Source: Laboratories of Margaret Workman and Kimberly Frye - Depaul University
Urban forests broadly include urban parks, street trees, landscaped boulevards, public gardens, river and coastal promenades, greenways, river corridors, wetlands, nature preserves, natural areas, shelterbelts of trees, and working trees at industrial brownfield sites. The history of urban trees begins with trees as landscape embellishment. Today, urban trees are seen as essential components of city infrastructure and critical to human life as food, housing, and other public utilities. Urban trees are now valued for the ecosystem services they provide (e.g., preventing erosion, air pollutant removal, oxygen, shade, etc.). Yet, to efficiently make use of these benefits, trees must reach maturity, as leaf number and size directly affect a tree’s ability to provide ecosystem services. Urban forestry has had to develop its own forestry methods to address the needs and challenges unique to urban trees as compared to their woodland counterparts.
The following excerpt from the USDA Forest Service illustrates the urban tree perspective and policies of federal government:
Urban forests are dynamic ecosystems that provide needed environmental services by cleaning air and water, helping to control storm water, and conserving energy. They add form, structure, beauty and breathing room to urban design, reduce noise, separate incompatible uses, provide places to recreate, strengthen social cohesion, leverage community revitalization, and add economic value to our communities... This natural life support system sustains clean air and water, biodiversity, habitat, nesting and travel corridors for wildlife, and connects people to nature.
The management of urban trees is an interdisciplinary practice involving architecture, landscaping, planning, development, horticulture, etc. One particular discipline involved in forestry is geography, especially through the use of geographical information systems (GIS). GIS is a broad name encompassing any type of database containing geographical or spatial data that can be used to create computer-generated visual representations (e.g., maps). GIS allows for extensive data collection and management through ever-improving user interfaces, increasing the user-friendly quality of very large sets of information that can be accessed by many users. GIS applications range from free software and open access protocols, such as Google Earth, to proprietary systems, like ESRI ArcGIS. Using GIS to create and store geographical information also allows for easy data maintenance, because maps can be quickly updated by adding new information to the database and regenerating the visual output.
An urban forest tree survey is conducted using parkway trees planted between sidewalks and curbs. Data is collected by city block, recording species, health condition, location, land use, and diameter at breast height (dbh) for each tree surveyed.
Tree condition is observational and based on visual assessments of six categories: trunk condition (missing bark and decay), growth rate (twig elongation and length of current year’s growth), structure (dead limbs), insects and disease, crown development (balanced appearance of branches, leaves, and reproductive structures), and life expectancy. Each category has a rating system based on the amount of unhealthy tree features summed together for an overall condition score, which corresponds to a categorical measurement of excellent, very good, good, fair, poor, and very poor.
Location is recorded by postal address and by using geodesic coordinates for longitude and latitude. A GPS receiver is used to determine geodesic locations based on satellite data transmitted to the receiver at each tree’s location.
To quantify the benefits of the urban forest around them, data is entered into a National Tree Benefits Calculator (easily found online and free to use) to determine the dollar value of annual environmental and aesthetic benefits: energy conservation, air quality improvement, CO2 reduction, storm water control, and property value of each tree.
Data is also entered into a Geographical Information System (GIS) for spatial and geospatial statistical analysis of surveyed tree characteristics.
1. Data Collection with GPS Receiver and dbh Tape
- In an open outdoor location, turn on the GPS receiver by pressing the power button. Wait 2-3 min while receiver connects to a minimum of three satellites.
- Walk to the survey area for data collection. Survey one city block segment at a time and number the parkway trees on a data sheet (Figure 1), resetting the numbering at the start of each block segment.
- At each tree in the survey, record species, postal address (e.g., 1253 W. Lill), and geodesic coordinates (longitude/latitude) provided by the GPS receiver (Figure 1). Ensure geodesic coordinates are collected from the same direction at each tree (e.g., north side of each tree).
- Measure the diameter of each tree at 4½ ft above the ground and record the diameter at breast height (dbh).
- Observe each tree condition by visually estimating and scoring according to the criteria (Table 1). Sum the points for one overall score for each tree and assign each tree to the corresponding health categories. Record health condition on the data sheet.
2. Entering Data into a GIS
- GIS using Google Earth: Type in the tree coordinates and save them in the “My Places” folder using the “Add Placemark” feature. Name each tree by species name. Once all tree data points are saved as placemarks, right-click on the My Places name, select “Save As,” and save to any location.
- GIS using ESRI ArcGIS 10.2: To import into ArcGIS, ensure all column headings have no spaces; any spaces should be replaced with underscores. If the coordinates are in latitude/longitude, they should be in decimal degrees (DD) format before importing into ArcGIS. Locations in degrees, minutes and seconds (DMS) or decimal minutes (DM) format should be converted to DD first. There are converters available on the internet (http://www.fcc.gov/encyclopedia/degrees-minutes-seconds-tofrom-decimal-degrees).
- Save/export the data as a comma-delimited text file (CSV format).
- Create a “layer” by adding the .csv file to ArcMap by using the Add data tool, either by expanding the submenu under File> Add Data or by clicking the Add Data tool on the Standard toolbar (Figure 2).
- Right-click on the new layer and choose Display XY Data. Ensure that the X and Y fields were selected correctly by ArcMap – they should be right if the names chosen reflect the coordinate positions (northing and easting or x and y).
- Click the Edit… button, then Select… to select the coordinate system for the points, Add… and OK (3x). The correct coordinate system to use may be obtained from the GPS unit (under map setup or units). For this data, select Coordinate Systems > Geographic Coordinate Systems > World > WGS1984.prj (default GPS datum).
- There should now be a point layer at the top of the Table of Contents with the same name as the .csv file and the word Events on the end of the name (Figure 3). This is an “event theme” and is a temporary layer. For a more permanent copy, right-click on the layer and choose Data > Export Data… Pick an output location — a geodatabase feature class or a directory for a shapefile — and enter a file name. Change the name from the default “Export_Output” to Urban_Forestry_Survey. Click OK.
3. National Tree Benefits Calculator
- Using this software, the benefits of street-side trees can be calculated. This includes a tree’s annual benefits for storm water management, property value, energy efficiency, and carbon sequestration. See the video Tree Identification: How to Use a Dichotomous Key for instructions on using the Tree Benefits Calculator.
Figure 1. Representative results for street trees found one block.
Figure 2. The Add Data tool on the Standard toolbar.
Figure 3. Point layer at the top of the Table of Contents with the same name as the CSV file.
Sound and solid
Sections of bark missing
Extensive decay & hollow
|Growth Rate (consider species)
More than 6" twig elongation
2-6" twig elongation
Less than 2" twig elongation
One major/several minor limbs dead, broken, missing
2 or more major limbs dead, broken, missing
|Insect & Disease
No pests present
1 pest present
2 or more pests present
Full & Balanced
Full but unbalanced
Unbalanced and lacking a full crown
Over 30 years
Less than 5
Very Poor: 9-6
Table 1. A table to calculate the condition class of a tree. Each condition score correlates with its description in each category, then all six scores are totaled for a final sum – the condition class.
Urban forests are valuable resources, and require care and management to ensure their health. The distribution of trees, their condition, and the scale and shape of urban forests can be mapped using GIS, or Geographical Information System, software.
Trees in urban areas may range from simple street trees to landscaped boulevards, greenways, public gardens, or those at industrial brownfield sites. Combined, these urban forests are essential components of city infrastructure. Urban forestry uses novel methods to address the needs and challenges unique to urban trees.
Urban forests fill valuable environmental niches, acting to clean air and water, control storm waters, prevent erosion, reduce noise, and conserve energy. Additionally, urban forests can provide important habitats for animals, provide shelter for nesting, or act as travel corridors for wildlife. Their value can also be social, connecting urban inhabitants to nature, enhancing architecture, and providing educational opportunities to learn about nature.
The structure, diversity, and resultant ecological value of urban forests can be quantified using GIS, or Geographical Information Systems, software. GIS allows for extensive data collection and management, which is an ideal framework for compiling urban forestry data. GIS allows users to combine geographical and tree-survey data to produce accurate maps of urban forests.
This video will illustrate the process of collecting tree survey data, incorporating this data into a GIS platform, and evaluating the environmental value of trees of interest.
Urban forest surveys may be conducted at a variety of locations. Commonly, parkway trees planted between sidewalks and curbs are recorded. Data is typically collected by city block, and records tree location, species, health, land use, and diameter at breast height, or DBH.
Tree condition is a visual assessment based on six categories. These are trunk condition, which takes into account factors including missing bark or decay; growth rate, examining twig elongation and length of current year's growth; and structure, taking note of dead or broken limbs. Insects and disease are noted; crown development, which assesses trees for a balanced appearance of branches, leaves, and reproductive structures; and finally life expectancy, based on the life expectancy for the species, minus the damage present.
Each category carries a rating system, based on the amount of unhealthy tree features summed together, which combined give an overall condition score for a tree. This can be attributed to a categorical tree health measurement ranging from excellent down through very good, good, fair, poor, to very poor.
A GPS receiver, which uses satellite positioning transmitted to the handset allows the user to record geodesic coordinates of latitude and longitude. Location is also noted by postal address. Once collected, data can be entered into a GIS software program, which allows spatial and geospatial statistical analysis of trees and tree characteristics. There are several such programs available, including ArcGIS or Google Earth.
The National Tree Benefits Calculator is a free online software tool that is used to provide an economic estimation of the monetary benefits provided by urban and suburban trees. Here, users can select their survey region, and choose from a list of trees in that area. Inputting a few simple details including tree trunk diameter and land-use type, will give an estimation of the value of the tree in several key areas, including property value, air quality, and storm water control.
Now that we are familiar with the concept of urban forestry surveys and the principles behind them, let's take a look at how these are carried out in the field.
To begin the survey, select a suitable outdoor location and turn on the GPS receiver. Wait for it to connect to satellites and detect the location. Proceed to the start of the survey area selected for data collection. Survey one city block segment at a time. At each tree encountered, record the species using this collection’s video on Tree Identification. Also record the longitude and latitude provided by the GPS receiver, and postal address.
Next, measure the diameter of each tree at 4.5 ft above the ground, which is taken as the diameter at breast height, or DBH. Assign each tree a number, resetting the numbering at the start of each block segment.
Observe the condition of each tree by visually estimating and scoring according to the criteria presented in the table shown. Sum the points to obtain an overall score for each tree, and assign each a corresponding health category.
Two programs that can be used to compile GIS data are Google Earth or ArcGIS. Using these programs, trees can be place-marked individually or their coordinates complied into a single file and uploaded.
To use the Google Earth GIS program, type in the tree coordinates and select new placemark, using the "Add Placemark" feature. Name each tree by species name, and once all tree data points are saved as placemarks, right click on the "My Places" label, select "Save As", then save to any location.
To use the ArcGIS program, when importing data endure all column headings are free of spaces. Underscores can be used instead. If the coordinates are in latitude and longitude format, they should be converted to decimal degrees format before import into ArcGIS.
Save the data as a comma-delimited CSV file. Next, create a layer by adding the CSV file to ArcMap using the Add Data tool, either under File: Add Data, or by clicking the "Add Data" tool on the standard toolbar.
Right-click on the new layer and choose "Display XY Data". Ensure that the X, or longitude, and Y, or latitude, fields were selected correctly by ArcMap.
Next, click "Edit", then "Select" to highlight the coordinate system for the points, then "Add", and "OK". The correct coordinate system to use can be obtained from the GPS unit itself.
There should now be a point layer at the top of the Table of Contents with the same name as the CSV file, and the word "Events" at the end of the name. This is an "event theme", and is a temporary layer. To make a permanent copy, right-click on the layer and choose "Data" then "Export Data". Pick an output location and enter a file name. Change the name from the default "Export Output" to "Urban_Forestry_Survey" and click "OK".
Data entered into the ArcGIS program will produce a map of urban forestry data. These maps can be used in a number of ways, including identifying patterns of DBH sizes, or determining if trees in a particular area are not reaching life expectancy.
Open up the iTree for Education software tool. For each tree, input the relevant data, and record the environmental benefits for each tree. Estimated benefits of urban trees can be calculated and assigned monetary values. This includes a tree's annual benefits for storm water management, property value, energy efficiency, and carbon sequestration.
The results from analysis with the National Tree Benefit Calculator can give a summary of the economic and environmental value of the trees surveyed. This can allow urban planners to decide upon any tree removals, or supplementary plantings that may benefit the area surveyed.
Urban Forestry maps can be used in a variety of applications, and some of these are explored here.
In urban residential or suburban areas, urban forestry will often have different requirements from trees than those in commercial or retail areas. Trees may be protected or selected for planting based on similar ecological properties such as wind or noise barriers, or water management concerns. However, aesthetic value may play a greater part in residential forestry.
Geographical Information Systems can also be used to map many other phenomena, natural or manmade. GIS maps of natural measurements of lead levels in soil can also be compiled and used to determine contamination levels, or safe versus unsafe regions for planting of food crops.
You've just watched JoVE's introduction to Urban Forestry using GIS. You should now understand the importance of urban forestry, how to use GIS to create maps for the study of urban trees, and how to use the National Tree Benefit Calculator to ascertain the value of surveyed trees. Thanks for watching!
Figure 1 shows the representative results for street trees found on one block, and a map from urban forestry data entered into GIS can be seen in Figure 4.
The results for using the Tree Benefit Calculator can be found in Table 2. This calculator provides an estimation of the benefits individual street-side trees provide. Once the data from the field investigation is inputted, including zip code, species, diameter, and land-use, the environmental and economic benefit provided by each tree can be seen.
Figure 4. Maps from urban forestry data entered into GIS.
|Tree Sample Number||Overall Benefit||Storm Water Management
|Property Value||Energy Efficiency
Table 2. The Tree Benefit Calculator results.
Applications and Summary
Once entered into a GIS, forestry data can be analyzed using geospatial statistics. For example, a Moran’s I geospatial statistical test is a widely used statistic that analyzes for significant geographical clustering of health variables. Moran’s I can be used for forestry data to report dbh values localized to particular areas, indicating different tree growth rates in different locations of the forest. If clustering is significant, a General G geospatial statistical test can additionally reveal whether it is the high or low values that are geographically clustered by reporting which end of the clustering values are concentrated in a geographical area (Figure 5). Significant Moran's I clusters are shown with the General G scores, indicating high dbh values clustered for the good trees and for each species. dbh values are represented by proportionally-sized symbols to illustrate the clustering of high values (large circles) and low values (small diamonds) (Figure 6). Features can be paired in maps to look for meaningful patterns, such as dbh and species, to identify which species tend to grow to maturity more successfully in an urban environment. Clusters of high dbh values indicate older trees that may present near future needs for tree removal or a higher risk area for tree damage caused by storms. High dbh clusters may also indicate areas where trees survive longer and regions of a city receiving higher ecosystem service benefits.
Figure 5. Clusters of dbh sizes for good trees on a map.
Figure 6. High dbh clusters identified on a map.