The effect of fertilization on sap flux and canopy conductance in a Eucalyptus saligna experimental forest

Land devoted to plantation forestry (50 million ha) has been increasing worldwide and the genus Eucalyptus is a popular plantation species (14 million ha) for its rapid growth and ability to grow well on a wide range of sites. Fertilization is a common silvicultural tool to improve tree growth with potential effects on stand water use, but the relationship between wood growth and water use in response to fertilization remains poorly quantified. Our objectives in this study were to determine the extent, timing and longevity of fertilization effects on water use and wood growth in a non-water limited Eucalyptus saligna experimental forest near Hilo, HI. We evaluated the short- and long-term effects of fertilization on water use by measuring sap flux per unit sapwood area, canopy conductance, transpiration per unit leaf area and water-use efficiency in control and fertilized stands. Short-term effects were assessed by comparing sap flux before and after fertilizer application. Long-term effects were assessed by comparing control plots and plots that had received nutrient additions for 5 years. For the short-term response, total water use in fertilized plots increased from 265 to 487 mm yr(-1) during the 5 months following fertilization. The increase was driven by an increase in stand leaf area accompanied by an increase in sap flux per unit sapwood area. Sap flux per unit leaf area and canopy conductance did not differ during the 5 months following fertilizer additions. For the last 2 months of our short-term measurements, fertilized trees used less water per unit carbon gain (361 compared with 751 kg H2O kg C-1 in control stands). Trees with 5 years of fertilization also used significantly more water than controls (401 vs. 302 mm yr(-1)) because of greater leaf area in the fertilized stands. Sap flux per unit sapwood area, sap flux per unit leaf area, and canopy conductance did not differ between control and fertilized trees in the long-term plots. In contrast to the short-term response, the long-term response of water use per unit wood growth was not significant. Overall, fertilization of E. saligna at our site increased stand water use by increasing leaf area. Fertilized trees grew more wood and used more water, but fertilization did not change wood growth per unit water use.