The Role of Height Increment and Marginal Height Cost in the Production Ecology of Eucalyptus Plantations

In forest production ecology, growth has been separated into a tree's capacity to absorb radiant energy, and its efficiency in converting absorbed radiant energy into biomass or stem volume. Stand production is also be expressed a function of height growth and the stem size required to physically and physiologically support height growth. Each additional increment in height requires a corresponding increment in stem size that can be considered a marginal cost. Since tree height is linked to light absorption, variation in height increment and marginal height cost should be related to the distribution of stem growth as well. This hypothesis was evaluated with data collected in a long-term study with fast-growing eucalyptus in a plantation setting. The experiment was designed to compare the effects of homogeneous and heterogeneous canopies created by varying planting date. Indigenous sources of variation due to genotypic, microsite variation, and spacing were controlled to the extent possible. Within-plot variation of stem growth (Sim), height increment (Sih), and marginal height cost (Sim/ih) were quantified with a symmetry index calculated as the cumulative proportional value of the growth variables for the tree with median height minus 0.5. Negative values indicate positively skewed distribution. In most cases the three symmetry indexes for the heterogeneous-planting treatment were more negative than those for the uniform treatment corresponding to the induced variation in height. Among the set of symmetry indexes for a canopy treatment, the symmetry index for height increment were the least negative and the indexes for stem growth were the most negative. A separate effect of canopy treatment was detected indicating the not all the variation in Sim could be explained by Sih and Sim/ih. Based on past studies, the independent effect of canopy heterogeneity on Sim could be due to a mix of mechanical perturbation and shade avoidance effects. For both canopy treatments, a constant difference between the symmetry indexes for height increment and marginal height cost was evident in the data, suggesting an interaction between height growth capacity and marginal height cost in these plantations. Previous studies also show interactions between resource-based components of capacity and efficiency variables. Identifying the physiological mechanisms responsible for these interactions may be a new direction for forest production ecology studies.