Nucleic acid constructs and methods are disclosed for modifying fiber length, plant height, and/or plant biomass in plant tissues. Plants are genetically engineered with constructs encoding an Arabidopsis thaliana wall-associated kinase gene, which alters fiber length and/or plant height when overexpressed under the control of a cambium/xylem preferred promoter. Plant transformants harboring a wall-associated kinase gene show increased fiber length, a trait that is thought to improve woody trees for pulping and papermaking.