Overhead and (Sometimes) Overlooked:

Knoxville’s Urban Forester Explores and Explains 10 Years of Change to the City’s Tree Canopy

By Kasey Krouse, City of Knoxville Urban Forester, with Bill Klingeman, UT Plant Sciences Department

In 2011, trees surveyed for an Urban Forest Management Plan documented that the Top 4 genera of trees within the City of Knoxville’s managed landscape consisted of maples (21%, Acer spp.), oaks (14%, Quercus spp.), dogwood (11%, Cornus spp.), and sycamore (6%, Platanus spp.), with the remaining 47% of trees spread among other genera. Yet the most abundant tree species within our landscapes are not always the most successful, either with establishment or maintaining tree health across time. Flash forward to 2019 and these genera still comprise the Top 4, yet their prominence as Knoxville tree canopy components has shifted: now to maples (19%), oaks (13%), dogwood (8%), and sycamore (4%). The story of these trees, and of the other 55% of Knoxville’s increasingly diverse tree canopy, is the focus of this article.

At about the same time that the 2011 Urban Forest Inventory was concluded, Knoxville and East Tennessee faced threats from Emerald ash borer (Agrilus planipennis) to ash trees, bacterial leaf scorch (Xylella fastidiosa) to sycamore trees and pin oak (Fig. 1), and thousand cankers disease (TCD) of walnut (Juglans spp.) caused by Pityophthorus juglandis bark beetles that spread Geosmithia morbida fungus. Although walnut trees that survived the initial stress of TCD have demonstrated recovery across the city, ash trees (Fraxinus spp.) have not fared as well. Knoxville has lost more than 90% of its ash tree population within the last 8 years.

Fig. 1A. Insect-vectored bacterial leaf scorch caused extensive decline in tree crowns, requiring removal of sycamore trees (Platanus occidentalis) that had been planted along the 3rd Creek Greenway Trail.

Photo by Kasey Krouse

Fig 1B. Leaf scorch symptoms caused by Xylella fastidiosa are apparent as necrosis on leaf margins

Photo by Kasey Krouse

In addition to new pests, 2018 was the 4th warmest year on record. Sustained shifts in regional climate conditions might extend the growth season for plants, but may also challenge health and survival of cool-weather tolerant plants and trees, like Spruce (Picea spp.) and fir (Abies spp.) trees. Of course, people also interact with trees in their urban forest environment in ways that aren’t always optimal to tree health and survival (Fig. 2). Given these pressures, change is inevitable. With knowledge and prior planning, adapting to that change can be strategic. And with slowgrowing trees, it pays to be proactive. Having the objectives of long-term canopy health and resilience of the trees in his team’s care, Kasey Krouse has enacted a roadmap for diversifying Knoxville’s urban forest. Six actions help Kasey navigate this roadmap: Assess, Prioritize, Organize, Plan, Build, and Sustain. Despite ongoing change to the exciting urban forest and health and persistence of individual trees within the Knoxville Tree Inventory, Knoxville’s urban forester and the horticultural team has documented species and assessed condition of about 90% of the trees that the city manages within its parks, roadways, and right-of-way areas. Every one of these trees are managed within a database that helps the team to track and schedule maintenance and manage tree health and evaluate tree populations (Fig. 3, 4). If we project the value of Annual Tree Benefits to the city using iTree, the economic inputs are staggering: these trees provide more than $1.28 million in benefits per year, including improvements to air quality, reduced energy inputs for heating and cooling, improved quality of life, and other ecosystem services.

Fig. 2. Humans can be credited with many (and diverse) mechanical injuries and cultural practices that threaten tree health and survival in the landscape.

Photos by Kasey Krouse

Fig. 3. Tree plantings in and around Knoxville’s Krutch Park.

Photo courtesy of Kasey Krouse

Fig. 4. Within neighborhoods, street-level evaluations can help identify locations where opportunities exist to plant new trees. The Knoxville Tree Inventory is 90% complete. Information on every tree is cataloged within a database that enables tracking and scheduling of maintenance.

Photo courtesy of Kasey Krouse

As a general guideline, the 10:15:20 Rule has persisted as a broadly applied principle used in decision-making for diversifying the urban forest. Briefly explained, the prevalence of a tree species within a “balanced” regional composition should not be relied on to contribute more than 10% to the total community of tree species present; that trees within a given genus contribute not more than 15% of tree composition; and that no one plant family make up more than 20% of all tree diversity. Because so many tree diseases and pest arthropods affect trees across one genus, it is the city’s goal to increase the diversity as much as possible at the genus level.

Categorizing urban tree canopy coverage and species composition is an ongoing effort. The progress that is made on keeping accurate records is challenged by storm events, improper tree maintenance, threats from invasive insect pests and plant pathogens, and urban development. Given the importance of valid information for making decisions about tree choices, a big goal during 2020 will be to update the city-wide tree canopy assessment. This project, which was last undertaken in 2010, has provided a baseline resource for prioritizing where management activity may be most needed, and directing where future tree plantings can have the greatest impact (Fig. 6).

Fig. 6. Tree canopy coverage in Knoxville can be rapidly visualized across the city.

Fig. 6. Tree canopy coverage in Knoxville can be rapidly visualized across the city.

A Proactive and Hands-On Approach to Prioritizing Knoxville’s Urban Forest

In 2012, a straightforward organizational change to the way that the city contracted planting and capital projects has had a tremendous impact on the benefits that Knoxville residents appreciate for trees that are purchased by the city. Among other key factors, city planners and the urban forester developed a relationship with suppliers in Tennessee and across the region. Together they could discuss needs for select plant species and cultivars, and plan ahead to ensure that the trees which would be most in need would be available three to five years ahead of planned transplanting. These relationships were critical in ensuring that commercial tree producers would have the available nursery stock ready in time to submit contract bids for the size and quality of trees that were needed by the city.

Once a well-grounded understanding of the baseline condition and status of Knoxville’s urban tree canopy was forged, it was time to build upon this information. Productive partnerships have been a key component to the success of these efforts. Kasey worked with the Director and staff at the Knoxville Botanic Garden and Arboretum to establish a Missouri Gravel Bed system, where hard-to-find trees can be maintained with limited resource inputs while trees continue to develop fine feeder roots for easier-toestablish root systems. Once the trees go dormant in late fall and early winter, they can be easily lifted and bare-root transplanted into prepared locations around the city (Fig. 7). Kasey also sought out and built relationships with several suppliers with whom he could ensure availability of diverse, high quality tree species for the city (Fig. 8). Between 2011 and 2019, Kasey and his team have planted more than 4,990 trees across Knoxville, adding to the more than 22,000 trees in the managed urban forest, and resulting in a 23% increase in the city’s tree canopy.

Fig. 7. The Missouri Gravel Bed system has been used to enable long-season holding, maintenance, and root generation on tree species that can be lifted in fall, when dormant, and bare-root transplanted into landscape locations around Knoxville.

Photo courtesy of Kasey Krouse

Fig. 8. As a Tree City USA member, the diverse and resilient urban tree community in Knoxville has been planned to provide benefits and enjoyment to future generations.

Graph courtesy of Kasey Krouse

Another milestone for the city was achieved in 2015, when Kasey Krouse and a group of local tree enthusiasts kick-started a local non-profit organization. Trees Knoxville is dedicated to increasing the urban tree canopy across Knoxville. The partnership has been important in assisting the city with additional resources, leveraging projects on private property, and engaging and educating the public on the importance of trees and species diversity. Trees Knoxville has helped to coordinate tree plantings and give away thousands of trees since 2015.

Once trees are planted, the work continues. Inventories are continually updated, failing and hazard trees are removed, and future problems are mitigated with careful pruning and training of central leaders that promote a healthy tree crown. All of these actions are scheduled or undertaken in response to storms (Fig. 9) with the long-term goal of sustaining the health and resilience of Knoxville’s urban forest. The dedicated work of urban foresters, like Kasey Krouse, ensures that generations to come will appreciate and value the benefits from the diverse community of trees that surrounds us.

Fig. 9. A growing record of purposeful management to Knoxville’s urban trees. Planned pruning is critical for limiting catastrophic loss of trees due to more and stronger storms across our region.

Graph courtesy of Kasey Krouse