Agroforestry – The integration of trees into a conventional agriculture system. This can be for timber or non-timber products such as fruit or nuts. When combined with livestock grazing this is called silvopastoral agroforestry, and when combined with arable crops it is called silvoarable.

BECCS – Bioenergy with Carbon Capture and Storage. The carbon dioxide emissions from combustion of carbon-based fuels are captured and pumped into long-term, usually geological, storage to prevent being lost to the atmosphere (CCS). If the fuel used is biomass which took CO₂ from the atmosphere during growth then the system may be viewed as having the effect of permanently removing CO₂ from the atmosphere.

Bioenergy – Energy generation using biomass-based fuel. Frequently used to refer to electricity generation, it can also refer to heat generation and, more broadly still, transport biofuels. The biomass fuel can be raw wood, or it can be processed in some way, such as densified into wood pellets or briquettes, or converted to a liquid or gas.

Biofuel – Any form of fuel derived from biomass. Often used to refer specifically to transport biofuels in liquid or gaseous form.

Biomass – Material that is derived from living, or recently living, biological organisms. In the energy context it is often used to refer to plant material, especially wood, however by-products and waste from livestock farming, food processing and preparation and domestic organic waste, can all form sources of biomass.

Carbon Dioxide Equivalent (CO₂e) – Expresses the equivalent amount of carbon dioxide which is either solid carbon, and/or the impact of different greenhouse gases in terms of the amount of CO₂ that would create the same amount of warming as CO₂. Since gases like methane (CH₄) and nitrous oxide (N₂O) have higher global warming potentials than CO₂,  CO₂e allows their effects to be compared and aggregated using a single metric. 

Carbon dioxide equivalent – the global warming impacts of different greenhouse gases (GHG) differ, such as in the strength of absorption of infra-red radiation, the length of time they remain in the atmosphere, etc. Comparing different GHGs is therefore complicated, but a set of standard figures have been developed that express the global warming potential (GWP) in terms of an equivalent mass of carbon dioxide. These are usually expressed in terms of the GWP over 100 years, but figures for other timescales are also available.

Carbon dioxide molecules consist of one atom of carbon (atomic weight 12) and two of oxygen (atomic weight 16), and therefore have a total molecular weight of 44. Consequently, to convert between weights of carbon and carbon dioxide it is necessary to divide by 12 and multiply by 44.

Carbon sequestration – The net uptake of carbon from the atmosphere and long-term storage as non-volatile carbon. This can be in the wood of living trees, in soil, or in long lived wood products.

(Forest) carbon stock – Fully dry wood is approximately 50% carbon by weight. Soil also contains a significant quantity of carbon, which can increase over time as leaves, needles and dead wood fall to the ground and decay naturally. The total amount of carbon on a site or defined area is called the carbon stock. When a forest is felled and most of the above-ground biomass is removed from the site, the carbon stock on that site decreases considerably, however much of it may be transferred to a range of wood products, with differing lifetimes, and these may be considered as part of the carbon pool.

Continuous Cover Forestry (CCF) – A system of forestry that is an alternative to clearfell forestry, with stands consisting of a mixture of trees of different ages. Felling is therefore performed as small groups or individual trees reach maturity, meaning that there is never a large area of bare ground following a felling.

Dbh – Diameter at breast height. A consistent measure of stem diameter, measured at a height of 1.3 m from ground level, usually expressed in centimetres.

Deadwood – As well as the living trees in a forest or woodland there are other important components, that contain carbon. Deadwood can include dead trees (“standing deadwood”), and broken stems and branches that lie on the forest floor and decay slowly over time.  The carbon contained in this deadwood  returns to the atmosphere as carbon dioxide as the deadwood decomposes, and some is transferred into the soil to contribute to the soil carbon stock. The timescale for this depends on local conditions and the size of the deadwood, but is typically of the order of several years; significantly longer than for litter.

Energy crops – Crops grown specifically for energy use. Typically grown as an agricultural crop, producing high yields of biomass with minimal inputs and usually on short or relatively short rotations. Short Rotation Coppice (SRC), Short Rotation Forestry (SRF) and some perennial grasses (such as Miscanthus) are all examples of energy crops.

Farm woodland – Woodland on farms but planted at densities higher than those typical of silvoarable and some silvopastoral agroforestry systems, which may combine low densities of trees with more conventional cropping or grazing. Farm woodland might include areas of conventional woodland, shelter belts, short rotation forestry (SRF), coppice, or orchards. Several of these options may also allow some grazing, but this is a secondary consideration as opposed to a more deliberate silvo-pastoral system.

Forest stand – A grouping of trees with consistent characteristics that differentiates it from surrounding areas.

GHG – Greenhouse gas. One of a number of different gases which contribute to global warming via the greenhouse effect. These gases have different global warming potentials, and transmit the majority of visible light from the sun, but absorb infra-red radiation, as emitted by the warmth of the Earth, thus tending to trap heat within the atmosphere.

Global warming potential – The differing amount to which different greenhouse gases trap heat within the atmosphere and over what timescale. These are usually compared using carbon dioxide equivalents.

IPCC – Intergovernmental Panel on Climate Change. An international, United Nations panel composed of technical experts representing 195 countries to “provide policymakers with regular scientific assessments on climate change, its implications and potential future risks, as well as to put forward adaptation and mitigation options.”

Litter – As leaves, needles, flowers, fruit and small branches and twigs fall from trees, they build up on the floor of the woodland or forest. Over time they decay and are assimilated into the soil with part of the carbon contained contributing to the soil carbon stock, typically over a timescale of a year or so. They also help to return key nutrients into the soil and contribute to its physical structure.

Natural disturbance – Periodically, significant mortality or loss of trees may occur as a result of pests, disease, storms and wind, drought or wildfire. These are collectively referred to as natural disturbances. In some circumstances the loss of trees and carbon can be very large.

Net zero target – On a national or global scale, emissions of carbon dioxide (CO₂) to the atmosphere from activities such as the burning of fossil fuels, decay of organic materials and the manufacture of cement, are considerably greater than the absorption of CO₂ by forests, crops, other vegetation, oceans and other processes, leading to gradually increasing atmospheric CO₂ concentrations. In order to prevent further increase it is necessary for the emissions and absorption of CO₂ to be in balance. This target results in no net emission of CO₂ and is referred to as “net zero”.

Service life – The service life of a wood product is the length of time for which it remains in use. It may be ended by the product breaking or ceasing to function; by its purpose being no longer required (such as the demolition or modification of a building in which it is a functional element); by rejection owing to changing fashions; or by its replacement by another product. Wood products may have more than one use over their lifetime, and at the end of their service life they may be re-purposed for another use or recycled within another product, both of which retain the carbon, or they may be used as wood fuel or disposed of in landfill, where they may or may not break down into a mixture of carbon dioxide and methane.

Silvo-arable – An agroforestry system that integrates trees with arable crop planting. A typical system consists of rows of trees planted at intervals across fields. These trees can have multiple benefits such as helping act as a wind break, helping protect crops and reduce loss of moisture and soil, providing habitats for natural predators for pests, as well as allowing additional crop(s) from the same land.

Silvo-pastoral – An agroforestry system that integrates trees with grazing livestock. As well as adding a second use on the land available, it has been suggested that many forms of livestock benefit from grazing between trees, giving protection from sun, wind and rain, helping retain warmth, reducing heat stress, protection from predators, and providing more diverse diets

Steady state carbon stock – Forests are dynamic environments. Individual stands of production forests are periodically felled, either all together as a clearfell, or just a few at a time under continuous cover forestry (CCF). Even before final felling, a small proportion may be periodically felled to allow the selection of better trees (thinning). Consequently, the amount of biomass, and hence carbon stock, on the site may either increase or decrease from one year to the next. When considered on a landscape scale, individual stands will be at different ages, and different stages of maturity. While one stand may be felled, others may be thinned, but the trees left will still be actively growing. Over time, the carbon removed from the landscape in felling, thinning and natural processes, may be balanced by that added by tree growth, leading to the carbon stock across the landscape achieving a steady state.

Sustainable forest management – The process by which an indefinite supply of forest products is provided over the long term by ensuring that trees are re-planted when they are felled, and the productivity of the site is not compromised for the future by ensuring that nutrient levels are not depleted, soil quality is maintained, and harvesting does not exceed productivity.

Thinning – In most conventional production forestry, trees are usually initially planted at relatively high density to encourage competition, so they grow straight and tall, to shorten the time before canopy closure when weed competition is suppressed and allow for natural losses. As the trees grow, some of them may be felled periodically, allowing the best examples to be retained and giving them more space, a process known as thinning. Thinning temporarily reduces the carbon stock on the site, however it can allow for the production of more good quality timber in the remaining trees.

Tree biomass – The total of material (biomass) in a tree. This includes the stem, but also the branches and foliage. The woody components are very roughly 50% water by weight, and approximately 50% of the dry weight, is carbon.

UNFCCC – United Nations Framework Convention on Climate Change. The UNFCCC secretariat (UN Climate Change) is the United Nations entity tasked with supporting the global response to the threat of climate change and has 198 members.

Wood-based products – Products manufactured from wood. These include solid wood products, such as beams, joists, furniture and wooden toys, but also composite wood products such as panel boards like MDF (medium density fibreboard), or particleboard, paper and card, or wood fuel. Some of these products may have a long service life, of perhaps several decades or even a century or more, while others may be disposed of or consumed within a year or two. Wood-based products contain carbon, and this is kept from returning to the atmosphere until the product is burned or decays in landfill or elsewhere. Even at the end of their lifetime, wood-based products may often be reused (sometimes for another purpose), recycled or converted to wood fuel.

Wood fuel – Solid fuel consisting of wood in one form another. This includes firewood, wood chips, wood pellets, charcoal and briquettes. Wood pellets – Wood pellets are a densified form of wood fuel, designed to flow easily to allow for trouble free, automatic feed into a modern pellet boiler or stove. They should usually be clean, dry, low ash, low dust and high density, to take up minimal space. Industrial pellets may be larger, and possibly slightly lower specification than domestic ones.

Yield class – an index used in the UK to describe the potential volume growth rate of an even-aged stand of trees which can be used to estimate the standing volume per hectare when the species, stand age, height initial planting density and management are known. As it is a measure of volume productivity, when comparing trees of different species for biomass and carbon content, it is also important to bear in mind the density of the wood.