Influence of age and planting density on the energy content of Eucalyptus benthamii, Eucalyptus dunnii and Eucalyptus grandis planted in Uruguay

According to some studies, the use of forest biomass for the generation of electric power and/or heat would result in a significant reduction in the emission of greenhouse gases. This would happen even in the case of soils substitution that currently have an agricultural use although its implementation would require more studies on the environmental impact that they could have in each particular case. In this study, the potential of electric power production is evaluated with several species of Eucalyptus planted in short rotation forestry systems. Three species were evaluated: E. benthamii, E. dunnii and E. grandis in four spacings: 3 x 1.5 m, 3 x 1 m, 3 x 0.75 m, 3 x 0.5 m, in two sites in the northern and western areas of Uruguay during a period of 76 and 75 months, respectively. During this period, the following parameters were evaluated: higher heating value, wood density, energy density and energy yield per hectare and per year. The results obtained show that the heating value changes slightly with the planting density and age. The average of higher heating values were 19,787 versus 19,454 J g(-1) and 19,867 versus 9909 J g(-1)for the planting densities of 2220 versus 6660 trees ha(-1) at Paysandu and Tacuarembo, respectively, at 76 months. Higher heating value increased from 19,665 to 19,745 J g(-1) at 18 months to 19,740 and 19,914 J g(-1) at 76 months for Paysandu and Tacuarembo, respectively. The wood density, depending on the site, is affected by age, species and planting density. On average, wood density increased (only at Paysandu) from 0.405 g cm(-3) at age 18 months to 0.497 g cm(-3) at age 76 months. There is a negative relationship between the heating value and the wood density. The energy yield is basically explained by the production of biomass. At Tacuarembo, a stronger relationship between energy production and planting density is obtained (1012 vs. 636 MW ha(-1) for 6660 and 2220 trees ha(-1), respectively) compared to the Paysandu site (1093 vs. 912 MW ha(-1) for 6660 and 2220 trees ha(-1), respectively). Harvest times are different in each site depending on the IMA values obtained.