Intersecting urban forestry and botanical gardens to address big challenges for healthier trees, people, and cities

Correspondence Nicole Cavender, The Morton Arboretum, 4100 Illinois Route 53, Lisle, Illinois 60532, USA. Email: ncavender@mortonarb.org Societal Impact Statement The need for urban greening increases with global urbanization. Trees are major assets to livable urban areas, providing valuable environmental services to combat challenges such as pollution, urban heat, and flooding, as well as to improve social cohesion, human health, and well‐being. Investments in tree planting and arboriculture yield valuable returns, but trees face many challenges in the unnatural and stressful urban environment and in a rapidly changing climate. Botanical gardens have expertise in growing plants in designed landscapes. We urge their increased involvement with urban forestry to improve sustainability of cities and human lives. Summary Improving urban forests is one of the solutions to achieving several of the United Nations Sustainable Development Goals and making cities healthier and more livable for people. Priority should be given to protecting mature trees and promoting long‐lived trees in the future. Achievement of this goal requires recognition of the myriad stresses trees face in built landscapes as well as the challenges related to climate change. Because all people living in communities are affected by the urban forest, developing solutions and forestry action plans should be a social endeavor and include diverse partnerships. Botanical gardens and arboreta can provide key resources in support of these efforts. They have a significant public reach, maintain a strong professional network, and can make important contributions to address key priorities including (a) protecting existing trees; (b) improving tree selection, diversity, and age structure; and (c) improving planning, standards, training, and management. A focus on below‐ground aspects, such as root development and soil composition, is a critical component for success. Horticultural and scientific knowledge combined with extensive public reach make botanical gardens and arboreta important potential partners in achieving urban forest objectives, but a greater call to action is needed.


Societal Impact Statement
The need for urban greening increases with global urbanization. Trees are major assets to livable urban areas, providing valuable environmental services to combat challenges such as pollution, urban heat, and flooding, as well as to improve social cohesion, human health, and well-being. Investments in tree planting and arboriculture yield valuable returns, but trees face many challenges in the unnatural and stressful urban environment and in a rapidly changing climate. Botanical gardens have expertise in growing plants in designed landscapes. We urge their increased involvement with urban forestry to improve sustainability of cities and human lives.

Summary
Improving urban forests is one of the solutions to achieving several of the United Nations Sustainable Development Goals and making cities healthier and more livable for people. Priority should be given to protecting mature trees and promoting long-lived trees in the future. Achievement of this goal requires recognition of the myriad stresses trees face in built landscapes as well as the challenges related to climate change. Because all people living in communities are affected by the urban forest, developing solutions and forestry action plans should be a social endeavor and include diverse partnerships. Botanical gardens and arboreta can provide key resources in support of these efforts. They have a significant public reach, maintain a strong professional network, and can make important contributions to address key priorities including (a) protecting existing trees; (b) improving tree selection, diversity, and age structure; and (c) improving planning, standards, training, and management.
A focus on below-ground aspects, such as root development and soil composition, is a critical component for success. Horticultural and scientific knowledge combined with extensive public reach make botanical gardens and arboreta important potential partners in achieving urban forest objectives, but a greater call to action is needed.

K E Y W O R D S
arboreta, botanical gardens, cities, climate change, environmental services, global urbanization, human health, human well-being, trees, urban forestry, urban greening

| URBANIZ ATI ON AND THE URBAN FO RE S T
Future world population growth in, and migration to, urban areas will redistribute the Earth's population in a way that will affect the natural systems of the Earth and the interactions between urban environments and populations (Torrey, (2004). Urban people alter their environment through their consumption of food, energy, water, and land. And in turn, the polluted urban environment affects the health and quality of life of the urban population. Urgent action and new partnerships are needed to work toward solutions for sustainability and the well-being of people and the environment in urban areas.
Urban greening with a focus on improving the longevity and health of trees in urban forests can enhance ecosystem services and living conditions of metropolitan areas (Endreny, 2018).
The world population increased more than 400 percent over the 20th century (Roser, 2017). In 2018, more than 50 percent of the world's population lived in urban areas and this segment is predicted to rise to 68 percent by 2050 (UN DESA, 2018). Rapid urbanization is often accompanied by environmental degradation (e.g., air pollution, heat island effects, soil erosion, habitat and wildlife loss, carbon emissions, noise levels, etc.) impacting human health, quality of life, and well-being (Gurjar, Butler, Lawrence, & Lelieveld, 2008;Roy, Byrne, & Pickering, 2012). These effects can be exacerbated by global climate change, requiring an urgent response. Efforts to improve human life are the focus of the United Nations Sustainable Development Goals, which include action steps for additional tree cover in cities to help ameliorate environmental, economic, and social conditions for urban people and communities (FAO, 2016).
The urban forest includes the sum of all trees growing in highly altered community environments where humans are the main drivers of influence and disturbance (Escobedo, Kroger, & Wagner, 2011). The urban forest encompasses trees on both public and private property, including individual trees along streets and in backyards, as well as stands of remnant forests (Nowak, Noble, Sisinni, & Dwyer, 2001). Furthermore, urban forestry is "the art, science, and technology of managing trees, forests, and natural systems in and around cities, suburbs, and towns for the health and well-being of all people" (Helms, 1998).  (McPherson, Simpson, Peper, Maco, & Xiao, 2005;Schwab, 2009). Urban trees lower energy costs of buildings through shading and evaporative cooling, reducing building energy consumption by up to 40 percent (Cameron & Blanuša, 2016;Huang, Akbari, & Taha, 1990;Livesley, McPherson, & Calfapietra, 2016;Norton et al., 2015). They also provide many health benefits to people (Donovan, 2017;Schwab, 2009). Views of trees are correlated with less pronounced ADHD (Attention Deficit Hyperactivity Disorder) in children, reduced violence in public housing communities (25 percent fewer incidents), and 23 percent fewer employee sick days for those with views of trees (Bengston & Dockry, 2014;Kuo & Sullivan, 2001;Roy et al., 2012).

| L ARG E TREE S OF THE URBAN FORE S T DELIVER THE MOS T B ENEFITS
Within the urban forest, the larger and more mature trees, with their fuller crowns and leaf surface areas, provide more carbon storage, economic benefits, and other ecosystem services than do smaller trees (Díaz-Porras, Gaston, & Evans, 2014;Lindenmayer & Laurance, 2017;Stephenson et al., 2014;Wolf, 2005). Mature trees provide more shade for people and more habitat and food for animal species (Remm & Löhmus, 2011;Stagoll, Lindenmayer, Knight, Fischer, & Manning, 2012). Mature trees often provide symbolic, religious, and historic value, and they provide iconic landscape elements in urban parks and centerpieces in public common spaces. People generally prefer views of large trees and favor large trees along streets and in neighborhoods (Blicharska & Mikusiński, 2014). Large, mature trees provide key infrastructure for the green cities and deliver the most benefits. Large trees, however, face the most threats and are in global decline (Lindenmayer, Laurance, & Franklin, 2012).

| THE PROB LEM: TREE S FACE ACUTE/ CHRONI C S TRE SS IN URBANIZED ENVIRONMENTS
Trees confront difficult and often extreme conditions in built environments that limit their ability to reach maturity, compromising benefits delivered to the environment and people. Simply planting trees does not equate to an increase in tree cover in the long-term (Roman, Battles, & McBride, 2014). A 50 percent loss of trees within 5 years of planting is not uncommon, and the average median lifespan of urban street trees is between 13 and 19 years (Watson & Himelick, 2013).
Trees are faced with a complex array of stressors that often prevent them from realizing their biological potential in terms of tree form, health, and life span (Jim, 2005). Much of this stress originates below ground in the soil and in the root system. Roots are often constrained by the alien physical, chemical, and biological properties of urban soils (Watson, Hewitt, Custic, & Lo, 2014). Trees growing in built environments must contend with limited sunlight in the shade of buildings, increased air pollution from vehicles, and limited room for crown growth due to the presence of buildings and utilities (See Figure 1). Furthermore, as average global temperatures climb and climate patterns shift, trees face increased pressures from pests, diseases, and invasive plants (Mainka & Howard, 2010).
To realize the full benefits of urban forests, ameliorate the effects of a growing urban population, and plan for a better future, people must intervene on behalf of trees and take strategic action. We must actively protect, plant, and properly care for trees throughout their lives. A global movement is needed to recognize the importance of trees and rally to protect and strategically plant more of them. People have much to gain from investing resources in trees, but more attention and resources are needed. Trees are long-lived and need time to mature, so we must not wait to act (McDonald et al., 2016).

| Protect existing trees
The first priority should be to protect existing trees, since they are delivering the most immediate benefits. Policies and protection regulations at local, regional, and national levels are needed to regulate and promote the protection of existing trees, especially those that are large or of historical value. The economic, environmental, social, and cultural loss is tremendous each time a mature tree is removed.
More civic engagement, advocacy, and legal protection is needed.
The most successful government tree protection policies are based on a solid community forest management plan that includes a forest inventory, a clear strategic direction, and the support of International Society of Arboriculture (ISA) Certified Arborists, or similar professional arboricultural accreditation, on staff. Permits should be required for significant tree removals and incentives offered to private property owners who proactively plant trees on their property.
Procedures and guidelines on planting, tree care, tree protection, legacy tree preservation, tree removal, tree replacement, and invasive species control are of essential value. Creating community "Tree Boards" of experts and interested citizens is recommended to provide assistance, direction, and expertise to government entities regarding the preservation, planting, management, and protection of trees. Tools are also available to help involve citizens in monitoring and protection of trees and forests (Crocker et al., 2019).

| Improve tree selection, diversity, and age structure
There is a lack of diversity and age structure in urban forests, especially the street tree component. Often, only three to five genera dominate urban areas (50 to 70 percent of all street trees) (Pauleit F I G U R E 1 The major below ground and above ground stressors trees face in urban and built environments that limit their ability to survive and reach maturity et al., 2002). Because age diversity is also limited, trees that are providing benefits for a large area may fail or need to be removed around the same time and benefits cannot be recovered for decades. Pests and diseases are threats to urban trees, especially under a changing climate, and it will be increasingly important that a more diverse palette of trees be used to mitigate concentrated risk and to improve resiliency.
Many plantings fail because the wrong trees are chosen. Trees need to be matched with the location and local growing conditions. A greater application of horticultural knowledge is needed for improved selection criteria of species, especially in a changing climate.
More evaluation and monitoring is needed for regional species se- lections. An understanding of the urban forest composition (i.e., audit or inventory) is required first to be able to develop sound strategies for improving species diversity and age structure. Botanical gardens can be excellent resources for improved plant selections (Hirons & Sjӧman, 2019; Chicagoland Grows®; www.chica golan dgrows.org) and for tools, such as the Northern Illinois Tree Selector (2019), which can help people select the appropriate tree for the appropriate site.
Even if the need for greater diversity in urban forests is recognized, where is this diversity going to come from? The supply chain of quality trees is limited across the world (Nyoka et al., 2015;Whittet, Cottrell, Cavers, Pecurul, & Ennos, 2016). More investment is needed to develop a diverse supply of trees so that appropriate and adaptable trees are available at the local level across the globe.

| Improve planning, standards, training, and management
Improving the urban forest is very much a social endeavor, because it requires people to take action. A strong case for trees needs to be made with community decision makers so that trees are placed on the agenda for public consideration. As the public and community decision makers become more aware that investing in urban forestry can be a solution for many pressing urban environmental problems, trees will be considered a higher priority. Multiple stakeholders need to come together to develop a vision and goals that serve the entire community and recognize that urban forestry is a long-term proposition. Plans should include an understanding of current conditions, an awareness of risks to the urban forest ecosystem, a strategy to involve public and private partnerships, an acknowledgment of the economic value that the urban forest is delivering, and a financial strategy for realistic investment over time (Darling, Custic, Scott, & Smith, 2017;Schwab, 2009). City planning should incorporate tree preservation laws, development regulations, design and planting standards, and long-term maintenance provisions. It is not enough to make short-term commitments to planting more trees, because success will be determined by survivorship and sustainability, which require improved site conditions, good planting and care standards, a trained workforce, and ongoing maintenance. Improving planting standards and horticultural practices will increase the longevity of trees so that longer term benefits can be derived.
Humans are responsible in a large part for creating the conditions under which trees will thrive, survive, or fail. A particular area that needs improved attention is the landscape below ground.
Considerably more is now known about soils and root growth, and what is required below ground for successful growth of trees (Watson, Costello, Scharenbroch, & Gilman, 2009;Watson, Gilman, Miesbauer, Morgenroth, & Scharenbroch, 2019), but this knowledge must now be put into practice.

| PARTNERING WITH BOTANI C AL G ARDENS
Urban forests can span entire regions across diverse demographics.
Sustaining and improving them is increasingly reliant on different interest groups sharing a common ambition and working together in partnership. Botanical gardens and arboreta have an important role to play in meeting the great modern need for urban greening. Their direct involvement in sustaining healthy forests is needed now more than ever to amplify best practices and to engage local citizens in a community-based process. Gardens can provide key resources for planners, architects, consultants, and government bodies and add major value and influence to urban forestry planning and implementation. They can provide meaningful contributions in a variety of ways, and many public gardens have already developed successful models and approaches. In many cases, botanical gardens and arboreta are already playing a role in sustaining urban forests by functioning as public green space-maintaining part of the urban forest-and delivering environmental, aesthetic, and social benefits (Ward, Parker, & Shackleton, 2010). Where botanical gardens have excelled most is in their botanical and horticultural knowledge and expertise. "Botanical gardens have unrivalled skills and knowledge of growing plants built up from many years of practical experience" (Heywood, 2017). Their horticultural and arboricultural knowledge, which has been amassed over decades or centuries, is needed now more than ever for greening and growing trees where people need them most: where they live, work, and play. With the growing need for urban greening, botanical gardens are positioned to respond to the conservation and sustainability needs of their local communities.

Tree collection performance records
Keep ongoing and long-term records that track tree provenance, phenology, disease threats, and overall performance Improved understanding of tree adaptability to local conditions, the urban environment, and climate change.

Tree selection and planting guides
Develop planting guides that identify trees appropriate for the region with information on tree traits, recommended planting sites, growth information, tolerances, and performance limitations Improved strategies for increasing diversity and the life span of trees in urban forests Tree breeding and evaluation Utilize tree collection germplasm to develop, test, and introduce resilient trees for built environments Increased tree diversity; new introductions of trees to the market with increased disease and pest resistance as well as overall adaptability Tree inventories and mapping Assist and provide guidance to cities and communities with urban forest inventories, including: species composition, locations, age distribution, canopy cover, health condition, ecosystem service valuation, and threats Improved understanding of the status, assets, benefits, and distribution of the urban forest so that better management strategies can be developed and implemented Urban forest management plans Assist in developing regionally specific plans that identify goals, needs, and prioritized activities to improve and care for the urban forest Improved decisions that favor a future with a robust, healthy, and long-lived urban forest for the region

Community tree planting and stewardship
Organize and participate in tree planting and care activities in communities, schools, public spaces, and private lands Improved and longer lived regional canopy; engaged constituency of tree advocates Restoration activities Assist with activities that enhance the overall functioning of forest ecosystems within the urban forest, such as removing invasive species, improving natural regeneration, prescribed fire management, and reducing harmful animal browsing

Enhanced functioning and ecosystem benefits of urban forests
Tree ordinance and protection policies Advocate and assist in developing local policies that regulate and provide incentives for the preservation of existing trees and their proper care Improved legal protection of green infrastructure that delivers essential environmental, economic, and social benefits to citizens Tree research Develop and participate in research that advances understanding of trees in built environments, the benefits they deliver, and methods of improving tree production, planting, and care Improved knowledge, methods, standards, and best practices Best management practices for homeowners Provide guides, tools, and services for visitors and the public that help them choose, source, plant, care for, identify pests/diseases, improve awareness of invasive species, and maintain trees in their yard and community Better selection of trees, improved care, and longer lived trees in the landscape

Arboriculture and urban forestry consulting
Provide consulting services such as tree inventories, risk assessments, development of tree management plans, tree pruning and care treatments, ordinance writing, inspection, and tree valuation Improved capacity and skilled workforce available for the community to support the urban forest Arboricultural training Offer training that advances education for students, volunteers, arborists, tree managers, and other professionals working with urban tree establishment and management. Below-ground aspects are of particular importance for arboriculture training and practice.
Increased, trained workforce to support a healthier and longer lived urban forest These goals aspire to improve tree health, improve urban forest policy, increase funding for urban forestry, and integrate science. The CRTI Master Plan, its partners, and examples of programs and resources for urban forestry can be found on the public website.
Botanical gardens have the opportunity and potential to make important contributions to urban forestry, but they are still underrepresented in such initiatives. An important barrier to consider is that although botanical gardens often have the knowledge and experience, they may not have the current capacity, and they would need additional support to be able to play a more influential role.
Due to their mission-based business models, they generally have a high rate of return on investment and further support could be effectively leveraged. If botanical gardens can overcome the barrier of capacity, their involvement has promising potential to lead to greater success. We suggest a dual call for action: (a) botanical gardens need to exert themselves more in the process and not passively wait on the sidelines to be asked; and (b) city planners, landscape architects, consultants, politicians, and urban forest professionals need to reach out to their regional botanical gardens and arboreta to involve them in urban forestry efforts.
Resources exist to help identify arboretum and botanical garden partners for urban forestry initiatives. ArbNet (www.arbnet. org) is a network of the world's arboreta that facilitates professionalism and involvement of arboreta in urban forestry activities. Greater influence to improve critical decisions pertaining to urban forestry and greening TA B L E 1 (Continued) Botanic Gardens Consortium, The Botanical Society of South Africa, and others. These networks and databases provide useful means for identifying botanical gardens as potential partners in the important work of urban greening and community forestry.

| CON CLUS IONS
There is a growing recognition that solutions are needed to ameliorate the environmental and social effects of an increasing urbanized world. Trees are critical components of green infrastructure and urban nature. Improving urban forests across metropolitan areas is a key solution that requires immediate and strategic action. To protect trees and ensure that people have the benefits trees provide, human intervention is needed. Priorities should focus on protecting existing trees, improving the selection and diversity of species, and improving planning, standards, and care for trees. Success in carrying out these priorities requires a more socially inclusive approach and is reliant on different interest groups. Botanical gardens and arboreta can greatly assist in these endeavors and many are already actively engaged. We encourage botanical gardens and arboreta to become more active and to take initiative to collaborate in urban forestry efforts, and we believe this will have growing relevance across the globe. Government officials, planners, and consultants should find and reach out to botanical gardens to collaborate at the local level.
Botanical gardens have particular strengths in practical, horticultural knowledge and training, as well as public credibility and outreach.
With their involvement, outcomes will likely be more successful.
Protecting, planting, and caring for the world's urban forests will deliver great benefits environmentally, socially, and economically.
Urban forest improvement is worth our attention and greater investment for a greener, healthier, and more beautiful world for people.

ACK N OWLED G EM ENTS
The authors thank Jessica Turner-Skoff, Claudia Wood, Alicia LaVire, and the three anonymous reviewers for their suggestions, as well as Caleb Phelps, Chai-Shian Kua, and Kris Bachtell for their contribution to the figure. We recognize Lydia Scott for her leadership of The Chicago Region Trees Initiative, from which we have learned much.
We thank The Morton Arboretum supporters and our colleagues who work to understand, plant, and protect trees for a healthier and more beautiful world.