Forest species replacement can influence significantly the amount of carbon stored in the several compartments that compose the terrestrial ecosystems (biomass, forest floor and mineral soil). This study intends to evaluate the influence of the replacement of the Quercus pyrenaica species (QP), which represents the climax vegetation of Serra da Nogueira, NE Portugal, by the Pseudotsuga menziesii (PM) and Pinus nigra (PN) plantations (fast-growing species). For this purpose, three plots of 314m2 were established in each stand (9 plots in total) and the height and diameter at the breast height of all trees were measured, in order to characterize the stands and estimate the tree biomass. Herbaceous vegetation and forest floor were collected in areas of 0.49m2 in 15 points under each tree species (5 per plot). At the same points, disturbed and undisturbed soil samples were collected at depths 0–5, 5–10, 10–15, 15–20 and 20–30 cm. Thirty years after the climax vegetation replacement, carbon gains are observed in forest species biomass and forest floor (1.3 Mg C ha−1 year−1 in PN and 4.0 Mg C ha−1 year−1 in PM) and significant losses were recorded on soil carbon pool (about 2.2 Mg C ha−1 year−1). Total carbon accumulated is significantly higher in PM (331 Mg C ha−1) compared to PN (246 Mg ha−1) and QP (273 Mg C ha−1), which present statistically similar values. Tree biomass and mineral soil constitute the major carbon pools.

Forest biomass used for energy or fuels can be sourced directly from land-use systems, indirectly from wood-based industries or recovered from other human activities outside the forest sector. The former, referring to primary biomass from forests, includes organic products or residues derived directly from living or recently dead trees or other forest vegetation. It constituted nearly half of world’s harvested forest biomass in 2021 and holds particularly importance in the Global South, where traditional biomass remains a vital energy source for many people. Besides direct wood fuel, secondary wood residues represent another substantial source of forest bioenergy. These organic residues, such as wood chips, sawdust or black liquor, are generated by the industries processing wood, especially primary forest industries. A large amount of these residues is well-suited for further material use and energy generation. However, wood suitable for energy is not solely generated by forest-based industries. Various other activities use wood products that eventually reach the end of their usable life and are discarded, such as wood waste from construction or demolition, furniture waste or end-of-life pallets and packaging used to transport goods. This chapter presents and characterises the different woody biomass streams that can provide feedstock for energy.