| Eucalypt is one of the main commercial tree species in southern China,the biological characteristics of fast growing plays an irreplaceable role in providing a large number of lumbers in short term which alleviate the pressure of timber resources shortage.However,with the expansion of eucalypt plantation areas,a controversial relationship between eucalypt plantations and water is a more and more prominent problem,especially five provinces in southwestern China drought since 2010,eucalypt water consumption issues were pushed up to the forefront which hindered the continuous development of eucalypt plantations.Hence,it is urgent need to research the relationship between eucalypt plantations and water for understanding the interaction of them,which is better guiding function for reasonable planning,scientific operating,high efficient water use and regional water conservation for managing eucalypt plantations.At present,the large areas of eucalypt plantations were planted many generations to directly cultivate for small fiber wood.Thus,in this paper,the second generation coppice of E.urophylla×E grandis was researched as a material,and we put up the scientific problem that is how to accurately measured canopy transpiration and stand evapotranspiration.Basing on the Nanning eucalypt ecological station,we monitored the data of the microclimate,rainfall input and water output of middle aged E.urophylla×E.grandis plantation,to discuss distribution of stand water for deepening understanding to water balance,and let us really understand the water consumption of Eucalypt plantations.The trial is a small closed watershed of hilly landform with 15.6 hm2.The forest is the second generation coppice of E.urophylla×E.grandis inside the small watershed,and no need manual clearance of weeds and fertilizer,therefore,the interference to stand is weak.Stand spacing is 1250 trees per hectare.LAI is 1.56~2.09.Diameter at breast height was 11.0 cm in August 2013 and 13.0 cm in August 2015.Tree height was 15.7 m in August 2013 and 18.7 m in August 2015.In 2014,mean height of shrub and herbal understory was 0.07~1.0 m,cover degree was above 80%.The soil in trial is latosol,the thickness of soil is less than 0.8 m,and the sediment content is about 23%.The observation period of this paper continued from July 2013 to June 2016.The main conclusions are as following.i.The microclimate inside and outside E.urophylla×E.grandis plantations suggested that wind speed,wind direction and wind structure were changed by stand and wind profile was warped which reduced the kinetic energy and wind speed.Due to the decrease of wind and turbulence exchange in the forest,the time of the evaporation and transpiration of vegetation transpiration in the near ground air was prolonged,at the same time the interception effect of the canopy reduces the air exchange between inside forest and outside forest,which makes the air relative humidity high in the forest.As a result of absorption,reflection and filtering effect of canopy on solar radiation weakened the intensity of solar radiation into forest,thus the temperature inside forest is lower than that outside forest,and it also reduced the forest surface evaporation and vegetation transpiration which made the soil moisture content higher inside forest than that outside forest.For nearly three years precipitation showed that the annual precipitation changed little in the short term in trial zone,they belonged to median water year,and annual precipitation and its variation characteristics were similar with Nanning district and the region of southern China.ii.Canopy rainfall redistribution over the past eleven months suggested that approximately 68% of precipitation went through canopy to form rain inside forest.Stemflow was very tiny only accounted for 1% of precipitation which could be ignored in the process of rainfall distribution calculation.Because of ignoring stemflow,canopy interception and throughfall formed a dependence relationship for each other.Throughfall was formed every precipitation,the throughfall and its ratio increased with the increase of precipitation.Canopy interception was mainly affected by precipitation,the amount of interception increased with the increase of precipitation,but the interception ratio decreased with the increase of precipitation.iii.The weakened manual interference to middle aged E.urophylla×E.grandis plantation,annual soil water storage changed faintly and the value was close to zero.As a result of large vegetation coverage,an amount of runoff was very few and the runoff coefficient was also very small.There was a runoff every rainfall more than 92 mm.Initial runoff lag behind initial precipitation.During the continuous precipitation condition,the runoff increased synchronously with precipitation and had the superposition effect.After rain stopped,the peak runoff can still maintain a period which show a maximum runoff stability.Runoff fading time lag precipitation and show a extended effect.The whole time of runoff fading was closely related to initial time of runoff and the last time of precipitation..iv.During typical sunny days in the four seasons,daily transpiration of middle aged E.urophylla×E.grandis was 1.58 mm·d-1,1.61 mm·d-1,1.23 mm·d-1 and 0.84 mm·d-1 respectivly.The differences showed sap flow start time in the morning,the peak,end time in the evening and duration during the whole day.Sap flow in cloudy days also showed a rule of day high and night low like change in sunny days,however,in cloudy days the speed of sap flow started was more slow and less peak,and also in the evening sap flow reduced to nearly zero,so flow duration was shorter in the day.During rainy days there was still a larger sap flow even at night,and it was increased with the increase of precipitation,sustained with continuous precipitation,and gradually reduced with rain stop.Average monthly canopy transpired 29 mm,except in October 2013 and January 2014,canopy transpiration was less than precipitation.Annual mean canopy transpiration was 349 mm,accounted for 27% of the precipitation,it was approximately a quarter of rainfall.By principal component analysis,stepwise linear regression analysis and path analysis,we found soil moisture content was a key factor to control the overall level of transpiration of middle aged E.urophylla×E.grandis,the contribution rate to canopy transpiration was amount to approximately 49%.Hence,soil water content was not a limiting factor for canopy transpiration because of abundant precipitation in southern China.Among all meteorological factors,solar radiation,relative humidity and air temperature were amount to a cumulative contribution rate to 38% of canopy transpiration,and solar radiation played a dominant role in impacting canopy transpiration.v.Application of water balance equation to estimate evapotranspiration indicated that total annual evapotranspiration was less than precipitation,which accounted for approximately 95% of rain.Among them,understory evapotranspiration was the largest,occupied 46% of precipitation,but E.urophylla×E.grandis themselves canopy transpiration was not very high,only occupied 27% of rain.In addition,the ratio of interception transpiration was slightly less than canopy transpiration,and there was only 5% of precipitation left to generate runoff.Use P-M model and test its suitable to estimate stand evapotranspiration.The results showed that daily evapotranspiration was 3.5 mm,monthly evapotranspiration was 96 mm and seasonal evapotranspiration was 280 mm,except spring,evapotranspiration was less than precipitation in others.Average annual evapotranspiration was 1156 mm,accounted for 82% of precipitation.Deviation ratio of Evapotranspiration between water balance equation and P-M model was 13%,the result of T test found no significant difference between them.Hence,it suggested that P-M model could be used to estimate roughly evapotranspiration of middle aged E.urophylla×E.grandis plantation.vi.Cultivating E.urophylla×E grandis aim to product lumbers,measuring diameter at breast height to estimate wood biomass is more convenient and quick than weighing full tree to calculate full tree biomass.Therefore,through the wood biomass to estimate water-wood productivity is more intuitive and has a practical significance.The result indicated water-wood productivity of middle aged E.urophylla×E.grandis was 5.65 g·kg-1 on single tree scale,that means 5.65 g wood could be formed every 1 kg water transpired on single tree scale.Water wood productivity of middle aged E.urophylla×E.grandis on stand scale was 1.9 g·kg-1,which means 1.9 g wood could be formed every 1 kg water evapotranspired on stand scale,this number was 20% less than that of E.urophylla in South-weast Australia.vii.In the condition of LAI 1.82,annual precipitation 1311 mm,the water balance characteristics of the second generation middle aged E.urophylla×E.grandis plantaton from July 2013 to June 2014 were that interception,throughfall and stemflow of precipitation input components accounting for precipitation were 77%,22% and 1% respectively.The runoff,understory evapotranspiration,canopy transpiration and intercepiton evaporation of the water output components were 5%,46%,27% and 22% respectively. |
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