Tree Survey: Point-Centered Quarter Sampling Method

In the point-centered quarter method, a point in the forest is identified and the area around it is separated into four quarters. In each quarter, the nearest tree with a diameter-at-breast-height (dbh) of ≥ 40 cm is identified. This is considered the “large tree” sample. In each quarter, the nearest tree with a dbh > 2.5 cm and < 40 cm is identified. This is considered the “small tree” sample. The dbh is the diameter (in cm) of a tree measured at 4½ feet above the existing grade. Identifying a large tree and small tree in each quadrant provides the ability to compare the overstory (the trees in a forest whose crowns constitute the highest layer of vegetation in a forest, typically forming the canopy) to the understory (vegetation growing beneath the forest canopy without penetrating it to any extent).

Using these measurements, the Basal Area and Importance Value of each tree species is calculated. The basal area is the cross-sectional area (in m²) of a single tree at breast height (4½ ft above the ground). The basal area of all trees of a species can be calculated to understand the species density in a site. This is used, instead of the number of trees per area, to take into account the size of the trees. The Importance Value of each species is calculated to understand the relative dominance of that species in a forest community. It is based on how commonly a species occurs across the forest, the total number of individuals of the species, and the total amount of forest area occupied by the species.

The point-centered quarter tree survey method produces three quantitative measures: the relative density, the relative frequency, and the relative basal area. These three values are added together to give the Importance Value of that species. This is then converted to a relative importance value (Table 5).

The importance value of a species can reach a maximum of 300 in a survey that only finds one species present. A high importance value does not necessarily mean that the species is important to the health of the forest; it merely means that the species currently dominates the forest structure (Figure 2).

Trees are an important natural resource that help a city’s environment, health, and overall quality of life. Therefore, having a good understanding of the composition of the forest is essential to maintaining this resource. For example, if the forest is very diverse, it can help minimize the impact from a species-specific insect or disease. If the understory shows a high frequency of invasive trees, it may indicate that they are beginning to outcompete and displace the native trees.

Figure 2. A bar graph of the Importance Value of trees in Sommes Woods.

Table 5. A table detailing representative results gathered from the point-centered tree survey method.

Tree surveys are an important technique for both private and public stakeholders. They can provide helpful information to allow land managers to make informed decisions. A community may want to do a tree inventory to determine if there is a need for a forestry program. For example, the survey may reveal many dead or diseased trees (Figure 3) and indicate the need for more plantings. The survey may also help the community set up a maintenance schedule to prevent damage from hazardous trees. Lastly, the survey can help communities with land management decisions. Knowing the species diversity in a forest can allow the managers develop a plan for planting (Figure 4). For example, they can set guidelines such as, “Do not plant trees from a species that comprise more than x% of the forest.”

Tree surveys help quantify a forest’s value as a natural resource. Knowing the forest structure allows forest managers to calculate the worth of the services that the trees provide, such as air pollution control, carbon capture and storage, and energy use reductions.

Figure 3. A photo of a forest with potentially diseased trees. A tree survey could help detect the presence of dying trees, so managers could plant new trees to maintain forest levels.

Figure 4. A photo of a healthy, diverse forest. A tree survey could help managers develop a plan for planting proper trees to maintain levels particular species numbers (so one tree type doesn’t take over a forest, for example).