| Water transport in forest ecosystem plays an important role in maintaining forest water and energy balance.Although there is abundant rainfall in the rocky desertification area of Western Hunan,the limestone geology has many karst caves and funnel topography,serious soil and water loss and insufficient soil moisture.The normal ecological restoration is limited by water resources.In order to solve the above problems,from October 2020 to September 2021,in the National Long-term Scientific Research Base for Comprehensive Control of Rocky Desertification in Wuling Mountain,Western Hunan,the mixed forest of Pinus elliottii,Liriodendron chinense and Cinnamomum camphora were selected as the research object to investigate and analyze the tree species composition and stand spatial structure of the test site;By measuring the physical and chemical properties of soil,the site conditions and soil and water conservation capacity of rocky desertification area in Western Hunan are revealed;Took different habitats and different tree species as the main variables,the sap flow of Liriodendron chinense,Cinnamomum camphora and Pinus elliottii were continuously monitored for one year by using heat dissipation probe method(TDP).The transpiration water consumption characteristics of Liriodendron chinense,Cinnamomum camphora and Pinus elliottii under typical weather,daily scale,monthly scale,continuous sunny days and continuous rainfall after drought were analyzed;The temperature(T),relative humidity(RH),saturated vapor pressure deficit(Vpd),solar radiation(Rs),wind speed(Ws)and other environmental factors of the test site were monitored synchronously,the variation law of meteorological factors and sap flow rate were analyzed,and the main meteorological factors affecting sap flow were determined.At the same time,the stand spatial structure parameters and forest competitiveness index were introduced to reveal the response between sap flow and stand ecological factors.The main conclusions are as follows:(1)The main forest layer is Pinus elliottii,with an average DBH of 34.4 cm and an important value of 34.67%;The number of trees is the largest,675 trees/hm2,with an important value of 35.31%;The second is camphor tree,with an important value of 19.63%.The stand presents random distribution,there is a certain degree of competition among trees,and the stand mixing condition is good.Soil physical and chemical properties have strong spatial heterogeneity.In addition to total K,the soil fertility is relatively poor,the soil porosity and water holding capacity are low,the soil in some areas is compact,and the soil ventilation and water permeability are poor,which is not conducive to soil water storage.(2)Sap flow mainly occurs during the day,and the diurnal variation of sap flow rate is low in the morning and evening and high at noon,showing an inverted "U"shape.The sap flow starts from 6:00 to 10:00 in the morning.The start-up time of trees with large DBH in the same kind of trees is earlier,reaches the peak from 12:00 to 14:00 at noon,and then decreases rapidly.It basically maintains a low and stable situation around 18:00 p.m.,and there is a weak sap flow flux at night.On sunny days in dry season,trees will have a photosynthetic lunch break,resulting in a short trough of sap flow density at noon.The sap flow rate of single plant with large DBH is higher than that of small DBH,and the sap flow rate in sunny days is higher than that in rainy days.(3)From June to September,the transpiration rate of Liriodendron chinense is higher than that of Cinnamomum camphora,but the sap flow rate of Cinnamomum camphora is higher than that of Liriodendron chinense in other months,which is highly correlated with the growth habits of the two trees.The change of sap flow rate of Pinus elliottii is relatively stable,and it is lower than that of the other two trees all year round.The sap flow rate of non bare rock habitat of the three trees is higher than that of bare rock habitat,and the difference of sap flow rate between the two different habitats is greater in sunny days.Compared with non bare rock habitat and bare rock habitat,the impact of climate change on tree transpiration is more obvious.(4)There is a highly correlated power function relationship between DBH and sapwood area,and there is a significant positive correlation between transpiration per plant and sapwood area.In the growing season(June to September),the daily transpiration water consumption of three kinds of trees is Pinus elliottii>Liriodendron chinense>Cinnamomum camphora;The annual water consumption per tree of the three trees is Pinus elliottii(11.04 m~3)>Cinnamomum camphora(2.58 m~3)>Liriodendron chinense(2.30 m~3).The huge sapwood area of Pinus elliottii determines its large water consumption per plant.On the seasonal scale,the transpiration per plant of the three trees is summer>autumn>spring>winter,which is consistent with the seasonal variation of the three trees.After continuous sunny days,the soil is relatively dry.Continuous rainfall reduces the sap flow rate and adds a lot of water to the forest soil.No matter in sunny or rainy days,the transpiration per plant of the three trees in non bare rock habitat is slightly higher than that in bare rock habitat.From the perspective of improving site conditions and low water consumption,the deciduous tree species Liriodendron chinense is more suitable to be planted in rocky desertification areas.(5)In the response mechanism between sap flow rate and stand spatial structure and competition index,the smaller the tree size ratio,the higher the degree of dominance,the greater the sap flow rate;The smaller the individual angular scale,the more uniform the spatial structure,the higher the ecological status in the environment and the greater the sap flow rate;The smaller the competition index of trees,the smaller the competition pressure they suffer,the greater the space for the utilization of environmental resources,and the greater the sap flow rate.In this study,transpiration per plant is significantly negatively correlated with size ratio,angular scale and competition index,but not with mixing degree.(6)The diurnal variation of T and Vpd in sunny days is consistent with the diurnal variation of sap flow rate.The diurnal variation of RH is opposite to the diurnal variation of sap flow rate in sunny days.The peak value of RH appears at the time of the lowest sap flow at night,and the peak value of Rs is more than 2 hours ahead of the peak value of sap flow rate;In rainy days,T,Vpd and Rs are relatively lower than those in sunny days,and RH is relatively large,so the sap flow rate is significantly lower than that in sunny days.The fluctuations of T,RH,Vpd and Rs are more complex than those in sunny days,and RH,Vpd and Rs show multi peak type;The influence of Ws on sap flow rate is low.The sap flow rate has a very significant positive correlation with T,Vpd and Rs(p<0.01),and a very significant negative correlation with RH(p<0.01).The saturated water vapor pressure deficit is the main driving factor of the sap flow rate. |
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