Exploring the relationship between stand growth, structure and growth dominance in Eucalyptus monoclonal plantations across a continent-wide environmental gradient in Brazil
Introduction
Stand structure is a key feature of forest stands that may have a considerable influence on forest growth and productivity. The relationship between structural diversity – including tree size distribution and genetic or species diversity – and forest productivity has been studied in various forest ecosystems and contrasting relationships have been found such as positive, neutral and negative relationships (Forrester and Bauhus, 2016).
Monoclonal stands are an especially useful framework to investigate how structure affects growth independent of the effects of genetic or species diversity. In monoclonal stands of Eucalyptus, increasing tree size inequality is associated with decreasing productivity (Stape et al., 2010, Soares et al., 2016, Resende et al., 2018). In non-monoclonal Eucalyptus plantations and monocultures of other species, where there is a certain degree of genetic diversity, this has also been shown (e.g. Bourdier et al., 2016, Sun et al., 2018).
Structural heterogeneity emerges from differences in trees’ growth rates within a stand throughout time. The growth of individual trees depends on their ability to acquire and use resources (i.e. light, nutrients and water) and how efficient trees are in using them (Binkley et al., 2010). For example, it has been shown that bigger Eucalyptus trees grow faster than smaller ones because they are able to capture a greater amount of resources and also because they use them more efficiently (Binkley et al., 2002, Binkley et al., 2010, Binkley et al., 2013, Campoe et al., 2013). At the stand level, the greater productivity of the biggest trees does not counterbalance the lesser growth of the smaller trees, resulting in an overall lower productivity of heterogeneous stands compared to more homogeneous stands (Binkley et al., 2013, Campoe et al., 2013, Luu et al., 2013).
Factors that change resource availability, resource use and or use efficiency can affect stand growth not only because of changes in mean tree growth, but also because of the different responses according to tree size (Forrester, 2019). For example, in Eucalyptus plantations in Brazil, increasing 1 °C in air temperature decreased growth by 2.2 Mg ha−1 yr−1 in the range of 19.5 to 23.5 °C, and for every 100 mm year−1 decrease in rainfall, growth declined by 0.5 Mg ha−1 year−1 (Binkley et al., 2020). Part of this effect of environmental stress on growth, might not only result from reductions in mean tree growth, but also because it leads to more unequal growth partitioning and consequently to a more heterogeneous structure with higher proportions of suppressed and smaller-sized trees, which, in turn, also reduces stand growth. In addition, since the relationship between stand structure and productivity has been shown to vary among species or genotypes (Soares et al., 2016, Resende et al., 2018), these different responses, according to tree size, may also have a genetic component.
Therefore, we used a continent-wide Eucalyptus experimental network in Brazil to explore the relationship between growth dominance and stand growth, and whether this relationship changes across a wide climatic gradient, represented by soil water deficit. We asked the specific questions: i) Is increasing growth dominance associated with decreasing stand growth? ii) Does increasing soil water deficit increase growth dominance? iii) If growth decreases with increasing growth dominance, does increasing soil water deficit intensify the negative effect of growth dominance on growth? iv) How do these relationships change with stand age?
Section snippets
Site description and experimental design
We used data from the TECHS project (Tolerance of Eucalyptus Clones to Hydric, Thermal and Biotic Stresses, www.ipef.br/techs/en), a continent wide experimental platform, in which different Eucalyptus genotypes were planted on sites with a wide range of climates and soils from north Brazil to Uruguay. Detailed descriptions of this project can be found in Binkley et al., 2017, Binkley et al., 2020, and findings related to various subjects are reported in this Especial Issue.
Out of the 36 sites
Data summary and exploratory analyses
Growth dominance coefficient (GDC) varied from −0.37 to 0.25 across all plots and measurement periods. Negative growth dominance, especially the lowest values, was highly concentrated in the early phase of stand development, typically before about 30 months. Throughout time, there was a shift in growth dominance sign. That is, the tree size relative contribution to stand increment shifted towards greater relative share of bigger trees and this increased with time (Fig. 3a).
The Gini coefficient
Discussion
Studies investigating the effect of stand structural heterogeneity or tree size inequality in monocultures or monoclonal stands associated increasing size inequality with decreasing stand level productivity due to increasing tree suppression (Binkley et al., 2013, Campoe et al., 2013, Luu et al., 2013). This effect was explained by the fact that suppressed trees are much less productive than bigger trees (Binkley et al., 2010). Therefore, a more heterogeneous stand would be less productive than
CRediT authorship contribution statement
Alvaro A.V. Soares: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Henrique F. Scolforo: Methodology, Writing - review & editing. David I. Forrester: Conceptualization, Writing - review & editing. Rafaela L. Carneiro: Data curation, Project administration, Supervision. Otavio C. Campoe: Project administration, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgements
We thank Dan Binkley and Daniele Arriel for their valuable insights and contributions to the discussion. We thank all the companies, universities and research institutions involved in the IPEF-TECHS Project. The project was coordinated by the Forestry Science and Research Institute (IPEF), funded by 26 companies, and supported by these universities and institutes: University of Sao Paulo, Sao Paulo State University, Federal University of Lavras, Federal University of Rio Grande do Norte,
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