Managing for water-use efficient wood production in Eucalyptus globulus plantations

• This is a long-term study of wood growth per evapotranspiration (PWPWOOD).
• PWPWOOD was positively correlated with evapotranspiration.
• Nitrogen increased PWPWOOD but also increased summer water stress.
• Thinning to 600 or 300 stems ha−1 did not affect PWPWOOD and decreased water stress.
• PWPWOOD decreased with stand age associated with increased soil water deficit.

This paper tests the hypothesis that thinning and nitrogen fertiliser can increase the mass of wood produced per volume of water used (evapotranspiration) by plantations of Eucalyptus globulus. We have called this plantation water productivity (PWPWOOD) and argue that, for a given genotype, this term integrates the effects of management, site and climate on both production and evapotranspiration. This is done using annual estimates of wood production and evapotranspiration from age three years to harvest age (∼age 10 years) in three E. globulus stocking density by nitrogen experiments. The ratio of annual rainfall to potential evaporation at these three sites varied from 0.85 to 0.45.Plantation water productivity (PWPWOOD) was calculated as the ratio of annual growth to annual evapotranspiration. In this study, the PWPWOOD of E. globulus varied from 0.2 to 3.1 g kg−1 and was significantly increased by the application of nitrogen at two sites where growth was nitrogen limited. In fertilised stands, soil stored water was depleted early in the summer while in contrast, unfertilised stands used the water more slowly, thereby extending the growth season to late summer when average daily evaporation was much higher. Increased PWPWOOD in response to nitrogen was associated with an increase in water stress that could be mitigated by reducing stocking density without affecting either production or PWPWOOD.Plantations are managed at the compartment scale while water resources are monitored and managed at the catchment scale or larger. At the compartment scale, growth and PWPWOOD are correlated with evapotranspiration; managing plantations to maximise water use can also minimise the impact of wood production on water resources.