**5.1. Current status**

seems to exist between density and rotation [101], frequently higher densities and shorter rotations [104, 114, 115, 121, 122], or lower densities and longer rotations [97, 118–120]. Harvest cycles depend on stump mortality and ability to resprout and cutting cycles of 10 to 30 years

*Site selection* is directly related to survival, growth, and yield of the tree species or clones. To obtain high productivities, sites should be of good quality with long growing seasons [83, 100, 101], and steep slopes should be avoided when mechanization is foreseen [99, 101, 104]. Control of natural vegetation to reduce competition between spontaneous vegetation and energy plantations is better suited during site preparation [101, 104, 115], though it might also be necessary

Two main options are available for the *selection of planting techniques*: plantation of cuttings or seedlings. While the former is use with *Salix* spp. [101, 104, 124–126], the latter is chosen for *Populus* spp. or *Eucalyptus* spp. [101, 124, 126]. Similarly, two approaches are available for management: the plantation with a cut after 1 year in order to promote coppicing or first

Other management practices include fertilization to promote yield [93, 101], though there is some controversy in the literature, with some authors stating that fertilization does not increase yield (*e.g.*, [124, 126, 127]), while others state the opposite (*e.g.*, [128, 129]). The control of pathogens should be primordially done by choosing resistant species or clones or by the increasing diversity (*e.g.*, [101, 130]) and, if this is not enough, with phytopharmaceuticals [93, 98, 101, 115]. Irrigation should be used when water stress and growth reduction are expected [93, 131, 132].

The main goal for stands managed for timber and other products and services is not biomass for energy. The latter is a secondary production, composed of residues, which are growing stock unused parts, such as tops, limbs, stems, stumps, and that result from harvest (cuttings or late thinnings) or silvicultural practices (noncommercial or early thinnings) [8, 133]. Regarding forest residues, two management options can be considered: their maintenance in the stand to preserve or improve stand productivity and site fertility or their removal when negative impacts are not expected [134–136]. The amount of forest residues depends on the species, stand structure, and stem quality, which generate a wide variability on their quantity (*e.g.*, [8, 89, 137]). Two constraints should be considered: the proportion of residues that is feasible to remove from a stand, which depends on its spatial distribution, 50% when scattered and 65% when stacked [8, 138]; and the distance between the stands and the places

Considering the different stand structures, the ones that potentially originate larger amounts of forest residues are even-aged, mixed managed stands, where some species are not well suited for timber or with timber of bad quality, and pure or mixed unmanaged stands, with high density, individuals of small diameter and bad timber quality [8]. Noteworthy are also the agroforestry systems, where the forest portfolio can include energy plantations [140, 141] and stands managed for timber and other nonwoody products and services from which forest residues can be obtained [140, 142–144]. The latter, frequently in rather small quantities, are

are indicated in the literature [83, 99, 104, 117].

harvest at the end of the rotation length [93, 104, 121, 122].

**4.2. Stands managed for timber and other products and services**

where it will be used, a 20–50 km radius is frequently used [8, 88, 137, 139].

after each harvest [93, 104, 123].

28 Renewable Resources and Biorefineries

Despite its advantages and despite being the most used renewable energy source, the current share of bioenergy in the world is still very limited. In 2015, bioenergy and renewable wastes accounted for 9.4% of the world's energy supply [2]. Among the various biomass sources, solid biofuels accounted for 63.7% of the global renewables supply (liquid biofuels, biogas, and renewable municipal waste accounted respectively for 4.3, 1.7, and 0.9% and the other renewable energy sources for the rest) [2]. In OECD countries, where biomass is mostly used in modern systems, the share of biomass and renewable wastes is even lower, with these fuels accounting for 5.2% of the total primary energy supply in 2015 and solid biomass accounting for 36.1% of the renewable energy supply [2].

Solid biofuels, which are almost entirely composed of wood, wood residues, and wood fuels, are used to produce electricity and heat. Direct heat is by far the most common application of solid biomass. In this case, biomass is used directly by the end users (*e.g.*, residential, industrial, commercial, agriculture) and not by the energy transformation sector (*e.g.*, power plants, combined heat and power (CHP) plants or heating plants). The dominance of the use of solid biomass for heating applications is mostly justified by its traditional use in the African and Asian countries for heating and cooking [1].

Looking at the situation in Europe, where biomass is mostly used in a modern way, the utilization of solid biomass by the energy transformation sector has a bigger prevalence. Power plants for the production of electricity have a 9% share, CHP plants both for the production of electricity and heat 16% and district heating plants 5% [148]. In total, the European energy transformation sector accounts for 30% of the solid biomass consumption, contrasting with the world average, which is around 9%.
