Study On Soil Water And Water Use Efficiency Of Eucalyptus Plantation On Multi-time Scale

Eucalyptus species,with its strategic significances for fast-growing plantations in southern China,produce great economic benefits.However,in view of high productivity of the tree species,Eucalyptus planting has being been criticized for whether it may have negatively ecological influences,particularly on hydro-ecology.The central and western Hainan island belongs to semi-humid and semi-arid region,where Eucalyptus planting is also relatively intensive,hence the study on the ecological impacts of Eucalyptus planting on soil hydro-ecology and on the water use efficiency of Eucalyptus is of great significance for assessing impacts of the Eucalyptus plantation on soil hydrology and ecology.Selecting an Eucalyptus plantation in Danzhou,Hainan as research location,the study applied the method of TDR and soil auger to continuously monitor soil moisture and meteorological factors in six typical plots,utilized Li-6400 photosynthesis system to monitor eco-physiological variables,regularly monitored increments of 100 Eucalyptus individuals,employed DD DBH meter to monitor real-time DBH changes,and used stem flow measuring system to monitor real-time flow rate changes,in order to clarify the characteristics of soil moisture variations and their relationships with surrounding environment factors,at different temporal scales(day,month,year),and to compare water use efficiency of Eucalyptus in dry and rainy season respectively at leaf level and organism level.The study provides theoretical basis for adequate utilization and scientific management of water in Eucalyptus plantation.The study results shows:(1)On a daily scale,soil moisture was nearly unchanged in sunny days,while increased non-linearly with the increase of precipitation in rainy days.If precipitation exceeds2.6mm,the rain can pass through eucalyptus canopy so that precipitation is effective to increase soil moisture in surface layer(0-10 cm).When precipitation exceeded 11.4mm,soil moisture in 0-10 cm layer increased little or stopped increasing.The response of soil moisture in soil layers of 10-30 cm and 30-60 cm to precipitation was similar.There was a significant positive correlation between soil moisture and relative humidity in sunny days(P<0.01),but no significant correlation with solar radiation and temperature(P>0.05).There was a significant positive correlation between soil moisture and precipitation in rainy days(P<0.01),but no significant correlation with solar radiation,temperature and relative humidity(P>0.05).(2)On a monthly scale,high soil moisture lasted for a relatively long time,during which the highest moisture occurred in the late rainy season(September),when the soil mass water content was 10.27%.The lowest soil moisture occurred at the end of the dry season(April),when the soil mass water content was 7.43%.The association between monthly soil water content and monthly precipitation was the most correlated,showing a high significance or significance(P<0.01 or P<0.05),while the correlation of the soil water content with solar radiation,atmospheric temperature and relative humidity was not significant(P>0.05).(3)From 2010 to 2017,there were significant differences among the annual precipitation(P<0.01),but the fluctuations of soil moisture in eucalyptus plantation were small across years with no significance(P>0.05).The coefficient of variation for annual soil moisture values gradually decreased with the increase of soil depth.It is thus known that eucalyptus planting has not caused continuous decrease of soil moisture in the context of tropical monsoonal climate in Hainan,imposing little impact on the soil water ecology.(4)It is found,for photosynthetic rate of eucalyptus,that there were similar variation patterns between dry and rainy season.During both periods,the first peak of photosynthetic rate occurred at about 10:30 am(17.73 ?mol·m-2·s-1 in dry season,20.67 ?mol·m-2·s-1 in rainy season),and the second peak occurred at about 16:00 PM(12.33 ?mol·m-2·s-1in dry season,15.41 ?mol·m-2·s-1 in rainy season).The transpiration rate of eucalyptus showed an indistinct bimodal curve in both dry season and rainy season.The first peak(maximum)appeared around 10:30(8.51 mmol·m-2·s-1 in the dry season,9.91 mmol·m-2·s-1 in the rainy season),and the second peak appeared around 16:00(4.15 mmol·m-2·s-1in the dry season,6 mmol·m-2·s-1in the rainy season).(5)The radial growth of eucalyptus was mainly affected by precipitation.In dry season,the radial growth of tree trunk had a significantly positive correlation with precipitation and a highly significant anti-correlation with temperature and solar radiation.In rainy season,the radial growth of tree trunk had a significantly positive correlation with precipitation and a significantly negative correlation with solar radiation.The daily water consumption of individual trees was the highest in sunny day,the second in cloudy day and the lowest in rainy day,and the monthly water consumption of individual trees was higher in rainy season than In dry season.In dry season,the daily water consumption was positively correlated with 30-60 cm soil moisture,temperature,solar radiation,and saturated vapor pressure difference with high significance,was negatively correlated with relative humidity with high significance,and was negatively correlated with 0-10 cm soil moisture with significance.In rainy season,the daily water consumption was positively correlated with solar radiation and saturation vapor pressure difference,and was negatively correlated with precipitation and relative humidity,both with high significance.(6)The diurnal variation patterns of water use efficiency in dry season and rainy season was similar.However,the efficiency at leaf level in rainy season was slightly higher than that in dry season,instead,the efficiency at organism level in rainy season was significantly higher than that in dry season,which was only 63.01% of that in rainy season.