Transpiration along an age series of Eucalyptus globulus plantations in southeastern Australia

Many of the world's Eucalyptus plantations are grown on short rotations of 15 years or less, which often covers the most rapid phase of stand development and peaks in growth rates and leaf areas. Since transpiration is related to stand leaf area these short rotations that make use of rapid early growth rates, may also maximise plantation water use, which has implications for predicting their water requirements and impacts on catchment hydrology. This study examined the transpiration, leaf area and growth rates of Eucalyptus globulus Labill. plantations aged 2–8 years. Transpiration (E), estimated using the heat pulse technique, increased from 0.4 mm day⁻¹ at age 2 years to a peak of about 1.6–1.9 mm day⁻¹ in stands aged 5–7 years. This was associated with similar trends for stand leaf area index (LAI) and periodic annual increments of aboveground biomass, which both peaked at about age 4–6 years resulting in a linear relationship between E and LAI. While stand sapwood areas were continuing to increase at age 8 years, E was already declining due to reductions in sap velocity, from 13.5 cm h⁻¹ at age 2 years to 6.3 cm h⁻¹ at age 8 years and reduced sapwood area growth rates. Trees compensated for this reduction in sap velocity with declines in the leaf area (A_L) to sapwood area (A_S) relationship (A_L:A_S) with age. There was also a reduction in growth efficiency (aboveground biomass increment per LAI) with age. However, reductions in WUE were small after age 4 years, which explained the linear relationship between E and LAI. If E continues to decline successive short rotation lengths may not only make use of rapid early growth rates but could also increase plantation water use compared to longer rotations over the same period of time.