Understanding the processes driving forest productivity is a critical element in our efforts to maximize production of biomass and wood products and more efficiently utilize resources required for plant growth. We examined above and belowground growth and productivity of four tree species – eastern cottonwood (Populus deltoides), American sycamore (Platanus occidentalis), sweetgum (Liquidambar styraciflua), and loblolly pine (Pinus taeda) – receiving irrigation and fertilization in the Upper Coastal Plain ecoregion of South Carolina, USA. Trees received treatments throughout an entire intensively-managed harvest rotation, which was nine years for cottonwood and sycamore, and 11 years for sweetgum and loblolly pine. Fertilization and irrigation positively affected growth and productivity of all tree species. Fertilization alone led to increases in stem volume index of up to 329% for cottonwood, 376% for sycamore, 261% for sweetgum, and 49% for loblolly pine. Loblolly pine grew the largest of all species tested, and sweetgum was the largest hardwood. Net primary productivity was driven by leaf and fine root tissue production. When accounting for the effect of tree size, belowground biomass decreased with increasing resource availability in sweetgum and loblolly pine, but not cottonwood or sycamore. These results help explain complex relationships between above and belowground tissues in woody species, and indicate that both ontogeny and resource availability can mediate allocation to belowground tissues.