Evaluating Stand Density Index in Southern Piedmont Silver Fir (Abies alba Mill.) stands

Abstract

Plant populations and their competitive dynamics are often described by forest structure and stand density estimates. Forest stand density can be described by absolute or relative measures; relative density indexes compare an absolute stand density with a reference density given by mathematical models of stand development. Reineke’s Stand Density Index (SDI) is a relative density index based on the self-thinning law, that is the inverse-proportional relationship between stand density and mean tree size in developing stands. A species’ maximum SDI expresses the upper limit of all possible combination of density and mean tree size that can be observed in monospecific stands and represents maximum achievable competition. Relative stand density comes from the ratio between individual stands’ SDI and maximum SDI for the species. The aim of this research is evaluating maximum Stand Density Index in southern Piedmont silver fir (Abies alba Mill.) stands, in order to assess their relative density and supply a suitable model of stand structure dynamics. Data consist of 173 plot areas sampled among silver fir stands during the last regional Forest Inventory; only monospecific stands (silver fir basal area greater than 80% of total basal area) were taken into account for the estimation of the species’ self-thinning line. Maximum density line was estimated by linear regression on a log-log scale between density and quadratic mean diameter in the selected stands. The slope of the line is –1,491; this value was used to calculate the Stand Density Index of each stand. The greater SDI in the sample stands is 1359, which is proposed as the species’ maximum value. Relative stand density ranges in most cases from 35% to 60%; this shows that the stands are still undertaking resource competition levels lower than the species’ achievable maximum. This description fits well to the structure and silvilcultural history of Southern Piedmont Silver Fir stands.