| Plantations of Eucalyptus and other exotic species have been extensively established in southern China during the past 20 years. Over 2.5 millions hm2 of Eucalyptus plantations have been established in southern China. In other countries, Eucalyptus have sometimes been shown to have high annual water use, which leads to depletion of water resource. This dissertation reports studies on whole-tree transpiration (sap flow), canopy interception, soil evaporation, soil moisture dynamics, weather conditions and tree growth conducted for 1 year in 4-year-old Eucalyptus urophylla trees grown at two sites (Hetou and Jijia) in the coastal flat area of the Leizhou peninsula and for 3 years in a of 4-6-year-old plantation of E. urophylla X grandis in hilly area in Gaoyao of Guangdong province, China.Investigatations the characteristics of sap flux density (SFD) of 4-year-old E. urophylla trees in Leizhou using heat pluse method and 4-year-old E. urophylla X grandis trees in Gaoyao using heat dissipiation probe method, respectively. Results indicated that SFD of outer sapwood moved faster than the inner one. The daily patterns of SFD of trees of different sizes of diameter were similar pattern, and larger trees had higher SFD than smaller ones probably due to better radiation exposure. Among meteorological factors, solar radiation and vapour pressure ddficit (VPD) were the main factors controlling the sap flow.The results also revealed that the relationship of between sapwood area and diameter at breast height (DBH) in Eucalyptus urophylla and E. urophylla X grandis trees were highly significant and therefore it was possible to use the sapwood area to scale up estimation of water use from individual trees to a stand level. The mean daily SFDs in Hetou and Jijia in dry season were 2477±86 and 1715±73 L·m-2·d-1, while in wet season 3241±88 and 1970±67 L·m-2·d-1, with annual averages of 2772±66 and 1839±86 L·m-2·d-1, respectively. Annual mean daily water use Eucalyptus plantations at Hetou and Jijia were 1.49 and 1.53 mm·d-1 (1.25 and 1.35 mm·d-1 in dry season, 1.83 and 1.69 mm·d-1 in wet season), respectively. Annual total water use calculated from sap flow were 542 and 559 mm, respectively, which were equivalent to 35 and 36 % of annual rainfall. Water use efficiency, defined as total stem volume growth per unit of water transpired, was 4.75×10-3 m3·m-3 at Hetou and 4.24×10-3 m3·m-3 at Jijia. Low water use in this region may have resulted from combined effects of several factors, including open canopy and low leaf area index, soil water decline in the dry season, and low VPD that limited water consumption of the trees.The annual evapotranspiration (ET) (October 1999 to September 2000) in Hetou and Jijia were 1040 and 1081 mm, which were equivalent 67.9 % and 70.2 % of annual rainfall. Dry season water balances showed ET exceeded or approached rainfall in the same period, indicating water use from soil water storages was associated with soil water depletion, particularly at Hetou. However, soil water storages were replenished by high water recharge in wet season. The differences in soil properties between the two sites resulted in greater soil water store at Jijia that provided a supply for soil evaporation. Sandier soils at Hetou with poor water holding capacity had greater drainage in wet season and higher abstraction in dry season from water storages of deep soil.The annual mean daily SFD of 4-6-year-old E. urophylla X grandis plantations in Gaoyao over the three years were respectively 1032±30, 867±22 and 659±26 L·m-2·d-1, and those in dry season were 782±32, 739±25 and in wet season 529±30 L·m-2·d-1 and 1279±41, 972±31 and 798±36 L·m-2·d-1, respectively. Mean values of daily water use in the three years were 1.22, 1.10 and 0.99 mm·d-1. Water use of plantations per annum was 443.6, 402.0 and 362.6 mm respectively, or 26.7 %, 24.5% and 19.3 % of annual rainfall, respectively. Annual water use were 402.8 mm, or 26.5 % of annual rainfall in the three years,of which 162.9 mm occurred in dry season, accounting for 77.8% of rainfall in the same period and 239.9 mm or 18.3 % of rainfall in wet season. The maximum water-capacity of the soil layer from 0 to 100 cm depth is 470.06 mm, and the non-capillary water-capacity accounts for 20.90 % (98.22 mm) of the total. Both of the total and non-capillary porosities show a decreasing trend with the increasing soil depth. The initial and steady infiltration rates in the surface (0-10 cm) are 25.03 and 8.83 mm·min-1 respectively, and also decrease with the increasing soil depth. Average annual precipitation was 1518 mm, and 42.0 %, 58.2 % and -0.2 % of which were allocated to total runoff (637.3 mm), ET (883.1 mm) and change of soil water storage (-2.6 mm). Annual values of ET were 944.3, 862.1 and 843.0 mm, which were equivalent to 56.9 %, 52.5 % and 67.3 % of annual rainfall. Annual ET was allocated to 202.2 mm (13.3 %) of canopy interception, 405.3 mm (26.7 %) of tree transpiration and 275.7 mm (19.3%) of soil evaporation。 Both the heat pulse method and heat dissipation probe (TDP) method were proved to be valid and accurate way to measure transpiration by eucaypt platnations and recommended for wider application in water balance studies of tree plantions.Comparison of the water balance of the above eucalypt plantations with other type of plantations in the same region did not support the opinion that eucalypt plantation will deplete ground water reserves and negatively affect regional water balance in regions with annual rainfall of≥1200 mm. |
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