Forest production responses to irrigation and fertilization are not explained by shifts in allocation

Production increases in intensively managed forests have been obtained by improving resource availability through water and nutrient amendments. Increased stem production has been attributed to shifts in growth from roots to shoot, and such shifts would have important implications for belowground carbon sequestration. We examined above and belowground growth and biomass accumulation and distribution in two eastern cottonwood clones and American sycamore receiving irrigation (I), fertilization (F), or irrigation + fertilization (IF) and compared with non-treated controls (C). Aboveground growth and biomass accumulation responded positively to both I and F in all genotypes. After three growing seasons, the largest trees were sycamore receiving the IF treatment, with a total mean annual biomass production of 9.4 Mg ha−1 yr−1. Total cottonwood mean annual biomass production in both clones was >5.0 Mg ha−1 yr−1 in the IF treatment. Aboveground biomass production was 6.3 and >3.1 Mg ha−1 yr−1 for sycamore and cottonwoods, respectively. Total root mass fraction was lower in high resource treatments and declined as stands aged. To compare allocation in trees of equal size, we used allometric relationships between above and belowground tissue. Allometric relationships between woody perennial tissues rarely differed among treatments. Improved resource availability caused large increases in growth and consequently accelerated development, but it had little effect on belowground allocation that was not explained by development. Reports of shifting belowground allocation due to soil resource availability must account for developmental effects if they are to accurately evaluate direct impacts of soil resources.