New York City Urban Field Station

Urban Tree Monitoring and Research
with Healthy Trees, Healthy Cities

Karine Aigner/Courtesy TNCHealthy Trees, Healthy Cities (HTHC) is a partnership between the US Forest Service and The Nature Conservancy that seeks to protect the health of our nation’s trees, forests, and communities. HTHC’s goal is to create a culture of stewardship that engages people in long-term stewardship and monitoring of the trees in their respective communities. Through our mobile app, users can participate in a nationwide citizen science effort studying urban tree health, learn how to care for trees in their community, and track stewardship efforts for trees.

Screenshot from Healthy Trees Healthy Cities mobile appThe Healthy Tree app assists with urban and suburban tree monitoring and care via four modules, which together create an all-encompassing tool for urban tree research, management, and stewardship. A browser-based dashboard is also available and described below. The modules in the Healthy Trees, Healthy Cities app include:

  • Add a Tree is an inventory tool which tracks new plantings and existing trees.  This module is linked to all the other modules’ data. Trees can be added individually or in bulk for projects using existing tree inventories. Once added, a tree will appear in the mobile app, on a project’s dashboard, and its associated data will be available for downloading.
  • Tree Care is a module that where a user can record and schedule tree stewardship activities like watering and mulching. Stewardship activities are logged in the app and can be easily downloaded from a project’s dashboard.  This allows homeowners with one tree, or neighborhood groups with blocks of street trees, to track and manage their trees.
  • Health Check is a module which allows users to conduct a comprehensive tree health assessment. The tree health assessment method is adapted and refined from those used in rural forest health protection and monitoring. This information may help with early detection of new pests and diseases.
  • Pest Detection is a module which lets users record signs and symptoms of several major forest pests. Tree stewards choose from a drop down list consisting of visual signs and symptoms of pests and diseases. Each pest symptom has an explainer page and each pest has an informational presentation.
  • HealthyTreesHealthyCitiesApp.org is the online dashboard for the app. Here, researchers and stewardship groups can set up projects that track tree health or provide care for trees. With the dashboard, users can easily manage projects, download data for analysis, and even assign trees to specific people.

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 insures that any new information is   linked to the correct tree. The app also allows users to add trees, by adopting existing trees not already in the app or by planting a new tree. Once added, the tree and its data become visible to all users and data managers.

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 find correlations 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.

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 serious 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.


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. The dashboard is currently live and will launch new features in time for the 2019 field season.

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.


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.

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

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.