New York City Urban Field Station

Urban Tree Monitoring and Research
with Healthy Trees, Healthy Cities

Karine Aigner/Courtesy TNCSince 2015, the USDA Forest Service and The Nature Conservancy have partnered on a national tree health monitoring initiative called Healthy Trees, Healthy Cities (HTHC). This initiative responds to the growing need to monitor the trees in our communities as a result of the increased threats to trees posed by insects, diseases, weather events and, especially, neglect. As community trees become more vulnerable to these threats, we are ever more dependent on the many benefits trees contribute to individual and community health like air quality improvements, shade and cooling, and reductions in greenhouse gas emissions.

In short, we need our trees to be healthy so we can count on them to keep us healthy in return. One of the best ways to do that is to keep an eye on them and care for them when needed.

HTHC offers virtually anyone with an interest in trees the opportunity to learn valuable information about the health and vulnerability of the trees they care about, and simultaneously contribute to national research and monitoring efforts. To accomplish these local and national goals, HTHC provides a free mobile device application (an “app”) for smartphones and tablets that serves as a data collection tool, an educational resource, and source of encouragement for tree stewardship.

Screenshot from Healthy Trees Healthy Cities mobile appThe app is intended for use nationwide and includes species found in all forest types throughout North America. It features four modules:

  • Add a Tree Inventory newly-planted trees, or existing trees by collecting basic information such as tree species, location, Diameter at Breast Height (DBH), notes and a photo (optional), as well as additional information for newly-planted trees. The data collected via this module can be “cross-walked” with iTree, a tree benefits software suite offered by the Forest Service. This module is completed in advance of each of the other three modules.
  • Tree Care Keep track of stewardship activities with this module which features a simple pull-down menu with tree care activities most frequently completed for both young and mature trees, with a notes section for additional activities. Users can also document the duration of time spent (in minutes) on these activities, an especially helpful feature for grant reporting requirements.
  • Health Check Monitor the overall health of your trees over time with a specially-designed non-stressor specific tree health protocol that includes six variables related to the tree’s crown (branches and leaves). The flagship assessment of this app, the health check does not require any specialized equipment - simply the power of visual observation!
  • Pest Detection Identify and document approximately one dozen signs/symptoms of harmful tree insects and diseases. This protocol includes two types of questions - pull-down menus and yes/no questions.
  • All modules include information pages accessible directly through the app via “i” buttons associated with each question or variable. These information pages offer helpful hints for completing assessments.

    Trees require a team of caregivers - municipal managers, volunteers, and especially licensed arborists. To build connections between app users and licensed professional arborists, the app features a connection to the International Society of Arboriculture’s “Find an Arborist” tool.

    To accompany the app, the HTHC web-based project management dashboard provides app users with a simple interface for analyzing and managing data. Users can also create and manage organized data collection projects through the dashboard.

    Read on for more information about these modules, and to learn about the science behind their inclusion in the app.

    Access the free app and dashboard by visiting http://www.healthytreeshealthycitiesapp.org/.

    Working with Tree Inventories

    Screenshot from Healthy Trees Healthy Cities appsThe Healthy Trees, Healthy Cities app can easily incorporate and work with existing tree inventories, and data can be cross-walked across multiple platforms. When an inventory is loaded, the app can auto-populate a user’s Nearby Trees list allowing a user to select a specific tree from the inventory. This ensures that any new information is linked to the correct tree. The app also allows users to add new trees (not already in the app) that were either newly planted (“Planted”) or previously planted (“Existing”). Once added, the tree and its data become visible to all users and data managers.

    "Tree Care" - Stewardship of Urban Trees

    Stewardship of trees planted in urban areas is a critical component of longevity (Lu et al., 2010). The Tree Care module of the app allows users to record stewardship activities, with a drop-down menu for the most common options and an open-ended notes section for unlisted activities and for project-specific details. This module can be a resource for local volunteer groups to help track and plan stewardship activities in their areas. The records of tree care can also be linked to tree health data and can be used to analyze the relationship between tree care and individual and community-wide tree health. The tools available for tree stewardship and the data generated in the app complement the research being done on environmental governance and civic engagement and will assist in research. Future plans include linking data from STEW-MAP to make it easy for local stewardship groups to find and manage the trees they care for.

    Health Assessment of Urban Trees

    Three people look at tree crown for signs of damage.The health check follows the methods laid out by Pontius & Hallett, 2014, using standardized ocular measurements that match the natural range of values for each variable to species specific distributions and normalize all variables so they can be combined into one continuous decline summary variable. The standardization process translates each value into a probability distribution score based on the mean and variability of values on a species level. In this way, several stress symptoms can be equally weighted, standardized to the same scale and combined into one continuous summary value (Pontius & Hallett, 2014). This continuous summary value is also used to generate a Stress Index value on a 0-10 scale, designed to be more easily translated by everyday users.

    To learn these tree health methods virtually, check out the following training videos:

    Early Detection of Tree Pests and Diseases

    Studying EAB damage to an urban tree.During the last century, urban and rural trees have been heavily impacted by introduced pests and pathogens including the chestnut blight (1904), Dutch elm disease (1928), hemlock woolly adelgid (1951), Asian long-horned beetle (1996), and now emerald ash borer (2002). All these pests are non-native and were introduced to North American cities inadvertently. Emerald ash borer was first detected near Detroit, Michigan, in 2002, but it probably arrived in the area in the early 1990s (Siegert et al. 2007). Earlier detection of these pests is the first critical step in developing an eradication strategy. Once populations are established and spreading, earlier detection remains important as we strive to delineate the impacted areas and develop mitigation strategies.

    To create an easy-to-use method for checking trees for pests, tree experts considered the common signs and symptoms of several of the most destructive pests currently impacting forests across the United States. With this information, a checklist of the signs and symptoms that would be readily visible to tree surveyors was created. This checklist uses multiple choice and yes/no questions, giving surveyors easy-to-assess questions that can be readily analyzed. Pest checks are automatically linked to species and location data, making searching for patterns, or signs of species-specific pests like emerald ash borer or sudden oak death simple.

    “Project Management Dashboard” - HealthyTreesHealthyCitiesApp.org

    Screenshot of Healthy Trees Healthy Cities dashboard shows location of trees in projectThe online dashboard for the app is how researchers, tree stewardship groups, and anyone who uses the app can manage data and projects. Projects can be as specific as one block of a city or include trees on a metro-wide scale. Any activity recorded in the app is paired to a tree, allowing users to track activities done to a tree over time. Researchers can track levels of stress in a tree from year to year, and stewardship groups can schedule key tree care – like watering during a drought – and ensure it happens regularly.  Additionally, the dashboard offers a Quality Assurance/ Quality Control (QA/QC) feature designed for project managers to select a representative subset of monitored trees to facilitate a field-based re-evaluation of these trees. These QA/QC data can then be compared to the original entry in the dashboard. This feature provides project managers the ability to ensure community scientists have the guidance they need to collect valid, accurate data.

    Research Using Healthy Trees, Healthy Cities Methods

    ​Hallett, Richard; Hallett, Tanner. 2018. Citizen science and tree health assessment: How useful are the data? Arboriculture & Urban Forestry. 44(6): 236–247.

    Hallett, Richard; Johnson, Michelle L.; Sonti, Nancy F. 2018. Assessing the tree health impacts of salt water flooding in coastal cities: A case study in New York City. Landscape and Urban Planning. 177: 171-177.


    Pontius, Jennifer; Hallett, Richard. 2014. Comprehensive methods for earlier detection and monitoring of forest decline. Forest Science. 60(6): 1156-1163.

    Lu, Jacqueline W.T.; Svendsen, Erika S.; Campbell, Lindsay K.; Greenfeld, Jennifer; Braden, Jessie; King, Kristen; Falxa-Raymond, Nancy. 2010. Biological, social, and urban design factors affecting young street tree mortality in New York City. Cities and the Environment. 3(1): 1-15.

    Siegert, Nathan W.; McCullough, Deborah G.; Liebhold, Andrew M. 2008. Dendrochronological reconstruction of the establishment and spread of emerald ash borer. In: Mastro, Victor; Lance, David; Reardon, Richard; Parra, Gregory, comps. Emerald ash borer research and development meeting; 2007 October 23-24; Pittsburgh, PA. FHTET 2008-07. Morgantown, WV: U.S. Department of Agriculture, Forest Service, Forest Health Technology Enterprise Team: 4-5.

    Hurricane Sandy Flooding Impacts

    Flooding from Hurricane Sandy has inspired a close look at the response of NYC’s trees to the disturbance. One of the costliest hurricanes in U.S. history, Sandy came ashore in New York City on October 29, 2012. Almost 20,000 public trees in NYC were destroyed as a result of the storm. The full impact of the storm on the city’s remaining trees, however, has yet to be determined. According to NYC Parks Department estimates, Sandy flooded 47,900 street trees in the city throughout the inundation areas. During June 2013 (eight months after the hurricane), NYC Parks employees surveyed the inundation zones and found almost 10,000 trees that had not fully leafed out and estimated whether the tree had 0, 25, 50, or 75% of its leaves. The four most impacted tree types were London Planetree, Maple species, Cherry species, and Oak species. Questions remain regarding whether these trees will continue to decline or recover over time. In a more focused study, NYC Urban Field Station staff assessed the health of red maple street trees in Queens that were flooded by the storm surge, comparing them to red maples that were not impacted by salt water. Preliminary analysis shows that flooded trees are significantly more stressed than those that were not flooded. These data will provide the basis for future work as we strive to understand the longer-term impacts of hurricanes and other disturbances on urban forests in coastal areas.

    Last Updated: October 13, 2020