Potential growth of poplar (Populus deltoidesP. nigra.) is highly dependent on the amount of applied irrigation and soil moisture. Hybrid poplar (cultivar OP-367) was planted at 222 trees/ac in April 1997 at the Oregon State University Malheur Experiment Station near Ontario, OR. Six irrigation treatments included a combination of soil water potentials as thresholds for initiating irrigation and varying water application rates. Water was applied via micro-sprinklers installed along the tree rows. Results indicated that for optimum poplar growth, soil water potential at an 8 in. depth should average above -20 kPa (kPa = cbar) during the growing season. This was achieved by irrigating when the soil water potential reached -25 kPa and applying 21 ac-in./ac of irrigation water during the first year, 35 ac-in./ac during the second year, and 44 ac-in./ac during the third year. By the end of the third year, trees receiving optimum irrigation averaged 26 ft tall and produced 256 ft3 of wood/ac. West. J. Appl. For. 17(1):46–53.
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Shock et al. (2002) studied this question.