Mixed short rotation plantations of Populus alba and Robinia pseudoacacia for biomass yield

•Biomass per ha was greatest in the 75P:25R mixture.•Treatment effects on basal area were different to those on biomass.•Within row mixing was more important than between row mixing.•Robinia pseudoacacia benefited from a higher presence of Populus alba.•P. alba growth was lower when both within row neighbours were R. pseudoacacia.