Successive treatments with fungus and alkali were proposed to reduce the recalcitrance and improved the enzymatic digestibility of triploid poplar. Biopretreatment with Trametes velutina 010149 for 0, 4, 8, 12 and 16 weeks gradually degraded hemicelluloses and lignin, and improved the digestibility of cellulose from 4.0% to 19.5% with the increasing dry mass loss of lignocelluloses from 15.5% to 53.4%. Combining with alkaline fractionation, biopretreatment for 4 weeks significantly enhanced the availability of cellulose and achieved a maximum glucose yield (38.8% of the original cellulose) with a dry mass loss of 24.4%. The BET surface area of lignocelluloses increased from 1.7 to 10.6 m(2)/g after combination of 8 weeks biopretreatment and alkaline fractionation. Moreover, alkaline fractionation removed amorphous and low molecular components, which incurred a higher crystalline index and narrower molecular weight distribution of residual carbohydrates in synergistically treated samples as compared to biopretreated samples. (C) 2012 Elsevier Ltd. All rights reserved.