Effects of planting density on the distribution of biomass in a douglas-fir plantation in southern Italy

The effects of initial planting densities on the distribution of above-ground biomass of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco var. menziesii) were investigated in a plantation in southern Italy. Allometric equations designed for the plantation under study were used to estimate above-ground biomass and its partitioning to stem and crown compartments. A comparison between biomass estimated with allometric equations and biomass estimated with a constant biomass expansion factor (BEF) from the Italian National Forest Inventory (INFC 2005) was carried out. Moreover, a BEF calculated as the ratio of total above-ground or compartment biomass to stem volume was used to define the sensitivity of BEF to age and to tree density. Variation of aboveground standing biomass estimated with allometric equations was evaluated according to 6 differing planting densities (833, 1000, 1250, 1667, 2000 and 2500 trees ha-1). In the first 20 years after planting, higher biomass stock was detected in high density plots, but after the age of 32 years differences between plots disappeared. When the plantation was 40 years old, the highest amount of total biomass was observed in plots of 2000 trees ha-1 (about 405 Mg ha-1), and the lowest in plots of 2500 trees ha-1 (about 381 Mg ha-1). The Douglas- fir plantation has a total above-ground carbon stock of 197 Mg C ha-1 at the age of 40 and a mean annual CO2 sequestration of 18 Mg ha-1 y-1. On average, constant BEF from INFC underestimated biomass by 11% at ages 15 and 25 and overestimated biomass by 16% at older ages. BEFs expressed as a ratio of biomass to stem volume significantly depended upon age and planting density, with decreasing trends for total, stem and crown compartments. Our results indicated that total above-ground biomass production is not influenced by different tree density if considered over a long period. If cutting cycles are short, planting density of 2000 trees ha-1 may ensure high biomass production rates; for longer cutting cycles, 1000-1200 trees ha-1 could also be a valid choice.