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Trees and crops in agroforestry system can make the best use of water,nutrient,radiance and heat.However,competition may occur when resources are scarce,especially for the water competition in the warm temperate semi-arid region.Due to arid climate,less rainfall and poor irrigation condition,water is one of the major factors limiting the development of agriculture and forestry in the low hilly area of North China.Stable carbon isotope ratio(δ13C) of walnut(Juglans regia L.)-wheat(Triticum aestivum) intercropping system and monoculture wheat as well as sap flow of walnut tree at seedling establishment,jointing,filling and maturing growth period of winter wheat in the low hilly area of North China were researched.Water use efficiency(WUE) and water use(WU) were calculated by using the δ13C value,crop biomass,walnut sap flow and meteorological data.The results showed that the WUE of intercropping walnut,monoculture wheat and intercropping wheat were 10.771—21.233,9.946—19.149 and 9.878—18.431 mmol C/mol H2O,respectively.The WUE of intercropping wheat were significantly greater than that in monoculture wheat at wheat jointing growth period.The total water use of the walnut-wheat intercropping system was 1755.19 t/hm2,which was 30.85% less than that in the monoculture wheat system(2538.13 t/hm2).Water consumption of the intercropping walnut accounted for 36.34% in the walnut-wheat intercropping system,respectively 62.09%,30.80%,35.58% and 27.24% at sowing-seedling establishment,jointing,flowering-filling and filling-maturing growth period of winter wheat.The walnut tree consumed the greatest amount of water at earlier growth period of winter wheat,and the less at vigorous growth stages of winter wheat.Contrary to the walnut,the water use of wheat was greater at its later growth period.Intercropping walnut and wheat staggered water demand periods and avoided temporal competition for water.Soil volumetric water content in the intercropping system was the greater the closer to the walnut tree in the shallow soil layer,and the less the closer to the walnut tree in the deep soil,which suggested that the walnut tree mainly absorbed deep water,while the intercropping wheat mainly absorb shallow water,it avoided spatial competition for water.The grain yield,total biomass and water use of monoculture wheat were respectively 26.79%,27.12% and 36.30%(P=0.001、P=0.033、P=0.050) higher than that of the intercropping wheat.The average fruit yield of intercropping and monoculture walnut were respectively 0.88 kg and 0.94 kg per tree(P=0.829).However,yield LER(Land equivalent ratio) and WUEe(WUE of economics) of the walnut-wheat intercropping system respectively reached 1.67 and 25.92RMB · mm-1 · hm-2,improved obviously comparing with monoculture system.The walnut-wheat intercropping system utilized water with high efficiency and exhibited both production advantage and economic advantage at the same time.

Trees,especially big ones,have huge and complicated vascular systems which make the long-distance water transport difficult to study.We used the heat dissipation method and deuterated water as tracers to characterize water transport and storage properties of Quercus variabilis trees under the different water stress treatment.The transit time for D2O transport from the trunk base to the upper branches and the tracer residence time were determined by measuring hydrogen isotope ratios in water extracted from branches sampled in regular intervals.The result indicated that the maxima δD of full irrigation(FI),mild water stress(LWS) and severe water stress(SWS) respectively increased to 586.67‰、997.33‰ and 1364.89‰ after injecting deuterium tracer.There were significant differences in maxima δD among the three treatments.The tracer velocities with LWS and SWS were significantly lower than FI treatment but the tracer half-life and residence time were longer than FI treatment.The branch PLC,sapflow rate,water potential and transpiration rate were also observed.The branch PLC treated with LWS and SWS was significantly higher than FI treatment and the sapflow rate,water potential and transpiration rate were lower.There significantly positive correlations between tracer velocity and leaf transpiration rate(P0.01,n=27),and between tracer velocity and sapflow rate(P0.01,n=9).The results indicated that water transport was determined by transpiration rate.The PLC and branch water potential were inversely correlated with tracer velocity(P0.01,n=27),suggesting that drought stress-induced embolism and lower water potential should increase resistance to water transport.Tracer half-life and residence time increased as water stress increased.Tracer half-life and residence time were significant exponential functions with the branch PLC(P0.01,n=27),but power functions with the leaf transpiration rate.The times needed for transporting the same volume tracer were increased,indicating that water transport efficiencies were reduced because of PLC,transpiration and storage water variations under water-stress condition.Analyses of sapflow showed that the calculative fluxes of drought treatments within tracer half-life and residence time were significantly higher than FI treatment.The amounts of water carrying the same volume tracer were increased as stress degree increased.The results indicated that the increased water was possibly stored in the sapwood and re-exchanged to transpiration stream when transpiration was strong in the daytime.Quercus variabilis trees should reduce the water transport velocity and efficiency to adapt water stress environment by xylem embolism and storage-water exchange.

This study took the Tianmu Mountain National Natural Preservation Area in western Zhejiang Province as an example 1) to quantify the species diversity with selected species indices including Shannon-Wiener index ( H ), Margalef index ( K ) and Evenness index ( E ) at different spatial scales, 2) to analyze the spatial distribution patterns of the species diversity by multifractal parameters such as the singularity index α , its fractal dimension f ( α ), the f ( α )– α spectrum range ( SR ) and its symmetry (Dist) using the multifractal theory, and 3) to determine their relationships. Results of nonlinear regression analysis with power functions showed that increasing spatial scale resulted in increasing H but decreasing E and K , indicating that the scale dependence of species diversity existed. By using the multifractal method, it was indicated that species spatial distribution had multifractal features. Moreover, strong linear relationships of the diversity indices H , E and K with α min and clear nonlinear associations of the diversity indices H , E and K with power functions for SR and Dist were found. Since interactions of the species diversity and the spatial characteristics are very complex, the above mentioned relationships need further validation along with precise explanations of any correlations among their ecological processes.