Basal area The sum cross-sectional area of tree boles at breast height divided by the plot area.

Canopy The above-ground biomass in a collection of several trees making up a stand. See crown. In terms of fuel, the canopy is considered to consist of the foliage plus one half of the 1-hour fuels.

Canopy base and ceiling height, CaBH and CaCH Canopy base height (CaBH) and canopy ceiling height (CaCH) are quantified in terms of CaBDs. CaBH is the first segment above the ground where CaBDs is greater than a critical value, currently thought to be ~11 g/m3. Similarly CaCH is the point at the top of the canopy where CaBDs drops below the critical value. Defining CaBH in terms of CaBDs puts the gap between surface and canopy fuel layers in the context of fire behavior rather than a somewhat arbitrary and observational measure of the height of the lowest foliage.

Canopy bulk density, CaBD The mass of available fuel in the canopy per unit of volume, generally expressed as kg/m3 or g/m3.

Canopy bulk density, Segmental, CaBDs The maximum of a series of "stacked" canopy bulk density segments or layers. Bulk density is calculated for each of a series of layers or segments generally 1 foot or 1 meter thick from the bottom of the canopy to the top. A running mean is sometimes used. The segment with the maximum value is reported as the segmental canopy bulk density. This method of calculating canopy bulk density is considered superior to volumetric canopy bulk density because tree mortality from hazard fuel reduction treatment, wind-throw, insect, or disease will never result in an increase in canopy bulk density. See volumetric canopy bulk density.
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Canopy bulk density, Volumetric, CaBDv Canopy bulk density calculated as canopy fuel load divided by some measure of canopy length. Simplistically the canopy is treated as a box. Although easy to calculate, there are several problems with volumetric canopy bulk density, chief of which is that, since it is a ratio of canopy fuel load to canopy length, fuel reduction that reduces the canopy fuel load and the canopy base height may leave volumetric canopy bulk density unchanged or actually leave it higher. See segmental canopy bulk density.

Canopy fuel load, CaFL The mass of available fuels in the canopy per unit of ground area.

Canopy length, CaL Canopy length (CaL) is canopy ceiling height minus canopy base height.

Crown The foliated portion of a single tree. The crown includes the foliage and/or some portion of the 1-hour twigs. See canopy.

Crown base height The height above the ground of the continuous foliage on a single tree, an observational field measure. See canopy base height.

Crown ratio The proportion of a tree that is clothed in continuous foliage.

Fuel bed depth, Herbaceous The average depth of the fuel bed consisting of forbs and grasses.

Fuel bed depth, Woody The average depth of the fuel bed consisting of dead woody material.

The following definitions follow Andrews and Bevins (2003).

1-hour Fuel The 1-hour timelag dead fuel category includes fuels from 0 to 0.25 inches (0.64 cm) in diameter. This includes needles, leaves, cured herbaceous plants and fine dead stems of plants.

10-hour The 10-hour timelag dead fuel category includes fuels from 0.25 to 1.0 inch (0.64 to 2.54 cm) in diameter.

100-hour The 100-hour timelag fuel category includes fuels from 1 to 3 inches (2.54 to 7.62 cm) in diameter.

Live herbaceous Live herbaceous fuels are grasses and forbs that are living. Herbaceous fuels can be either annual or perennial.

Live woody Live woody fuels are shrubs that are living.