Gibberellins (GAs) are involved in many aspects of plant development, including shoot growth, flowering and wood formation. Increased levels of bioactive GAs are known to induce xylogenesis and xylem fiber elongation in aspen. However, there is currently little information on the response pathway(s) that mediate GA effects on wood formation. Here we characterize an important element of the GA pathway in hybrid aspen: the GA receptor, GID1. Four orthologs ofGID1were identified inPopulus tremula×P. tremuloides(PttGID1.1–1.4). These were functional when expressed inArabidopsis thaliana, and appear to present a degree of sub-functionalization in hybrid aspen.PttGID1.1andPttGID1.3were over-expressed in independent lines of hybrid aspen using either the 35S promoter or a xylem-specific promoter (LMX5). The35S:PttGID1over-expressors shared several phenotypic traits previously described in35S:AtGA20ox1over-expressors, including rapid growth, increased elongation, and increased xylogenesis. However, their xylem fibers were not elongated, unlike those of35S:AtGA20ox1plants. Similar differences in the xylem fiber phenotype were observed whenPttGID1.1,PttGID1.3orAtGA20ox1were expressed under the control of theLMX5promoter, suggesting either thatPttGID1.1andPttGID1.3play no role in fiber elongation or that GA homeostasis is strongly controlled when GA signaling is altered. Our data suggest that GAs are required in two distinct wood-formation processes that have tissue-specific signaling pathways: xylogenesis, as mediated by GA signaling in the cambium, and fiber elongation in the developing xylem.