Variation Patterns of Forest Structure Diversity after Set-Aside in Rarau-Giumalau Mountains, Romania

The present study aims to analyze the set-aside effect on the current structure diversity of mountain temperate forests from the Natura 2000 site Rarau-Giumalau. In the past 80 years, the area of entirely protected forests successively increased to up to 77% of the site. The description of past structure diversity was based on the analysis of management plans drawn up for these ecosystems after 1940, while their current structure diversity was based on a tree census carried out in 2015. The forests' structure diversity was described in relation to: tree dimensional heterogeneity; wood volumes homogeneity of the living trees throughout the site; variability of the standing and lying dead wood volume; number and basal area of large trees; natural regeneration. The results show that forest stands where no harvest has ever been registered record the highest level of tree size heterogeneity, while in previously managed forests, the current structure diversity was influenced by the harvesting intensity. The dimensional diversity of trees also depends on the structure, density and age of forest stands at the moment when they are set aside. We observed that the volume of dead wood on the ground greatly increases after abandonment of timber production and that there is a progressive decrease in the number and percentage of large trees in the first 40 years after the last timber harvest, accompanied by a significant decrease in living trees volume. Nevertheless, the number of large trees in stands where the last timber harvesting occurred more than six decades ago is 1.8 times higher than that of the corresponding number in stands where no harvesting was ever performed. The time elapsed since the last harvest generated important changes in the regeneration process, which seems to stabilize after three decades. The forest stands' reaction after set-aside very much depends on their characteristics at the time of exclusion from timber production, especially their age and structure. After 80 years since set-aside, the ecosystem processes and descriptors begin to look very much like those in the forests unaffected by human actions, but the old-growth characteristics have not entirely recovered.