The effect of organic-matter management on the productivity of Eucalyptus globulus stands in Spain and Portugal: Tree growth and harvest residue decomposition in relation to site and treatment

Sustainable management of forest resources, including nutrient retention and protection of the soil structure, is required to ensure long-term soil fertility and productivity of later rotations. Potential depletion of soil nutrients is particularly likely in production systems based on fast-growing trees, such as eucalypts. We have studied production of second rotation Eucalyptus globulus stands at two sites in northern Spain and two in central Portugal, after replanting or coppicing, under four treatments, in which plant residues from the first crop were utilised. The residues were either removed from the sites (Treatment R), spread over the soil surface (Treatments S, uniform spreading, and W, in which the woody debris was confined to rows between the trees) or incorporated into the soil by harrowing to 15-cm depth (Treatment I). We measured tree height and girth at intervals over three growing seasons, and root biomass at the Portuguese sites. Decomposition of three residue components: leaves-plus-bark, twigs and branches, was measured in litter bags placed in the position corresponding to the placement of the organic residues. By the end of the experiment, tree height was significantly greater in Treatment I than in Treatment R at both Spanish sites, if planted as seedlings, with intermediate growth in S and W. In Portugal, tree height was smaller in R, though not significantly. DBH showed similar trends, although treatment differences were not significant. Coppiced trees grew faster than seedling trees, but a significant treatment effect on the growth was only observed at the inland Portuguese site, where it was better in Treatment I by the end of the experiment. The residues decomposed significantly faster in I than S or W at the Portuguese sites, but not in Spain. Leaves-plus-bark decomposed faster than twigs, and twigs faster than branches. The results are discussed in relation to recommended management options.