Yield pattern of eucalypt clones across tropical Brazil: An approach to clonal grouping
Introduction
Intensively managed plantations supply 33% of the world's non-fuelwood demand, even though their area correspond to only 1.5% of the forests in the world (INDUFOR, 2012). These productive forests have alleviated the historical pressure on native forests in some places (Hayes et al., 2005), and eucalyptus plantations emerge as the pinnacle of fast-growing forests. This genus is well known for the highest growth rate among the hardwoods in the world, where the productivity in Brazil has increased about 3-fold in the past 40 years (Stape et al., 2010). This dramatic increase of eucalyptus productivity is a consequence of the summation of key factors: development of superior clones and silvicultural practices including site preparation, fertilization, weed control and spacing (Stape et al., 2010, Gonçalves et al., 2013).
Intensively managed plantations focus on the manipulation of soil and stand conditions in order to minimize the environmental constraints that may limit tree growth (Fox, 2000). The climate effect, however, cannot be controlled, which highlights the importance to understand its effect on forest production. As noted by several authors, for example Almeida et al., 2010, Scolforo et al., 2016, droughts can dramatically reduce eucalyptus productivity. This climate phenomenon makes plantations more susceptible to attack of pests, disease, and catastrophic mortality (Netherer et al., 2008, Pinkard et al., 2015).
Traditional areas of eucalyptus plantations in Brazil have faced unusual droughts in the past few years (Otto et al., 2015). The market demand for bio products however, has increased and prompted the expansion of eucalyptus plantations even to drier sites (Binkley et al., 2017). These facts have challenged the development and selection of silvicultural regimes and clones to successfully keep commercial plantations with high growth rate in these new conditions.
Changes in silvicultural regimes, such as the reduction in the number of planted trees per hectare (Hakamada et al., 2017), can reduce tree mortality by mitigating the competition for water resources. On the other hand, clonal deployment seems to be more challenging, since the genotype × environment (G × E) interaction, may be substantial, especially in regard to climate variation (Binkley et al., 2017). On one hand, the development of breeding programs has increased eucalyptus productivity and wood quality (Lemos, 2012), but on the other hand, little effort has been extended to verify how the large variety of clones interact with climate (Scolforo et al., 2017). Questions always arise, especially the ones concerning: (1) how does clonal productivity vary across different sites; (2) are there clones with similar volume yield pattern spanning large areas? These questions are crucial in the context of the selection of the most proper clones to match specific sites in order to avoid plantation failure (Gapare et al., 2015).
Some studies have started to address some of these questions, such as Scolforo et al., 2017, Marcatti et al., 2017. These authors used statistical models for recommending the appropriate places where different clones should be planted. Their methodology focused on the gain of forest productivity by using climate information. Calegario et al. (2005) suggested the combination of climate and/or soil data with mixed effects modeling to improve clonal selection for maximizing forest productivity. Almeida et al. (2010) proposed clonal selection to increase eucalyptus productivity through the use of ecophysiological models.
It is still necessary to acquire better understanding of how clonal productivity varies along a national climate gradient, and if certain clones can be grouped according to similar volume yield patterns. Furthermore, observing how volume yield patterns vary with clonal group and climate is crucial for proposing site-specific management. Clonal grouping may be used by geneticists by searching for a few physiologic characteristics that explain how different clones have similar environmental interaction (Scolforo et al., 2017). This may serve as baseline information for developing superior clones in Brazil.
This paper provides an approach of clonal grouping based on the volume yield pattern of 11 widely planted eucalypt clones, evenly distributed across 17 sites in tropical Brazil. The approach enables assessment of how annual water deficit index variation impacts clonal productivity in tropical Brazil.
Section snippets
Characterization of the sites and database
The database is composed of remeasurement information from 17 research sites that span tropical Brazil (sites: 2, 4, 5, 7, 8, 9, 11, 13, 14, 20, 22, 24, 26, 29, 30, 31 and 33). These sites are part of the TECHS Project (Tolerance of Eucalyptus Clones to Hydric, Thermal and Biotic Stresses), which was launched in 2011 in Brazil and northern Uruguay (Binkley et al., 2017). The database of this study, however, is concentrated in tropical Brazil, which ranges from Paraná to Pará State (Fig. 1).
The
Fitted equation for standardizing the volume yields of the clones
A polynomial of integer and real powers was chosen to model the volume growth of the clones in order to allow for the standardization of the volume yield at a common age of 5 years for all the plots and TECHS sites. The fixed effect slope coefficients are all positive demonstrating a monotonic increase of volume over time (Table 2).
Therefore, the specific fitted coefficients generated for each nested combination [plots (TECHS site) and TECHS site] through linear mixed effect modeling were
Discussion
This paper provides a useful and straightforward approach to grouping clones regarding their volume yield pattern across a very large area in Brazil. The dataset used in the analysis is unique and provides a balanced experimental observation of clonal yield pattern over Brazil, ranging over all climate zones and several soil types (Binkley et al., 2017).
Studies evaluating G x E interactions are often found in the literature. Recently, Silva et al. (2006) evaluated the G × E interaction for
Conclusions
A simple and useful approach for grouping eucalypt clones was proposed and successfully implemented. Consequently, the developed approach can: (1) provide extra information for the deployment of superior eucalypt clones; (2) improve site-specific productivity estimates for a variety of clones with similar yield, which results in increase forest resilience and is an important component against plantation failure.
The clonal groups highlighted different clonal productivities along the Brazilian
Acknowledgments
The authors are especially grateful to the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq - Brazil) for the scholarship provided to develop this research (249979/2013-6), to the Instituto de Pesquisas e Estudos Florestais (IPEF – Brazil) and to the TECHS project. We thank all the companies, universities and research institutions involved in the TECHS Project. The project was funded by the following 26 companies, with a main researcher: Anglo American (Andre Machado), Arauco
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