Influence of contrasting stocking densities on the dynamics of above-ground biomass and wood density of Eucalyptus benthamii, Eucalyptus dunnii, and Eucalyptus grandis for bioenergy in Uruguay

Short-rotation plantations have been suggested to develop renewable energies in South America. Our study evaluated the biomass production of Eucalyptus benthamii, E. dunnii and E. grandis plantations at stocking densities of 2220, 3330, 4440 and 6660 trees ha−1 in the North (Tacuarembo) and West (Paysandu) of Uruguay, over a 76-month period. The species survival was not related to planting density, and the highest mortality rates occurred at Tacuarembo. The effects of tree competition were more evident for E. grandis. At Tacuarembo site, the average survival of the species were: 57, 57 and 46% at age 76 months for E. benthamii, E. dunnii and E. grandis, respectively. Survival rates were similar for the different planting densities, with ranges between 50 and 57%. At Paysandu site, the average survival of the species were: 83, 86 and 75% for E. benthamii, E. dunnii and E. grandis, respectively, with a range from 78 to 84% for different planting densities. Weather conditions (temperature and rainfall) were similar at both sites throughout the evaluation period. Wood density and individual and total biomass weight (kg tree−1 and Mg ha−1, respectively) were evaluated sampling destructively 2592 trees and logarithmic biomass equations were developed using the diameter at breast height (cm) and total height (m) of the trees. Wood density increased with the age of the crop only at Paysandu. On average, wood density increased from 0.405 g cm−3 at age 18 months to 0.497 g cm−3 at age 76 months. The highest wood density was observed at 76 months on E. benthamii (0.413 and 0.431 g cm−3) and E. dunnii (0.496 and 0.539 g cm−3) at Tacuarembo and Paysandu, respectively. Age had no effect on the wood density at Tacuarembo. The stem biomass showed an inversely-proportional relationship with the planting density. Individual stem biomass was higher for E. grandis (81 and 74 kg tree−1 on average at Tacuarembo and Paysandu, respectively) than for the other species. The highest biomass per hectare was achieved for the highest planting density, specifically for E. dunnii (193 Mg ha−1) and E. grandis (203 Mg ha−1) at Paysandu and for E. dunnii (157 Mg ha−1) at Tacuarembo. At both locations, the effects of contrasting planting densities on biomass production increased throughout the rotation.