An inventory of trees and planting spaces is a prerequisite in planning for and making sound management decisions including budget strategies and priorities. An inventory can provide the locations of trees that require pruning or removal to reduce risk, the number of trees located within the public right-of-way, the value of ecosystem services trees provide, and the number of available planting sites. In addition, an inventory can help to identify insect or disease problems or young trees that require irrigation, pruning, and other maintenance.
With this information, tree commissions can annually plan, prioritize, and budget tree removals, maintenance work, and plantings. They can also determine the value of their community's trees for increasing property values and stormwater management which helps emphasize a tree program's importance to community leaders. Over the years, changes in a community forest can be seen in the number, age, and species of trees. A well maintained inventory can be used in cases of liability to demonstrate there was no negligence in the inspection or care of public trees. An inventory will also improve the chances of receiving grants and other assistance by documenting the condition of and care for the community forest.
The process to conduct a tree inventory can be divided into four phases: planning, implementation, application, and maintenance.
- Identify the types of information needed and how that information will be used.
- Assess the availability of computers and software and people to maintain the inventory.
- Determine the method of data collection (drive or walk).
- Assess the requirements for labor, equipment, and funding.
- Training people for gathering data.
- Collect data and check accuracy of data collectors.
- Input and maintain data in a computer or GIS system.
- Analyze data and use information.
- Establish objectives for tree removal and planting that increase species and age diversity.
- Prepare annual work plans and budgets for removal, planting, and pruning.
- Re-inventory the trees to maintain the information at a current level; or
- Continuously update the information when permits are issued and tree work is completed.
The only way to acquire needed data is by inspecting individual trees and recording the information on handheld computers or other electronic devices. Recent inventories of Harrisburg, Lancaster, and other cities have used Trimble Nomad handheld computers running ESRI ArcPad. Those conducting the inventory must decide how much of the community will be inventoried, which areas will be completed first, who will collect the data (volunteers, interns, staff, or consultants), and what information is needed. Once these decisions are made, municipalities must determine how to use, maintain, and analyze the information. Tree inventory data can be placed in Excel, Access, and other software. The most commonly collected and used data fields in a tree inventory are:
- The location of the tree (by GPS and street name and building number);
- The name of the tree species or abbreviation;
- The diameter of the tree's trunk;
- The condition of the tree (good, fair, poor, dead/dying);
- Any trees that require urgent pruning or removal;
- Any tree that requires maintenance in a timely fashion;
- The location and quality of potential planting sites for new trees;
- Potential constraints on planting spaces such as utility wires and narrow tree lawns; and
- The location and extent of tree damage to sidewalks and curbs.
Another important decision is whether to walk or ride when collecting data. Driving in a car with one person driving and one person recording data is faster and more comfortable, but a thorough and accurate inspection requires walking. A driving or "windshield" inventory does not allow the recorders to notice all the indicators of tree health or structure. The windshield method is better used for a general overview of trees and quick estimates of tree numbers, planting vacancies, or special problems like topped trees and Emerald Ash Borer. Regardless of whether they walk or ride, data gatherers must have good knowledge and training to identify tree species, recognize planting sites, and evaluate pruning needs, tree risk, and other tree conditions. Research has shown that with good training and supervision, community volunteers can collect information on tree species, condition, and maintenance needs. Volunteers cannot be relied on to evaluate and identify decay, poor branching habit, and other indicators of tree risk, so professional arborists should be consulted when assessing hazardous trees and tree risk.
Summarizing and Using Data
A personal computer offers an efficient means of managing and manipulating tree inventory data. Computerized systems can be easily maintained and changes made as soon as tree removals, maintenance work, and plantings occur. More importantly, the computer allows the user to quickly organize and tabulate the data into a variety of formats or reports.
In order to monitor the progress of a community forest, it is necessary to perform periodic inventory updates. In municipalities with an active tree program, maintaining a street tree inventory should become an annual activity. A rotational schedule should be developed so that a portion of the community is re-inventoried every two to three years. This will result in a total inventory update every five to ten years. Community workers can also take the opportunity to update the tree inventory as they complete annual tree assessments and work in different areas.
Finding Help to Complete a Tree Inventory
Tree inventory assistance is available from organizations such as the Penn State Extension, the Pennsylvania Bureau of Forestry, and resource consultants. There are many local consulting arborists and foresters who can help communities complete inventories. A completed street and park tree inventory can lead to a clearer picture of the condition of a community's trees and forest. After the conditions are known, citizens can encourage needed actions and prioritize use of time, equipment, and funds. Enlisting the help of leaders and citizens in a tree inventory is a good way to energize a community tree program. The information that is collected can be used to start a municipal tree commission, justify budget requests, and develop a tree plan that will guide future management decisions and activities.