Carbon stocks and net ecosystem production changes with time in two Italian forest chronosequences

Forest management influences several ecosystem processes, including carbon exchange between forest ecosystem and atmosphere. The aim of this paper was to study the carbon cycle over different age classes of two managed forests in the Italian Alps through direct measurements and modelling. For this purpose, ecosystem carbon dynamics of a beech forest (Fagus sylvatica L.) and of a spruce forest (Picea abies (L.) Karst.) were investigated using a chronosequence approach. In both forests, five forest development stages were identified (thicket, pole wood, young forest, mature forest and the regeneration phase) with an age spanning from 42 to 163 years for the beech forest and from 35 to 161 years for the spruce forest. Measured total ecosystem carbon stock increased up to 80-100 years, with a mean of 232 MgC ha(-1) in the beech forest and of 299 MgC ha(-1) in the spruce forest. Calculated net ecosystem production (NEP) was found to decrease linearly with age and had an average value of 2.2 and 4.4 MgC ha(-1) year(-1) for beech and spruce forest, respectively. Model simulations reported an increase in NEP till 50-60 years followed by a decrease thereafter. The model also predicted a negative NEP for a short period (8-11 years) after the seed cut. Aboveground biomass was the main driver of carbon accumulation while soil carbon was not significantly influenced by both age and management system. Moreover, measured data and model showed that the applied shelterwood system allowed for a rapid recovery of the ecosystem after the disturbance (i.e. seed cut), bringing back forest to act as C sink in few years.