the removal of understory vegetation at the 0-10 and 10-20 cm soil depths.These results indicated that the change of organic carbon between soil aggregates could temporarily alleviate the effects of short-term drought stress and understory vegetation on total soil organic carbon stocks.2)In 2015,the soil volumetric water content at the 0-1 m soil depth was 25%and 26%under drought stress and control treatments,respectively,and there was no significant difference between them?p>0.05?.In 2016,the soil volumetric water content at the 0-1 m soil depth was 23%and 26%under drought stress and control treatments,respectively,and drought stress significantly reduced the soil water content?p<0.05?.For the 1-4 m soil depths,there was no significant difference?p>0.05?between drought stress and control treatments.3)The effect of drought stress on the transpiration of Robinia pseudoacacia showed a certain"hysteresis".Namely,drought stress significantly reduced the transpiration of Robinia pseudoacacia?p<0.05?in 2016,but not in 2015.Although drought stress reduced the transpiration,it did not change the seasonal variation characteristics of the water consumption of Robinia pseudoacacia.During the growing season,the overall trend of transpiration of Robinia pseudoacacia was lower in early stage?April to May?,higher in middle stage?June to August?and lower in late stage?September to November?.In addition,the drought stress significantly reduced the leaf area index?p<0.05?.The average monthly transpiration of Robinia pseudoacacia had a significant linear correlation with the leaf area index?p<0.01?,and the determination coefficient was 0.689.Therefore,it could be concluded that the decrease in leaf area index may lead to the decrease in water consumption of Robinia pseudoacacia.4)The hydraulic conductivity of the branches and roots of Robinia pseudoacacia reached the highest value in August and the lowest value in November.Drought stress significantly reduced the hydraulic conductivity of the branches and roots?p<0.01?,but the sampling date and their interaction did not significantly affect the hydraulic conductivity of shoots and roots?p>0.05?.The regression analysis of sapwood area and hydraulic conductivity showed that the hydraulic conductivity of the branches and roots showed a good exponential relationship with the cross-sectional area of sapwood.This suggested that the hydraulic conductivity of branches and roots is mainly affected by the cross-sectional area of sapwood.The greater the cross-sectional area of sapwood,the higher the hydraulic conductivity,the stronger the ability to transport water.5)Drought stress significantly changed the anatomical structure of the xylem of stem of Robinia pseudoacacia.The xylem vessel diameter in the drought?105.3?m?was significantly lower than that of the control?156.7?m??p<0.05?.The xylem vessel density in the drought(61.8 mm-2)was significantly higher than that of the control(39.6 mm-2)?p<0.05?.The wall thickness in the control?12.4?m?was significantly lower than that of the drought stress?19.6?m??p<0.05?.These results showed that soil moisture was an important factor affecting the anatomical structure of artificial plantation of Robinia pseudoacacia.6)The leaf water potential of Robinia pseudoacacia gradually decreased with the growing season and reached the lowest value in October.Drought stress significantly reduced the leaf water potential of Robinia pseudoacacia?p<0.05?.The linear regression analysis showed that there was a significant negative correlation between leaf water potential and soil volumetric water content?p<0.01?and transpiration?p<0.05?.The higher the transpiration,the lower the leaf water potential,indicating that Robinia pseudoacacia in this region may be subject to certain water stress.7)Drought stress significantly reduced the soluble sugar,starch and total NSC concentration in leaves,shoots,stems and roots of Robinia pseudoacacia?p<0.01?.However,drought stress did not significantly change the proportion of each organ in individual?p>0.05?.That is,for soluble sugar,leaf>root>shoot>stem;for starch,root>stem>shoot>leave;for total NSC,root>stem>shoot>leave.The results showed that Robinia pseudoacacia deposited the soluble sugar in these organs aboveground,and the starch was main stored in the non-photosynthetic organs during the growing season.In addition,the NSC and its components of each organ had significant seasonal changes.8)The NSC concentration in each organ of Robinia pseudoacacia increased gradually with stand age.The root and stem was the main storage organs of NSC,and the proportion of leaf and bark wass relatively small.In addition,the concentration of NSCs of dying Robinia pseudoacacia was close to zero,indicating that the death process of Robinia pseudoacacia seriously consumed NSC in vivo.The NSC concentration at different stand ages significant seasonal changes.That is,the metabolism of Robinia pseudoacacia was more vigorous and consumed more NSC during the growing season?May?;while in the dormancy period?November?,Robinia pseudoacacia grew slowly and lots of NSC?mainly starch?are stored in roots and stems,which can be used to maintain the respiration in winter and the growth of organs and tissues in the spring.In this paper,the temporal variation characteristics of sap-flow,leaf area index,diameter at breast height,non-structural carbohydrate and xylem hydraulic structure parameters were analyzed and their responses to drought stress were studied.The results provided a scientific basis for understanding the mechanism of artificial vegetation and soil moisture on the Loess Plateau and the ability to evaluate the extreme drought in the plantation ecosystem in this area. |
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