Eucalyptus plantations: combining silviculture and genetics to achieve rational yield increases

The rapid growth rate of eucalyptus and the quality of its wood make it the world's leading industrial reforestation species. Since the 1960s, plantation productivity has quadrupled, reaching a current average of 40 m3 per hectare, per year, thanks to investment in research, in close relation with development. Eucalyptus forests are now part of a more systemic vision, ranging from wood production to the biodiversity of ecosystems.

However, there is much still to do about environmental impact, sustainable management, and adaptation to global change. This was the central question for the international conference of the International Union of Forest Research Organizations (IUFRO), Joining silvicultural and genetic strategies to minimize Eucalyptus environmental stresses: from research to practice , co-organized by CIRAD in Porto-Seguro, Brazil, from 14 to 18 November 2011*. This concern is illustrated in the work done by CIRAD, which was presented at the conference.

Reconciling production and resource preservation

In particular, researchers have shown that it is now possible to quantify the high water consumption of plantations in line with biomass production. Precise knowledge of that consumption new serves to rationalize plantation management on a landscape scale (non-planted areas between reforested areas, protection of the heads of catchment areas, etc) and thus to reconcile production and resource preservation.

In terms of fertilization, it is now possible to partially replace potassium by sodium. This ensures better preservation of the potassium resource. The rates applied apparently do not cause soil salinity or degradation. This type of fertilization is also much more accessible for smallholders, since the cost of sodium chloride is just 15% of that of potassium chloride.

Researchers have also demonstrated the ability of eucalyptus hair roots to penetrate the soil to a depth of up to 10 m in two years, and to make use of large volumes of soil very rapidly. This enables eucalyptus to grow in very poor soils and avoids the risk of water table pollution by fertilizers.

Lastly, in the case of nitrogen-deficient tropical soils, a common occurrence in the event of low water stress, researchers have shown that production can be boosted by 30% if the eucalyptus trees are planted with acacias rather than as a monoculture. On the one hand, the nitrogen fixed by the acacias is transferred to the eucalyptus trees, and on the other, the complementarity of the two species ensures better capture of the available water and light.

Genetic research: major progress for a model species

Eucalyptus is also a model forest species for genetics research, by virtue of the fact that its whole genome sequence is available, and of the number of teams involved on an international level. In terms of genetics, researchers are working to determine how the varieties express their characters in more stressful environments. In the work presented at the conference, they showed that the variations in lignin and cellulose composition of different stands depends either on the environment, or on their genes, or on both factors at once. Near-infrared spectroscopy has been used to facilitate understanding of the degree of genetic responsibility for the variability of ligno-cellulose characteristics and the way in which environmental stresses may affect these characters. In particular, it has been demonstrated that stressful environments with hydric constraints have a substantial impact on tree growth characters but very little effect on ligno-cellulose properties.

Genetics serve to develop strategies for improving and creating varieties better suited to marginal areas. New breeding methods, also presented at the conference, guarantee greater efficacy. These very new methods, known as genomic breeding, use a high-throughput genotyping technology and new statistical models. They were presented in relation to the release into the public domain of the eucalyptus genome sequence. These methods raise many questions about their applicability in terms of eucalyptus genetic improvement. However, they should facilitate greater integration of adaptation, production and quality characters into varietal creation operations, and help boost the synergy between the various lines of research.

The conference thus demonstrated the importance of combining silviculture and genetic research, with a view to achieving economic, social and environmental sustainability.

More than 250 scientists and forest managers from 20 countries attended the event. At the suggestion of CIRAD and its private- and public-sector partners in Europe, the conference scheduled for 2017 is to be held in Montpellier, France, after China in 2014.

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Eucalyptus plantations in Brazil

As things stand, eucalyptus is generally planted to replace pasture or crops in tropical and sub-tropical zones. It thus plays a major role in the carbon and water balances of these landscapes, thanks to ecological, economic and social interfaces. In Brazil, eucalyptus is the dominant and the most productive planted forest species, covering some 4.7 million ha. 60% of the wood produced is certified under international standards. Eucalyptus is grown in short and medium-length rotations, to produce paper pulp, charcoal, fuelwood, and reconstituted and solid wood. The forestry sector in Brazil generates some 4% of the country's GDP and employs around 4.5 million people.

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* The conference was organized by USP (Universidade de São Paulo), ESALQ (Escola Superior de Agricultura Luis de Queroz), CIRAD, EMBRAPA (Empresa Brasileira de Pesquisa Agropecuaria), IPEF (Instituto de Estudos e Pesquisas Florestais) and NCU (North Carolina University).