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Subsurface drip irrigation(SDI) is an intensive silvicultural practice which has the potential to increase productivity of triploid Populus tomentosa pulpwood plantations.In order to efficiently apply SDI in the cultivation of P.tomentosa pulpwood plantations,it is necessary to determine irrigation timing.A field experiment was conducted to investigate the effect of soil water potential(ψsoil) on the growth and physiological characteristics of 6—7 year old trees under SDI in a P.tomentosa pulpwood plantation.The experiment included three ψsoil treatments,which initiated irrigation when the ψsoil at 20 cm soil depth and 10 cm distance from a drip emitter reached-25,-50,and-75 kPa,respectively.A control non-irrigation treatment(CK) was also included.Meteorological factors,ψsoil,groundwater level,and tree growth were monitored hourly,daily,every 1—10 days,and monthly for two years,respectively.Soil water content at 20 and 50 cm depth,pre-dawn leaf water potential(ψpd),and trunk sap flow rate were measured in selected periods.Results showed that relative to CK,SDI could improve diameter,height,and individual volume increment of P.tomentosa.Irrespective of SDI treatment,SDI on average increased annual volume growth of the 6-and 7-year-old plantations by 24% and 28%,respectively over the CK treatment.Annual volume growth of the 6-year-old plantation following the-25 kPa treatment reached 39.90 m3 hm-2a-1,which was 43.5% higher than the CK treatment(P0.01).Relative to the-50 and-75 kPa treatments,annual volume growth in the-25 kPa treatment was 20% and 31% higher(P0.01) in the 6-year-old plantation,respectively,and 13% and 14% higher(P0.05) in the 7-year-old plantation,respectively.Relative to CK during the fast growing period(April—July) of P.tomentosa,SDI increased the soil water content at 20 and 50 cm depth by 35% and 27%,respectively;increased average daily trunk sap flow rate(SFmean) by 46%;and increased ψpd by 41%.The ψsoil threshold for initiating irrigation had no significant effect(P0.05) on the SFmean and ψpd of P.tomentosa.However,among the three ψsoil treatments,the average SFmean and ψpd values were highest following the-25 kPa treatment.The increase of soil water availability in the 0—20 cm soil layer(i.e.,active rooting zone) was also greatest in the-25 kPa treatment.These results may explain why the-25 kPa treatment had the highest tree growth rate among the three ψsoil treatments.Overall,the observed increase in soil water content,SFmean,and ψpd indicate that these are the mechanisms by which SDI significantly improves P.tomentosa tree growth.In conclusion,our findings suggest that SDI should be promoted in the cultivation of P.tomentosa pulpwood plantations.To realize optimum tree growth,irrigation should be initiated when the ψsoil at 20 cm depth and 10 cm distance from a drip emitter reaches-25 kPa.Furthermore,it is recommended that irrigation should be applied between April and July,and terminated between August and October when planting P.tomentosa in regions similar to our experimental plantation.