Increasing stand structural heterogeneity reduces productivity in Brazilian Eucalyptus monoclonal stands

The effect of stand structural heterogeneity on production was examined in the northeastern region of Brazil using a set of spacing × genotype trials of Eucalyptus along a large gradient in site productivity. This experimental platform enabled an analysis of relationships between productivity and structural heterogeneity for entire rotations while controlling the confounding effects of species and genetic diversity. Stand heterogeneity was negatively correlated with productivity. A 10-unit increase in heterogeneity, quantified using Gini's coefficient, was associated with a loss of approximately 17 m3 ha−1 to 23 m3 ha−1 for the lowest planting density (667 trees ha−1) and highest planting density (1667 trees ha−1), respectively, by the end of a 7-year rotation. The most productive genotypes were generally the most homogeneous. While stand density increased productivity, it also increased structural heterogeneity. In general, the positive effect on productivity of increasing density was greater than the negative effect of heterogeneity, but we found that the contrary can also occur. The relationship between planting density and heterogeneity differed between genotypes, with some much less plastic than others. The results show that structural heterogeneity per se, in the absence of genetic diversity and species diversity, can have a strong negative effect on productivity, and an understanding of the mechanisms causing these contrasting patterns (with versus without genetic diversity) will be important when engineering forest reforestation projects and plantations for wood production, carbon sequestration and many ecosystem functions correlated with productivity.