This chapter investigates biomass, net primary productivity (NPP), and net ecosystem productivity (NEP) of boreal and temperate forest ecosystems in relation to stand density and age. Forests may accumulate woody biomass at constant rate for centuries and there is little evidence of an age-related decline in productivity. Self thinning and management may lead to a loss of tree-individuals to the extent that the available ground surface is no longer covered, thus leading to a decline in productivity per unit ground area. Carbon-accumulation of old forests is similar to that of young forests at the same yield class and of the same species. However, due to the accumulated mass per area and the increased spread of fungal heartwood rot, old forests become unstable and collapse due to external forces, mainly wind. Since carbon accumulation and collapse are highly asymmetric in time, old stands are rarer than young stands. Forest structure and management, rather than stand age, determine NPP. There is no clear distinction in productivity between primary and managed forest, except that managed forests are generally harvested at an age below 100 years. Although unmanaged forests sustain natural processes, biodiversity – expressed as species richness – is not necessarily higher in unmanaged compared to managed forests.