Eucalyptus production and the supply, use and efficiency of use of water, light and nitrogen across a geographic gradient in Brazil

Millions of hectares of Eucalyptus are intensively managed for wood production in the tropics, but little is known about the physiological processes that control growth and their regulation. We examined the main environmental factors controlling growth and resource use across a geographic gradient with clonal E. grandis × urophylla in north-eastern Brazil. Rates of production and resource use were estimated for 14 stands that spanned a four-fold range in production. The supply of water appeared to be the most limiting resource in these fertilized plantations. Above-ground net primary production (ANPP) increased by 2.3 Mg ha^-1 per year for each 100 mm per year increase in rainfall. Higher water supply was also associated with increased use of light and nitrogen (N). The efficiency of resource use (ANPP per unit of resource used) increased with increasing productivity along the gradient. The most efficient stands produced 3.21 kg ANPP m^-3 of transpired water, 1.14 kg ANPP GJ^-1 absorbed photosynthetically active radiation (PAR), and 381 kg ANPP kg^-1 N taken up. The stands with high resource use and high efficiency also had lower mean vapor pressure deficits, less soil water stress, and smaller coarse root to above-ground biomass ratios. Our study indicates that the productivity of fertilized tropical plantations of Eucalyptus is most likely constrained by water supply, and that water supply substantially affects the efficiency of resource use as well as biomass allocation to roots, stems, and leaves. At a regional scale, our results indicate that high productivity stands could produce wood in a 6-year rotation on half the land area required for low productivity stands, using only half as much water.