Tree density is a quantitative measure of tree cover within an area or the amount of tree material per unit area.
In tree ordinances that regulate trees during the process of land development, a minimum amount of tree density is usually required and defined on a per acre basis. This minimum tree density requirement takes one of four common forms–number of trees, tree density units, DBH inches or tree canopy cover.
NUMBER OF TREES
Tree ordinances commonly require tree density based on the number of trees, especially in subdivision and commercial developments. A specific number of trees are often required for:
- Street frontages, with 1 tree required for a specific linear distance, such as “one (1) tree per 30 linear feet”;
- Landscape strips, with 1 tree required for a specific linear distance as with street frontages;
- Buffers, with 1 tree required for a specific linear distance, usually less distance between trees than that required for frontages and landscape strips;
- Parking lots, usually with 1 tree required for a specific number of parking spaces [5 to 10], a maximum distance to a tree [50 or 60 feet], and 1 tree required for each parking lot island; in addition, the requirement may include that the parking lot be covered with at least 50 to 60 percent tree canopy cover; and,
- Residential lots, with 2 trees required in the front yard, or similar requirement; residential lots may also be required to have a specific amount of tree canopy cover, often 10 to 20 percent.
TREE DENSITY UNITS
Both the use of tree density units and DBH inches (DBH is the tree trunk diameter at breast height, 4.5 feet above the ground) have their origins in a stand density measurement commonly used by foresters known as basal area.
Basal area is based on the cross-sectional area in square feet of a tree. Basal area for a site is the total cross-sectional area of all tree trunks on an acre of land, or the average per acre within a wooded stand. Both the number of trees and the size of the trees are factors in the amount of basal area that exists. You can have either many small trees or a few large trees on a tract of land and still have the same basal area, and the same total DBH inches.
Basal area is calculated by using the formula for the area of a circle, which is the radius squared multiplied by pi, or 3.14, a constant. The cross-sectional area of a 10 inch DBH tree trunk would be 5 inches (radius equals half the diameter) squared times 3.14, equal to 78.5 square inches.
Divided by 144 square inches (the number of square inches in a square foot), the result is a basal area of .55 for the tree. If there were twenty 20 10-inch DBH trees on a wooded site 1/3 acre in size, we would multiply .55 times 20, to get a basal area of 11 for the site. We would multiply by 3 to get the average basal area per acre, which is 33.
Tree density units are derived from these basal area calculations, but are often adjusted from community to community to reduce or increase the amount of units for a tree of a specific DBH. This means that from community to community the tree density units assigned to a 10-DBH inch tree could vary considerably. Many ordinances that use tree density units include a table in their ordinance that lists the number of tree density units by DBH for conserved trees and by caliper for planted trees.
If 15 tree density units per acre are required, a 1/3 acre site would need 5 units. If .55 units are assigned to a 10-inch DBH tree, then nine (9) 10-inch DBH trees are required on the site, or some combination of trees of various sizes that would equal the 5 tree density units.
There are some ordinances in Georgia that require minimum tree density based on a measurement of DBH inches. This is similar to using tree density units, but no conversion from DBH to tree density units is made.
If 100 DBH inches per acre are required on a site, the requirements could be met on one site by four (4) 25-inch DBH willow oak trees and on another site by 50 2-inch DBH dogwood and redbud trees. The character of the sites would vary widely, but would both meet requirements. If the 50 2-inch DBH trees were oak or maple trees that would potentially get quite large at maturity, instead of small flowering trees, the site could eventually be significantly overplanted.
We can also look at this variability in terms of the amount of tree canopy that would be produced on the site. If each of the 25-inch DBH willow oak trees have a canopy spread, or tree canopy cover, of 2,200 square feet each, then the 1-acre site would have 8,800 square feet of tree canopy cover. On a 1-acre site of 43,560 square feet, this would result in a tree canopy cover of 20 percent (8,800/43,560).
On the same site, if 50 2-inch DBH dogwood and redbud trees are planted, with a crown that is approximately 4 feet by 4 feet, then their tree canopy would be about 12.5 square feet at the time of planting, using the formula for the area of a circle and a radius of 2 feet. The tree canopy cover on the 1-acre site would be 625 square feet at the time of planting, but eventually, if each tree reaches a tree canopy cover of 400 feet or so at maturity, the cover on the site could be as much as 20,000 square feet, or 46 percent.
If those 50 2-inch DBH trees were oaks, and if each had an average of 1,600 square feet (a standard credit used by Athens-Clarke County) at maturity, then the 1-acre site would eventually have 80,000 square feet of canopy. The crowns of these trees would very likely intermingle, covering the entire 1-acre site for a potential tree canopy cover of 100 percent.
TREE CANOPY COVER
Because there can be significant variability in the tree canopy cover on a site if tree density is based on the number of trees, tree density units, or DBH inches, some communities have chosen to instead adopt requirements for a minimum amount of tree canopy cover that can be satisfied by a combination of conserved and planted trees. See our Tree Ordinance of the Month page, and the Athens-Clarke County Community Tree Management Ordinance as an example of a tree canopy ordinance.
Because the contributions trees make to important issues such as stormwater management and air quality are directly related to the size (and age) of a tree and its crown, regulating the amount of tree canopy cover makes sense. The conservation of large, existing trees is the most effective, since they have spreading canopies. The conservation of forested areas increases the contributions to an even greater degree.
Existing individual trees, groups of trees and forested areas in tree canopy cover ordinances are given the actual amount of cover provided by their crowns, as projected and measured on the ground. For an individual tree, by measuring the average radius of the tree’s crown, actual canopy cover can be estimated, using again the formula for the area of a circle. The 25-inch willow oak tree with 2,200 square feet of tree canopy cover has an average radius of 26.5 feet.
Actual tree canopy cover can also be measured using aerial photography on a geographic information system. You can outline a tree’s crown or a group of trees as a polygon and the system will calculate the area. You can do this for multiple trees or areas on a site to get the total tree canopy cover, then divide by the total area of the site to get percent tree canopy cover.
Newly planted trees in tree canopy cover ordinances are given credit for the minimum canopy expected at maturity. In Athens-Clarke County, these credits are based on the mature size category of the tree as follows:
- Large trees – 1,600 square feet (such as oaks, elms, and hickories)
- Medium trees – 900 square feet (such as red maple, blue beech, and river birch)
- Small trees – 400 square feet (such as dogwoods and redbuds)
- Very small trees – 150 square feet (such as crapemyrtle, fringetree or yaupon holly)
COMBINATIONS OF REQUIREMENTS
Communities may require tree density using more than one of these common methods. For example, a community may require both 10 percent tree cover on a new residential lot and 2 trees along the street frontage. In a commercial development, they may require 15 tree density units per acre, 1 tree for every 7 parking spaces in the parking areas and 1 tree per 30 linear feet along the street frontage.
How many trees should you require?
The amount of tree density that can be required on a site is limited by the amount of growing space available for trees. For trees to survive and thrive, they need an adequate volume of soil for their roots and a pervious surface that allows water and oxygen to infiltrate into the soil and gas exchange between the soil and atmosphere.
On a building site or in a larger development, the construction of roads, buildings, driveways, parking lots and other infrastructure, and the installation of other paved and impervious surfaces reduces the amount of open space and pervious area remaining on the site for trees.
The pervious area remaining for trees is specifically determined by development standards. In the zoning or unified development ordinance, a communities usually regulate the:
- The minimum lot size;
- The minimum lot width;
- The minimum width of front, side, and rear yards;
- The minimum heated floor area;
- The maximum lot coverage (for a building or for all impervious surfaces); and,
- Minimum width of required landscape strips, landscape islands and buffers.
The area within which structures and paving may be constructed is often defined as the “buildable area”. Conversely, the buildable area is sometimes defined as “all of the areas outside of required yards, landscape strips and buffers”.
Alternatively, tree ordinances may call the combined area of these required yards, landscape strips and buffers the “non-buildable area”. It may be best to use the definition found in your community’s zoning or unified development ordinance to avoid confusion.
How much space is available for trees?
The amount of space in the unbuildable portion of the lot will depend on the combination of development standards for the district, which basically divides the lot into areas in which structures and paving may not be built and areas in which they may be built. And, divides the lot into areas in which trees can be conserved and planted (unbuildable area), and areas in which they cannot (buildable area).
The amount and configuration of the remaining unbuildable portion of the lot will determine if trees can be reasonably conserved on the site. If the lot is small and the yards and landscape strips required are narrow, it may be difficult to protect and conserve any trees, except for individual trees, small groups of trees, or trees in required buffers.
The configuration and quality of the unbuildable portion of the lot remaining after tree conservation will determine how many more trees can reasonably be planted on the site. Each planted tree should have a minimum amount of soil area and volume, with more needed for larger maturing trees and less for smaller maturing trees. The amount of open soil surface area required for planted trees by the Athens-Clarke County tree ordinance are:
- Large trees – 400 square feet
- Medium trees – 225 square feet
- Small trees – 100 square feet
- Very small trees – 75 square feet
Calculate the area within required open space, buffers, yards, and landscape strips and islands and divide by the minimum open soil surface areas you require to see how many of each size tree you reasonably can fit into the total unbuildable portion of a lot.
Other factors to consider that may decrease the available growing space for trees include:
- Septic systems and size and location of drain fields;
- Overhead and underground utility line locations;
- Specialized uses of the site requiring open space (vehicle sales lots); and,
- Building heights.
Where should trees be required?
Many tree ordinances require not only a minimum amount of tree density across the site, but also require that trees be placed in specific areas to perform specific functions.
Trees may be required within street frontages to create shady streets, provide canopy over walkways and pavement, reduce ozone formation and intercept particulate matter and stormwater. Trees are often required in and around parking lots for similar reasons–to provide shade, cooling, reductions in ozone formation and interception of stormwater.
Trees may be required in buffers to provide visual and noise buffers, or to protect streams by moderating water temperatures and stream banks from erosion and sedimentation.
Even when the amount of trees conserved on a site meet the requirements, additional trees may be required in these areas.
Which trees should make up the required tree density?
Tree ordinances, regardless of the tree density measurement method used, often also include requirements for minimum amounts of:
- Conserved trees
- Specimen trees
- Planted trees
- Native trees
- Overstory, canopy or large shade trees
Communities requiring tree density almost always have a list of approved tree species, usually categorized by mature size, to support the requirements for certain types of trees, and where they should be planted.
Another requirement often included to increase the diversity of trees planted on a site is that no more than a certain percent of any one species (such as red maple, overcup oak, or slash pine) may be planted to meet tree density requirements, or any one genus (such as maples, oaks or pines).
How do you decide what method is right for your community?
Consider the following factors in deciding which method is right for your community:
- Community forest vision and goals;
- Level of tree density desired in the community, in developments, and on individual lots;
- Level of tree conservation desired in the community;
- Existing zoning regulations, development standards and tree spacing requirements and the resulting available space for trees; and,
- Level of technical expertise of individuals administering the tree ordinance and those monitoring compliance.
Talk to surrounding communities to find out how they define and require tree density. Visit some recently completed development sites with your tree ordinance development team members to see the outcomes of their tree density and other tree ordinance requirements.
Posted November 2017