Wood production varies substantially with stand age, fertility, water availability and stand structure or evenness. Changes in wood production can result from changes in canopy photosynthesis, changes in the fraction of photosynthesis used by different sinks (partitioning or carbon allocation), or both. However, because wood production is only a small fraction (20-30%) of the carbon fixed annually in photosynthesis, to uncover the mechanism underlying the changes in wood production requires information on the complete carbon budget of a stand—so that photosynthesis and carbon allocation can be calculated and tracked. We measured annual carbon flows over a six-year rotation for three sites with different productivities and climates (Aracruz, IP Brazil and Veracel) with treatments designed to change fertility, water availability and stand structure. We measured carbon flows to wood production, foliage production and total belowground carbon allocation (the sum of root production, root respiration, and mycorrhizal production and respiration) and estimated foliage and wood respiration for the BEPP study using relationships from Eucalyptus saligna in Hawaii.
Our results show that increased resource availability (in this case mostly water) increases canopy photosynthesis and changes carbon allocation—with wood production getting a higher annual fraction of photosynthesis and a lower fraction going below ground. At age six, irrigated treatments had 30-40% greater woody biomass than non-irrigated treatments. Soil respiration and belowground allocation in the newest clones was ~700 g C m-2 yr-1, substantially lower than older clones or seed origin stands (~2000 g C m-2 yr-1). Wood net primary production received 40% of gross primary productivity (GPP) in the best clone, but only 25% of GPP in seed origin stands, under the same resource availability. Apparently genetic improvement has altered partitioning of annual photosynthesis from belowground to wood production. GPP was closely related to annual foliage net primary productivity.