Acre
An area of land measuring about 43,560 square feet. One acre is equal to 0.4046873 hectares.

Biodegradable
Capable of being broken down, especially into harmless organic matter, by the action of living things.

Biological diversity (Biodiversity)
The variety of living organisms from all sources, including land and water, and the ecosystems of which they are part. This includes diversity within species, between species and of ecosystems.

Cambium
The growing part of a tree trunk.

Canopy
A layer or multiple layers of branches and foliage at the top of a forest.

Chlorophyll
The chemical that makes a tree's leaves green.

Conifer
A tree, usually an evergreen, with cones and needle-shaped or scale-like leaves; also known as softwood.

Conservation
The preservation and careful management of the environment and natural resources.

Crown
The upper part of a tree that includes the branches and foliage. 

Deciduous
Refers to trees that lose their leaves some time during the year, usually autumn, and grow them back the following season. 

Ecosystem
A system formed by the interaction of a community of plants and animals within their natural environment.

Endangered species (threatened species)
A species is considered endangered when the number of remaining members may not be sufficient to reproduce enough offspring to ensure its continued survival. A threatened species exhibits declining or dangerously low populations but still has enough members to maintain or increase its numbers. 

Fire danger
Measure of the likelihood of a forest fire, based on temperature, relative humidity, wind force, wind direction and the dryness of a forest.

Firebreak
Any nonflammable barrier used to slow or stop fires such as mineral soil, green or slow-burning vegetation, or mechanically cleared areas.

Forest
An ecosystem characterized by trees, with a unique combination of plants, animals, microbes, soil and climate.

Forest ownership
While the majority of US forests are privately owned, 94 percent of Canada's forests are publicly owned, mainly by provincial governments, which manage 71 percent, and by the federal government, which manages 23 percent.

Forest product certification
Certified forest products come from forests that have been carefully evaluated by an independent, third-party to ensure they are sustainably managed. Read more...

Forest age
The amount of time since the last major disturbance that cleared all or most of the trees in a forested area. Wildfires, volcanic eruptions and timber harvests are all examples of disturbance.

Forest zones
Tropical, boreal and temperate forests are the three main forest zones in the world. Tropical forests have an annual rainfall of at least 98 inches (250 cm) and evergreen trees that form a continuous canopy. Boreal forests are located in the northern regions of Canada, Russia and Scandinavia, and are made up mostly of coniferous or evergreen, cone-bearing trees. Temperate forests are composed mainly of deciduous trees.

Forestland
Land that is covered with trees. This ranges from closed-canopy forests where the trees form a continuous cover, to savanna forests where the trees are scattered. The transition from savanna forests to grassland is usually gradual, with no distinct boundary between the two.

Global warming
The gradual rise of the earth's surface temperature thought to be caused by the “greenhouse effect” and responsible for changes in global climate patterns. Global warming most often refers to the warming predicted to occur as a result of increased emissions of greenhouse gases through the burning of fossil fuels such as gas, oil and coal.

Greenhouse effect
The natural process by which the earth's atmosphere holds both radiation from the sun and infrared radiation from the earth's surface. The greenhouse effect sustains life on earth and has kept the average temperature about 59°F warmer than it would otherwise be.

Greenhouse gases
The gases in the earth's atmosphere that absorb infrared radiation. Greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, halogenated fluorocarbons, ozone, perfluorinated carbons (PFCs), and hydrofluorocarbons (HFCs).

Habitat
An area in which a specific plant or animal can naturally live, grow and reproduce. For wildlife, habitat is the combination of food, water and land.

Heartwood
The dead wood in the center of a tree that gives it strength. 

Intermediate trees
Trees or shrubs receiving little direct light from above. They are characterized by small crowns, considerably crowded on the sides, which are generally either below or extending into the canopy formed by co-dominant trees or shrubs.

Limb wood
Part of the tree above the stump that is not qualified as an upper stem portion.

Log
A section of a tree trunk at least 8 feet long.

Mixed stand
A timber stand in which less than 80 percent of the trees in the main canopy are of a single species. 

Needles or leaves
The parts of the tree that make sugar from air and water through a process called photosynthesis.

Old growth trees
As a popular term, old growth describes forests containing trees that are big and old, usually older than 200 years. Scientifically, it means forests that have reached the age of maturity. For alder or birch, this would be 50 years, while for redwood or Douglas-fir it would be 500.

Open forests
Forests where the trees are relatively sparse and small.

Outer bark
The part of a tree that protects it from fire and insects, and insulates it from extreme cold or heat.

Phloem
Cells that form a pipeline to carry sugars and nutrients from the various layers of the tree.

Photosynthesis
The process by which plants containing chlorophyll (including trees) combine light energy with carbon dioxide and water molecules to make sugar and oxygen.

Reforestation
Reforestation is the process of renewing a forest after harvest. Deforestation is the opposite of reforestation.

Roadless forests
Areas of national forest land in which companies are prohibited from building roads.

Roots
The part of the tree that anchors it to the earth and also absorbs water and nutrients.

Sapling
A small tree with a trunk usually between two and four inches in diameter.

Sapwood
The pipeline of a tree that carries water and nutrients from the roots up to the leaves.

Seedling
A very young plant or a nursery plant not yet transplanted in the ground.

Stand
A group of trees, similar in composition, age or condition, distinguished from the overall forest or other adjoining areas.

Stomates
Tiny openings or pores in leaves and young stems, which allow the exchange of gases between the outside air and the system of air canals within the leaf.

Sustainable Forest Management (SFM)
The planning and activities that maintain and enhance the long-term health of forest ecosystems, while providing environmental, economic and social opportunities for the benefit of current and future generations.

Timberlands
Forestland used for the production of wood products and for the repeated growing and harvesting of trees.

Tree
A woody plant usually having a single, elongated stem and with few or no branches on its lower part.

Trunk
The base of a tree where the roots enter the ground.

Understory
The trees or foliage beneath the forest canopy.

Vegetation density
The number of individual plants of a given species in an area.

Wetland
Areas, such as tidal flats or swamps, containing high soil moisture.

Wilderness
Land set aside by the government for forest conservation or recreational use.

Wildfire
Unplanned fire requiring action to suppress or extinguish.

Wood pulp
Fiber from wood that is used for the production of paper or paperboard.