| At present, in China, more than30%of the domestically consumed timber are imported from abroad, and the predicted total timber shortage will reach200million m3during the ’12th five year’ period. Thus, the problem of timber security is very serious in China. One important solution to this problem is to vigorously develop fast-growing and high-yield poplar plantations. However, at present, the average productivity of poplar plantations in China is still below the global average level, which can be mainly attributed to its inefficient intensive silvicultural practices such as water management. In this research, the theories of irrigation management and key techniques of high efficient subsurface drip irrigation (SDI) were investigated for plantations of triploid Populus tomentosa, which is an important tree species for timber production in the North China Plain. The objective of this study was to improve the water management efficiency in P.tomentosa plantations, subsequently increasing their productivities and helping to ease the pressure of timber importation in China. Meanwhile, this study will also provide specific guidelines for water management in plantations of other poplar species. The main results and conclusions of this research are listed below:(1) Subsurface drip irrigation (SDI) was applied in the6-and7-year-old P.tomentosa plantations. The SDI was initiated when the soil water potential (ψs) at20cm depth and10cm distance from a drip emitter reached-25,-50, and-75kPa, respectively. Trunk sap flow rate, pre-dawn leaf water potential (ψpd),ψs, soil water content, meteorological factors, and groundwater level (GWL) were monitored continuously or measured in selected periods. Results showed that relative to non-irrigation treatment (CK), SDI on average increased annual volume growth of the6-and7-year-old plantations by24%and28%. Annual volume growth of the6-year-old plantation following the-25kPa treatment reached39.9m3·hm-2·a-1, which was44%higher than the CK treatment (P<0.01). Relative to the-50and-75kPa treatments, annual volume growth in the-25kPa treatment was20%and31%higher (P<0.01) in the6-year-old plantation, and13%and14%higher (P>0.05) in the7-year-old plantation, respectively. The fast growing period of P.tomentosa was from May to July, during which time the cumulative DBH (diameter at breast height) increment accounted for84%of the total year increment and the GWL was very deep. From August to October every year, the growth rate of P.tomentosa was very slow, the GWL was relatively high, and the soil water availability was high even though there was no water recharge from irrigation. Relative to CK during the fast growing period (April-July) of P.tomentosa, SDI increased the soil water content at20and50cm depth by35%and27%, respectively;increased average daily trunk sap flow rate and ψpd by46%and41%, respectively. These are the mechanisms by which SDI significantly improves P.tomentosa tree growth.(2)In the6-and7-year-old P.tomentosa plantations under full irrigation condition (i.e.-25kPa treatment), tree transpiration (Tr), soil evaporation (Es), and leal area index (LAI) were measured consecutively using thermal dissipation technique (TDP), micro-lysimeter (ML), and WinSCANOPY canopy analyser instrument, respectively. Results showed that, for the6-and7-year-old P.tomentosa plantations, the daily Tr, Es, and evapotranspiration (ETa) were2.37and2.49mnrd"1,0.77and0.87mm·d-1, and3.12and3.33mm·d-1, respectively; the whole year Tr, Es, and ETa were422and493mm,134and167mm, and556and660mm, respectively. In the initial and final growing period, Es was the dominant component and accounted for about30%-97%of ETa, Whereas, in the middle growing period, Tr became significant, accounting for about70%-93%of ETa. Relative to the6-year-old plantations, initiating the first irrigation one week earlier in the7-year-old plantations resulted in obviously faster canopy development and higher sap flow rate in initial leaf expansion period, and147%higher cumulative Tr in April. On average, cumulative T, between May and Junly accounted for68%of the whole year value. Relationship between Kcb and LAI could be simulated by a negative exponential function (R2=0.834), and the same for relationship between Kc and LAI (R2=0.627). The Kcb and Kc of P.tomentosa plantations in initial growing period (25d) were0.02-0.96and0.04-1.27, respectively; in middle growing period (135d) were0.96and1.27, respectively; and in final growing period (45d) were0.96-0.4and1.27-1.26, respectively. Therefore, every year, from May to July was the main water use period of P.tomentosa, and the key water management period for P.tomentosa plantations was from April to July. The fitted functions of Kcb(LAI) and Kc(LAI) could be used to predict Kcb and Kc of P.tomentosa plantations with the measured LAI. For mature pure P.tomentosa plantations in regions similar to our experimental site, their Tr, and ETa could be predicted by using our constructed Kcb>and Kc curves and commonly availabile meteorological data.(3)2106root samples were collected in a5-year-old P.tomentosa plantation using soil coring method. Dry dig method was used to trace the root spread in plantaions with different ages (4-,5-, and7-year-old) and planting densities (1250,1404, and2500tree·ha-1), and to get the whole coarse root system of one5-year-old tree with average size. Resultes showed that, in the5-year-old P.tomentosa plantation, lateral root distribution was even, but root distribution tended to be shallower with increasing distance from tree trunk. In contrast, the vertical root profile showed an unusual pattern (nearly an ’S’ shape). Nearly half (44%) of line roots corresponed to0.2-0.5mm diameter. Dense fine roots (25%) occurred in surface soil (0-20cm) and nearly one third (28%) of total line roots occurred below100cm depth. Mean line root diameter was significantly larger (P<0.05) below120cm. The main lateral distribution areas of coarse root length and biomass were within40and20cm distance from tree trunk, respectively, whereas their main vertical distribution soil layers were0-20and0-50cm depth, respectively. Root spread of P.tomentosa was1.9times as high as its canopy spread. The maximum rooting depth of4-,5-,and7-year-old trees were2,2, and2.7in, respectively.79%,6%, and15%of the 1-order lateral roots were horizontal, oblique, and vertical lateral roots, respectively. Thus, a dimorphic root system had developed in the5-year-old P.tomentosa plantation. The form of P.tomentosa root system was horizontal type with vertical roots.(4) Two sample trees with average size were seclected from the5-year-old P.tomentosa plantation, and an experimental plot was established around each of them. Tranpiration of the sample trees, and soil evaporation and soil water content within the experimental plots were measured concurrently for four months using TDP, ML, and Trime-IPH, respectively. Then, soil water balance method was used to deduce the root water uptake (RWP) rate and pattern. Results showed that RWP in the0-20cm layer contributed58%of that within the0-90cm soil layer, suggesting surface roots played the major water uptake role in shallow soil (<90cm). On average, P.tomentosa extracted57%of transpired water from deep soil (>90cm), implying deep roots can contribute significantly to the water relations of mature P.tomentosa plantations. When the soil water availability increased in surface soil, the RWP mainly happened in surface soil. The water uptake contribution of deep roots increased when the water availability in surface soil decreased. When water condtion of the soil profile was highly heterogeneous, RWP mainly occurred in zones with both high fine roots density and high water availability. However, RWP pattern was in good agreement with the fine roots distribution when the soil water distribution was even and there was no water stress. Consequently, the RWP pattern of P.tomentosa was primarily determined by the fine root distribution, but would vary with the soil water availability distribution.(5) HYDRUS-1D and HYDRUS-2D/3D were used to simulate one dimentional soil water dynamics under natural rainfall condition (NC)(i.e. CK treatment) and two dimentional soil water dynamics under SDI (-25kPa), respectively. Performace of these models were validated using measured soil water content data of two years. Then, HYDRUS models were used to simulate soil water availability dynamics in P.tomentosa plantations under NC and SDI for two growing seasons. Results showed that the root mean square error (RMSE) and relatively mean absolute error (RMAE) of the HYDRUS-1D simulation results were0.004-0.060cm3·cm-3and0.7%-13.7%, respectively. As to the simulation results of HYDRUS-2D/3D, the average RMSE and RMAE in different soil layers were0.005-0.038cm3·cm-3and0.9%-9.7%, respectively, and the average RMSE and RMAE within the domain around the dripper were0.032cm3·cm3and8.6%, respectively. The influence of SDI on soil water content was mainly limited to0-90cm soil.Soil water availability (r0) in all soil layers were significantly correlated with the fractional ABH (area at breast height) growth rate (P<0.001). However, the difference in tree growth that could be explained by ro decreased with increasing soil depth.Tree growth would not be limited when the roof root zone (0-150cm soil layer) was kept above90%.The effect of soil water on tree growth was positively correlated with the availabile nitrogen and organic matter content of soil and the tissue nitrogen content of fine roots (P<0.01), was negatively correlated with the mean fine root diameter (P<0.05), and was not correlated with the soil availabile phosphorus content and the root length density and specific root length of fine roots (P>0.05). Consequently, HYDRUS model can be used to simulate one and two dimentional soil water dynamics in P.tomentosa plantations under NC (similar to flood irrigation) and SDI, respectively, and can also be used to predict the influence of different irrigation treatments on the growth of P.tomentosa plantations. Relative to the NC treatment, one of the reasons that SDI could increase tree growth was that it could greatly increase the r0of0-90cm soil. The difference in influence of min different soil layers on tree growth should be considered when establishing the quantitative relationship between r0and tree growth rate.(6) In conclusion, when apply irrigation in P.tomentosa plantations in regions similar to our experimental site:①ψ of-25kPa at20cm depth and10cm distance from a drip emitter can be used as the irrigation threshold of SDI;②the constructed Kcb, and Kc curves and commonly meteorological data can be used to estimate the water use of P.tomentosa plantations, subsequently helping to determine the irrigation amount;③irrigation should be applied between April and July, and terminated between August and October;④irrigation schedules should be devised based on periodic measurement of the depth to water table;⑤irrigation water should be mainly provided to and maintained in the surface40cm soil, the zone within1m from the tree, and the soil zone with higher availabile nitrogen and organic matter content;⑥HYDRUS can be used to design irrigation management strategies;⑦tree growth will not be limited when the r0of root zone is kept above90%.(7) As this research was conducted in maure P.tomentosa plantations, the conclusions will inevitably have limitations. Thus, similar researches should also be conducted in young P.tomentosa plantations to supplement and optimize our conclusions, so that they can be applied in the whole rotation of P.tomentosa. |
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