Light use efficiency declines with water deficit and age in Eucalyptus plantations across Brazil

Eucalyptus plantations in Brazil are among the world's most productive forests. We examined how differences in growth across an 1100 km environmental gradient related to light use and light use efficiency for 11 tropical Eucalyptus plantations, with the same 11 clones planted at each site. The long geographic gradient led to differences in precipitation, temperature, and soils, giving annual water deficits ranging from 40 to 500 mm yr- 1. Mean annual increment (MAI) declined by about 2.2 Mg ha-1 yr- 1 for each 100 mm yr- 1 increase in water deficit. Across sites, about one-fourth of the differences in MAI related to light use (absorbed photosynthetically active radiation, APAR), and three-fourths to light use efficiency (LUE, wood growth per unit of light absorbed),. Light use did not relate to water deficit, so the decline in MAI resulted from a 0.1 g MJ-1 decline in LUE for each 100 mm yr- 1 increase in water deficit. Though APAR accounted for a minor part of the decline in growth across sites, differences in both APAR and LUE among clones were important within each site. The pattern in wood growth per unit of light use likely includes the effects of water deficit on both total carbon (C) gain per unit of light use and partitioning of C belowground. A next step in harnessing production ecology insights for silviculture and breeding may be direct assessments of belowground partitioning (and perhaps respiration), and how opportunities vary with site conditions.