Observed and modelled leaf area index in Eucalyptus globulus plantations: tests of optimality and equilibrium hypotheses

This paper reports on variation in leaf area index (L) in five Eucalyptus globulus Labill. plantations in response to application of nitrogen, thinning at age 2 years and variation in climate wetness index (the ratio of rainfall to potential evaporation). Observed L is compared with: (i) L predicted to optimize net primary productivity for a given average annual temperature, annual water use and potential evaporation (L-opt) and (ii) L calculated as a linear function of climate wetness index (L-eq). L peaked in fertilized plots at between 4 and 5 years of age or immediately after canopy closure. The value of L from canopy closure to age 8 years was not strongly related to annual rainfall or climate wetness index. At two sites with total soil nitrogen < 1.2 mg g(-1), L in fertilized plots was about two units greater than in unfertilized plots. This difference persisted until measurements ended in 2004 when the trees were 8 years old. The L of plots thinned to 300 and 600 stems ha(-1) at age 2 years recovered quickly and was not significantly different from L in unthinned plots when the trees were 8 years old. L-opt was a good predictor of the leaf area index of 8-year-old plots of E. globulus when nitrogen and phosphorus were non-limiting (model efficiency (EF) was 0.5). For the same plots, L-eq underestimated observed L by an average of two units, and the model efficiency was low (-3.25). Data from two nitrogen-limited sites demonstrated that for fertilized plots L-opt (EF = 0.6) was a much better predictor of L than L-eq (EF = -3.36). At the same sites, L-eq (EF = 0.42) was a better model for predicting L of unfertilized plots than L-opt (-3.59). These results provide evidence that comparing observed L with L-opt can identify stands limited by factors other than growing climate.