Increasing stand structural heterogeneity reduces productivity in Brazilian Eucalyptus monoclonal stands

Abstract

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 m 3 ha À1 to 23 m 3 ha À1 for the lowest planting density (667 trees ha À1) and highest planting density (1667 trees ha À1), 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.

Covariation between tree size and shade tolerance modulates mixed-forest productivity

“…Our results showed a significant negative effect of size inequality on productivity after controlling species diversity, stocking and stand development stage effects, which is in agreement with the results obtained by several authors in pure stands, through experimental, observational or more theoretical studies Caspersen et al 2011;Cordonnier and Kunstler 2015;Soares et al 2016;Bourdier et al 2016). For instance, the negative effect of structural diversity can be as much as a 14% growth reduction in a young eucalyptus plantation .…”

“…However, studies that explored the direct effect of tree size inequality on stand productivity in multispecies forests show more contrasted results. Some studies found an increase of productivity when tree size inequality increased (Lei et al 2009;Hardiman et al 2011;Dănescu et al 2016;Silva Pedro et al 2017;Zeller et al 2018 for USA), while other studies found neutral or even negative effects (Liang et al2007;Bohn and Huth 2017;Zeller et al 2018 for Germany) of tree size inequality, in line with results obtained in monospecific stands Long and Shaw 2010;Cordonnier and Kunstler 2015;Soares et al 2016;Bourdier et al 2016). For instance, Liang et al (2007) showed that basal area growth was negatively correlated with tree size inequality in Douglas fir-western hemlock mixtures as well as in mixed conifer forests.…”

Forest biodiversity, ecosystem functioning and the provision of ecosystem services

“…This suggests that many of these processes and interactions could be important in monocultures that are heterogeneous, especially uneven-aged monocultures. However, just as tree species diversity does not necessarily increase productivity, structural diversity has also been found to have negative effects on productivity, and in some forest types increasing size heterogeneity can reduce productivity by 20% Ryan et al 2010;Stape et al 2010;Aspinwall et al 2011;Luu et al 2013;Bourdier et al 2016;Soares et al 2016). In such stands, the heterogeneous size structure enables larger trees to acquire higher quantities of a given resource than smaller trees, which they also use more efficiently than smaller trees.…”

Tree species richness enhances stand productivity while stand structure can have opposite effects, based on forest inventory data from Germany and the United States of America

“…Similar trends were also found by Bourdier et al (2016), who show that tree size heterogeneity would also decrease productivity, depending on the shade-tolerance of the tree species. Also, Luu et al (2013) and Soares et al (2016) reported a negative effect of tree size heterogeneity on stand productivity.…”

“…Bourdier et al (2016) discovered that a negative relationship between tree size heterogeneity and productivity could result from lower light interception and use efficiency in the case of an enhanced stand structure. Others have discovered negative relationships between structural diversity and productivity (Edgar and Burk 2001) in temperate forests or tropical Eucalyptus stands when structural diversity is present, but genetic and species diversity are absent (Soares et al 2016). Chen and Klinka (2003) however, did not find any relationship between structure and productivity.…”