Urban Vegetation: Anthropogenic Influences, Public Perceptions, and Wildlife Implications

Abstract

Urban environments are becoming more common as cities grow and proliferate. Subsequently, their ecosystem services are becoming increasingly more important as climate change impacts urban dwellers, their benefits to human psychological and physical wellbeing are better documented, and biodiversity elsewhere is declining. However, while urban wildlife have received growing attention in recent decades, the importance of urban vegetation has mostly been emphasized in narrow literature niches related to horticulture and landscape architecture. Here, we review literature on multiple uses of urban horticulture, not solely limited to beautification of personal space; the importance of urban vegetation to urban wildlife conservation and sustainability; and how urban vegetation is portrayed in the media.

1. Introduction

1.1 Vegetation classifications based on urban uses

Urbanization has caused the global expansion of human-occupied, artificial, or non-natural landscapes in the place of natural, native habitats. While land-use conversion is inevitably associated with the expansion of city boundaries, these novel landscapes can offer beneficial places not just for humans but also for some plants and wildlife. Urban landscapes have many forms, functions, and features which vary based on geographic region and infrastructure. These landscapes consist of many types of vegetation that impact the environment surrounding them and humans in different ways [1]. Plant choice, function, maintenance, and biodiversity create urban landscapes that vary in their sustainability. We begin our discussion of how vegetation is used in urban environments by classifying vegetation types into four primary groups: esthetic/ornamental, functional, and undedicated vegetation.

1.1.1 Aesthetic/ornamental

Over 2000 genera are currently used as ornamental plants [2], defined by their appearance and perceived beauty within a landscape, garden, or interior plant environment and where their primary function is decoration and beautification. Their popularity and use in various landscapes, climates, and gardens throughout the world has risen to make the ornamental plant industry worth over EUR 35.5 billion, a number projected to expand [3]. Ornamental plants are also a fundamental part of human history, having been grown for thousands of years worldwide. However, when it comes to esthetic or ornamental use within an urbanized landscape, diversity in plant selection is limited. A wide range of plants in a garden or landscape may not be practical for homeowners or municipally-managed properties because of cost or availability. Sourcing large numbers of diverse plants is also a limitation, as wholesalers and retail nurseries tend to focus on plants that are popular and profitable [4]. Nurseries also may be limited in plant selection by vendors that only produce certain plants in large quantities. Yet in recent years, a growing number of retail and wholesale stores are offering plants that are more suitable for local environments, climate conditions, and landscape needs. The development of water-wise, native, and specialty landscape plants such as the Texas Superstar^® or Proven Winners^® plants has further supported interest in species that are better suited to local ecosystems or environments while balancing esthetic appeal and landscape performance.

1.1.1.1 Landscapes

Ornamental plants shape spaces within a landscape and serve to beautify and enhance homes, businesses, and larger, privately-owned areas. In home landscapes, ornamental plants are chosen for their esthetic attributes in relation to a house or building along with their function within the landscape (i.e. shade, color, windbreak). The plants selected by homeowners or landscapers are often chosen based on availability, durability, and cost, rather than ecological or environmental purposes. Landscapes also range in size from large to small and therefore further influence plant selection. However, landscape value and esthetics are subjective. For example, St Hilaire [5] found that landscape preference can be dictated by environment (i.e., desert vs. mesic climates), water shortages, expense of water, and length of time living in a landscape. Because types of landscapes vary widely, such as xeriscape, prairie scape, formal, English, wild, etc., personal preferences account for many of the differences across cities, environments, and between urban regions. Cultural or ethnic background can also account for many landscape preferences. Buijs et al. [6] showed that cultural differences affected landscape preferences in the Netherlands, where immigrants had greater preferences for urban landscapes while native Dutch people gravitated toward wilder settings. In addition to these factors, personal values can also influence plant choices in landscapes. Selection of native plants, pollinator friendly plants, or bird attracting plants has been made easier by the breeding, selection, and marketing of many varieties of landscape plants that cater to these needs. Figure 1A illustrates common landscape choices found in Lubbock, Texas, USA, compared to more ecosystem-service centered landscape designs with more native and wildlife friendly plant choices (Figure 1B–D). The owners of the yard illustrated in D specifically have pollinators as their management goal. The effort is documented at https://www.inaturalist.org/projects/urban-garden. As of January 14, 2023, there were 734 species documented, including 562 insect species, 56 birds, 48 plants, 38 arachnids, 5 mollusks, 4 mammals, and 2 reptiles. Those responsible for the front (B) and back (C) yards shown are more focused on native, water-wise vegetation and habitats attractive to native and urban species as a whole.

Figure 1.

Native vs. non-native urban landscapes. (A) Non-native lawn with few landscape plants, (B) bird friendly plantings, (C) yard with native plants and natural landscaping, and (D) pollinator garden. Photos courtesy of Gad Perry (A and B), Kate LeVering (C) and Catherine Galley (D).

1.1.1.2 Public/botanical gardens

In contrast to private landscapes, botanical gardens are mostly available for the general public to view and enjoy (Figure 2). They can be privately sponsored, funded through the government, or supported through grant funding or donations. The primary mission of most botanical gardens is to educate and exhibit plants. More recently, there has been a shift in the education mission of these gardens to include conservation, sustainability education, and public exposure [7]. However, this has had both positive and negative impacts on the organization and operational functionality of these gardens. Many sustainable practices create significant challenges in implementation or financial barriers to adoption [7]. For example, pesticides that help horticulturists suppress pest populations have been phased out because of public perceptions and commitment to sustainable endeavors [7]. Alternatively, increased public adoption of sustainable practices and education initiatives has increased public awareness of “green” practices [7, 8]. Botanical gardens have also spearheaded community and citizen science programs to involve the public in conducting research that contributes to the mission of sustainability as well as understanding responses to climate change [8]. They can also provide networking opportunities for scientists, volunteers, botanists, and ecologists that are conducting projects on similar subjects. Fundamentally, botanical gardens serve a significant role in educating the public about sustainability and creating environmental awareness through science communication efforts.

Figure 2.

The Chicago Botanical Garden located in Chicago, Illinois, USA. Photo courtesy of Catherine Simpson.

1.1.1.3 City beautification

Public greenspaces are often used for beautification and as functional recreational spaces [9]. Green infrastructure components include diverse plant materials, green spaces, green streets, stormwater runoff systems, permeable pavements, green roofs, etc. Green spaces like parks, green streets, and street plantings can positively impact urban ecosystems by reducing temperatures and effects of extreme weather, improving biodiversity and climate resiliency, and mental well-being of humans [10, 11, 12]. Thus, green infrastructure is also an important part of urban sustainability [13] . Another concept that can influence city beautification is biophilic design, which emphasizes nature-based systems and design fundamentals to improve sustainability in built environments [14]. Many cities participate in greening and beautification efforts. For example, Keep America Beautiful^® is a non-profit organization that partners with communities to build and maintain green spaces (kab.org). To date, they have 695 affiliates across America that participate in clean-up, beautification, and other efforts to educate and bring awareness to environmental concerns. Other communities have taken further steps to improve city beautification by enacting ordinances for landscaping, maintenance, and plant selection for companies that build within city limits. Cities like Austin, Texas, USA, require a Sustainable SITES Initiative certification for Parks and Recreation Department projects to ensure projects are sustainable [15]. Furthermore, some cities have planting lists that require city-maintained areas to select and plant plants that are non-invasive and will not cause significant damage to surrounding infrastructure [16]. The adoption of such policies may not only improve city beautification, but also biodiversity, plant health, and more widespread urban green space adoption [11, 14, 17, 18].

1.1.1.4 Roadways

While the primary purpose of urban streets is transportation of people and goods, throughways provide space for other public services including exercising, recreation, shopping, and congregating. As such, transportation officials’ ideas of proper streetscape composition and management may conflict with city planners’ and residents’ visions of visually appealing streetscapes. Drivers prefer roadsides with greater amounts of vegetation and increased tree height and density, relative to adjacent commercial properties [19]. Pedestrians and cyclists also select routes based on the amount and type of vegetation present; the presence of large trees and structural diversity in vegetation especially increases preference [20, 21]. Roadside vegetation is correlated with slower driving speeds and reductions in drivers’ stress [22]. Roadside vegetation can serve as hazards, but vehicle crashes into vegetation are less common in urban areas compared to rural areas due to lower speed limits and more clearly defined boundaries of urban roads [22, 23].

1.2 Functional

While esthetics can be considered a function of plants on its own, urban plants can also serve other purposes to the broader environment. Among these uses are ecological or conservation, food production, recreation, and roadways/streetscapes.

1.2.1 Ecological or conservation

Though ecological and conservation functions may not be the original intended function of urban plants, urban vegetation provides critical ecosystem services including cooling, heat mitigation, air filtration, storm water retention and filtration, and habitat and food sources for urban wildlife. Urban vegetation contributes to human health and well-being through therapeutic and calming effects, outdoor recreation, and reducing exposure to environmental stressors (e.g., ultraviolet radiation, heat exposure, air and water pollution). These benefits are discussed in detail in the “Quality of Life” subsection of this chapter. Due to increased recognition of ecosystem services, ecological and conservation purposes have grown in popularity as intended functions of urban vegetation. Some specific examples of urban vegetation established for ecological and conservation functions include rain gardens, pollinator gardens, green roofs, and xeriscaping.

1.2.2 Food production

Community gardens differ from private gardens, landscapes, and largescale food production systems in being accessible by members of the public and serving as areas for food production in urban or peri-urban locations [24]. Yet, community gardens vary widely in their purpose and function. These purposes vary from bringing community members together to garden; promoting healthy practices; engaging children; promoting food security (Figure 3), land conservation or restoration; or allowing users to connect with nature. Community gardens can help people access more diverse foods, which aids in social justice causes and reduces undernutrition caused by calorie dense, nutrient poor diets [25]. Furthermore, Alaimo et al. [26], Carney et al. [27], and Litt et al. [28] showed that adults who participated in community gardens significantly increased vegetable and fruit intake. They can also serve to address conservation and sustainability on a community level [9]. Additionally, the ecosystem services that community gardens can provide are not limited to humans alone. They provide habitats for pollinators, beneficial insects, wildlife, and benefit the environment as well. A recent meta-analysis shows that the quantity of crops (ex. 129–200% yield increases per m²) grown in urban settings can be substantial [29, 30]. Another approach gaining ground in the food production sector is urban vertical farming (e.g., [31]). Typically using hydroponic or aeroponic techniques, these high-tech efforts offer the promise of producing food close to urban consumers. Using reduced footprints and inputs, including reduced water usage, increased yields per m², and reduced food miles, they can be quite profitable [32]. However, they still only produce a narrow variety of foods and require advanced technologies that may not be suitable for conditions in many developing countries [33].

Figure 3.

Heart of Lubbock Community Garden located in Lubbock, Texas, USA. Photo courtesy of Jonah Trevino.

1.2.3 Recreational

While some may consider community gardens to be recreational space, these gardens are generally limited to people who enjoy gardening as an activity. However, recreational uses of plants are not limited only to gardens. Recreational greenspaces can include parks, sports fields, playgrounds, public seating areas, lots, and other areas where nature can be experienced [34]. These functional spaces are maintained to fulfill particular needs within society, such as exercise, etc. The plants selected for the spaces that are designated for sports typically are less diverse than those in greenbelts or natural areas. Grass species dominate sports fields and athletic areas [1] and are more aggressively maintained than non-sports areas. However, functional spaces may offer a large, planted area for wildlife and other species to inhabit (Figure 4). For example, an extensive review by Petrosillo et al. [35] found that while golf courses may negatively impact water and soil environments, they increase biodiversity and ecosystem services, particularly in amphibian, bird, turtle, and bee species. These findings are further supported by Tanner and Gange [36] and Guzy et al. [37] who observed increased biodiversity and species richness near golf courses in urban areas. Other areas that are designated as recreational may include urban remnant prairies or forests and refer to patches of space cities use as a natural park but contain minimal built structures. This increases the likelihood of ecological plant and animal diversity within these spaces, but also provides opportunities for species to encroach into urbanized areas where they may not be welcome (Figure 4B and D).

Figure 4.

Recreational spaces in Lubbock, Texas, USA. (A) Play area for children located in a park, (B) park that has been minimally maintained and has led to prairie dog repopulation, (C) walking trail with limited accessibility for handicapped persons, and (D) grey fox seen near a building on Texas Tech University campus. Photos A–C courtesy of Gad Perry, photo D courtesy of Emily Stamm.

1.3 Undedicated vegetation

1.3.1 Lots

Though these areas do not have a dedicated function, they provide ecosystem services for urban humans and animals that would not otherwise be provided if the space was filled with buildings or impervious surfaces. In an early example, Darlington [38] summarized the flora associated with urban walls, primarily in the United Kingdom, and the animals that rely on them and help them disperse. Vacant or abandoned areas comprise about 15% of the land area in cities [39]. These areas receive little to no maintenance and are dominated by spontaneous vegetation comprised of a variety of species. Plants occurring in these areas may include remnant species purposefully planted when the area had a designated function, in addition to species that were unintentionally introduced through a variety of abiotic (i.e., wind, water) or biotic (i.e., endozoochory, epizoochory by animals) dispersal mechanisms. The vegetative composition and structure in vacant areas is highly influenced by biophysical and social factors. In a study of vacant lots in Chicago, for example, species richness was most affected by the amount of trash in the lot, home ownership around the lot, proportion of the lot covered by turfgrass, and the proportion of area around the lot covered by built structures [40].

2. Applications of vegetation in urban environments

Vegetation functionality changes in different urban environments. For example, vegetation can serve as pollutant filters, erosion control, stormwater buffers, as well as spaces for wildlife habitats or human recreation. Often this relies upon the selection of plants that have particular roles in the ecosystem. Selection of plants for environmental or water conservation, carbon sequestration, pollutant mitigation, wildlife attractants, or the improvement of quality of life are some of the primary roles that plants can play in an urban landscape.

2.1 Sustainable and conservation practices

2.1.1 Native plants

Native plant species have grown in popularity for urban gardens due to a variety of reasons such as being adapted to local climate conditions and providing critical habitat and ecosystem services [41, 42, 43]. Additionally, native species are increasingly adopted to replace the large amounts of non-native species that have come to characterize urban vegetation; between 30% and 50% of urban plant species are non-native [44, 45, 46]. Several cities have adopted ordinances mandating that native plants be used in and around new commercial developments and public landscapes and promoting management practices that make urban vegetation more hospitable to wildlife and conducive to natural ecosystem functions [47, 48, 49]. Additional details about the effects of urban vegetation on wildlife are provided elsewhere in this chapter, particularly in the sections titled “Importance of Vegetation for Urban Wildlife” and “Case Study.” Native plantings vary in scale; homeowners and business owners may dedicate flowerbeds or entire lawn areas to native plants [50, 51]. In contrast to typical turfgrass lawns, native plantings are typically managed as low- to moderate-intensity, “near-natural” states with little to no fertilizer or irrigation inputs (Figure 1B and C).

2.1.2 Water conservation and management

Garden and lawn irrigation is estimated to comprise approximately 30% of residential urban water use in the United States, totaling nearly 30 billion liters per day [52]. Outdoor water use may be as high as 60% in arid climates [52]. Sustainable gardening encourages the use of water conservation and adoption of species that require less water. “Xeriscaping”, “water-smart”, and “water-wise” landscaping are similar concepts with the common end goal of attractive water-efficient landscapes [53, 54]. The inclusion of low water-using plants that are tolerant of the regional climate is a key component, particularly in light of climate change challenges [55]. As mentioned above, adaptation to local climate and soils is one of the drivers for adopting native vegetation. Species selection should balance water usage with water infiltration. A study by Chang et al. [54] reported that xeriscaping, although water use-efficient, had less infiltration and greater runoff than turf lawns. Ideally species selected for water-efficient landscaping will require little irrigation while maintaining moderate to high infiltration rates and low runoff.

2.2 Services plants provide

2.2.1 Climate and pollution mitigation

Due to their large amounts of impermeable surfaces, cities are especially vulnerable to extreme precipitation flooding events. Impervious surfaces result in low infiltration and high runoff; subsequently, the potential for flooding and runoff containing large quantities of chemical and physical hazards increases [56]. Vegetated areas provide critically important permeable surfaces within the largely impervious urban environment. The replacement of impervious surfaces by vegetation slows the speed and reduces the total amount of runoff. Zabret and Šraj [57] calculated that reducing asphalt surfaces in a parking lot by 35.5% through tree planting would result in a 7–17% reduction in runoff. Additionally, vegetated areas assist in stormwater treatment and improve water quality through bioretention processes [58]. Green infrastructure specifically used for stormwater retention and treatment includes rain gardens, constructed wetlands, bioretention basins, roadside swales, filter strips, and green roofs [56, 58, 59]. Chinese researchers proposed the “sponge city” concept in 2013 as a systematic method to retain, slow, and clean water in urban areas [60]. In 2015, 16 cities were selected to pilot the program; the program expanded to another 14 cities in 2016. Goals included retaining and reusing 70% of urban stormwater by 2020 and reusing up to 80% of stormwater by the 2030s. Though the infrastructure and concepts mentioned in the previous sentences are specifically deployed to capture and treat stormwater, vegetation in residential yards and private properties can serve similar functions, though they may be restrained. Vegetable and herb gardens are not typically thought of in terms of their ability to retain stormwater or improve runoff water quality. However, Whittinghill et al. [61] reported that vegetable-producing green roofs had similar water retention capabilities and reduced NO₃⁻ concentrations in runoff as green roofs with Sedum spp., a succulent ground cover commonly used on green roofs.

Urban vegetation improves air quality by modifying deposition and dispersion of gaseous molecules and particulate matter. Because plants have a high surface area to volume ratio, deposition on vegetative material is enhanced relative to smooth, synthetic surfaces [62]. Vegetative species with hairy, thin, and/or waxy leaves have increased deposition relative to species without these characteristics [63]. Urban vegetation alters wind direction and speed, therefore affecting the dispersion pattern and dilution of pollutants. In a review of urban vegetation effects on deposition and dispersion, Janhäll [64] recommended vegetation be shorter and/or closer to surfaces to allow dilution of emissions by clean air. They also recommended vegetation be planted close to the source of pollutants to increase deposition, such as planting vegetative barriers near roadways. Lastly, because polluted air passing above vegetation is not filtered, vegetative barriers should be porous enough for air to pass through and deposit pollutants, but solid enough to encourage air to pass close to the surface.

2.2.2 Quality of life

The health, well-being, and quality of life benefits of vegetation are extensive. Urban vegetation plays a critical role in urban residents’ well-being. Integrating biophilic design principles can reduce energy consumption, lower the urban heat island effect, and improve human resilience in the face of climate change [11, 14]. Integration of these concepts can also make humans more aware of nature and, as a result, encourage them to care more about protecting natural spaces [65]. Vegetated spaces provide the public with cultural ecosystem services, as greenspaces contribute to increases in property value, tourism, improving population mental and physical health, as well as supporting social cohesion and interactions [10, 66, 67, 68]. Urban green spaces have also been linked to reduction of violent crimes, improvement of cognitive development, reduction of noise levels, and other social benefits [69, 70, 71, 72, 73]. These benefits also translate to specialized green spaces such as university campuses, where McFarland et al., Speake et al., Van Den Bogerd et al., and Trevino et al. [74, 75, 76, 77], among others, have shown that green spaces can improve perceptions of the campus, academic performance, and mental health. Frumkin et al. [78] reviewed many studies on the health benefits of nature contact and reported better sleep, improved mental health, life satisfaction, social connections, healing, birth outcomes, child development, pain control, immune function, and general health as well as reduced aggression, stress, blood pressure, heart failure, obesity, diabetes, asthma, and mortality. These observations have led to the use of plants in complementary therapies such as horticultural therapy (registered horticultural therapist directed horticultural activities with the aim to achieve specific goals within an established plan or treatment) or therapeutic horticulture (use of horticultural activities as a therapeutic modality facilitated by registered horticultural therapists or trained horticulturalists) to improve quality of life for different populations [79]. Therapeutic horticulture can involve different modalities, generally active or passive interactions with plants. When plants are used as an active tool in therapies, improvements to physical and mental health have been seen. This can be particularly useful in vulnerable populations such as children, elderly, military veterans, and the mentally ill because of the lack of significant, adverse impacts [80, 81, 82, 83, 84, 85, 86, 87]. In a study by Fleming [88], U.S. military veterans reported that ‘veteran to farmers’ programs that utilized interactions with plants helped alleviate symptoms of Post Traumatic Stress Disorders (PTSD) while giving participants a source of meaningful work and skills. These findings have been echoed all over the world and throughout history, from Zen gardens, to nature assisted therapy programs established after World War I and II [82]. This further supports the theory that nature and vegetation appeals to humans on a fundamental level and improves our quality of life.

2.2.3 Wildlife and invertebrate habitats

Vegetation specifically utilized for ecological and conservation functions is the most conducive to providing habitat for wildlife and pollinators. However, various studies have documented the benefits of other vegetation classes and functional types on urban vertebrates and invertebrates. Recreational spaces, botanical gardens, ornamental plants, roadway vegetation, community and private gardens, and undedicated vegetation have all been shown to have positive effects on urban wildlife and invertebrates [89, 90, 91, 92]. The degree of positive effects and potential negative effects vary with the specific function and plant species utilized.

Diverse urban plant assemblages with a greater abundance of plants and high species richness do not always equate to suitable quality habitat for urban wildlife. Urbanized environments support increased non-native plant species abundance and richness [93]. This increase in non-native plant species is primarily driven by human-mediated trade and transport, whether intentional or unintentional [94]. Effects of alien vegetation on urban wildlife and invertebrates range from positive, to negative, to negligible [43, 95, 96, 97]. Generally, native plant species benefit urban fauna with benefits (e.g., greater abundance, diversity, occupancy, and richness) to native animal species attributed more frequently to native than exotic plant species [43]. Non-native animals tend to benefit more from introduced plant species. Wildlife responses to plants’ “nativeness” are quite complex though. Requirements for habitat and food resources vary considerably across species, and urban vegetation’s delivery of necessary resources is ultimately more important than plant origin [98, 99]. The success of wildlife in cities depends on species’ ability to acclimate to suboptimal resources in the urban environment. Additionally, even if an urban space contains primarily native plant species, native wildlife abundance and richness may be low due to competition with nonnative biota which are better adapted to the urban surroundings. Relationships between urban vegetation and urban wildlife are discussed in detail under “Importance of Vegetation for Urban Wildlife” below.

2.3 Challenges

2.3.1 Availability or access

While the benefits of green spaces in urban areas are extensive, a major challenge regarding urban vegetation and green spaces is equitable access and availability for all communities. In urban centers where availability of green space is already scarce, preferential distribution has been shown to benefit more affluent communities [17]. Low-income communities have more limited access to green spaces, or lower quality and less maintained green spaces [67]. This may contribute to socioeconomic inequality and inequity and can result in poorer health and psychological impacts as well. Moreover, city planners and horticulturists must be aware that inequitable green space construction can further marginalize low-income populations and communities of color. There have been efforts to combat this by installing more green spaces in communities where there are none, but this may lead to gentrification, resident displacement, and risk further marginalizing these communities [17]. To avoid “ecological or green gentrification,” developers and city officials must consider the impact of improvements on property values and focus environmental cleanup and green development according to community needs rather than applying generic approaches [17, 18]. These approaches can include community gardens, restoration, or small-scale sites that are “just green enough” to provide community benefits without displacing the residents they were originally meant to serve [17]. Additionally, better access to green spaces can narrow the magnitude of differences between socioeconomic classes [67]. Access can also refer to accommodation of disabled populations. Health inequality is an issue of concern in most urban centers, and by addressing access to green spaces and risks associated with poorly maintained spaces, overall public health can be improved. Fortunately, laws regarding accommodations for disabled persons such as wheelchair access, smooth pathways, etc. have become more common. However, not all public greenspaces are equipped with these modifications due to infrastructure, expense, and labor limitations. This further supports the need of city officials to include adaptations to improve access to green spaces to include a more diverse population with variable needs.

2.3.2 Adoption and acceptance

Alternative garden and yard designs and adoption of sustainable practices (e.g., using native plants, reduced fertilizer, reduced water, locally-sourced materials) may require fewer external inputs and be more ecologically friendly [55]. However, these landscapes may be less attractive to traditional landowners than conventional yards, due to perceived increases in maintenance [100] and personal environmental attitudes [101]. Additionally, adoption of alternative designs and sustainable practices may have high costs up front [102, 103], and it may take months to years to see the benefits [104] whereas homeowners expect fairly immediate and obvious benefits. If results are slow or ambiguous, people lose interest and support for the practice or species diminishes. Although people report wanting to increase sustainability and the associated benefits of vegetated areas in cities, many are less willing to personally adopt certain practices [105, 106]. Other studies have reported that residents are willing to adopt sustainable practices and alternative garden and yard designs if the perceived benefits outweigh the costs and any downsides associated. For example, Helfand et al. [107] reported that consumers were willing to pay more for esthetically pleasing prairie gardens which provided more ecological benefits than traditional turf lawns, despite increases in costs associated with the native plantings. Turf replacement participation is significantly correlated with replacement rebate rates, owner-occupancy, median income, and residents’ environmental attitudes [101, 108]. Residents who adopt environmentally friendly landscapes can increase adoption in their neighborhoods; Pincetl et al. [108] reported that 36% of properties with turf replacement had nearby properties with partially- or fully-replaced landscapes. Personal preferences may deem some species and maintenance practices as suitable for a resident’s personal yard, but unsuitable for public green spaces, and vice versa. For example, dense vegetation around residents’ personal property may provide them with a sense of privacy, but dense vegetation in public spaces such as parks may decrease residents’ sense of safety, especially at night [109, 110].

2.3.3 Invasiveness and escape

Non-native plant species are typically more tolerant of the altered microclimates, hydrology, and soil structure that characterize urban areas. Non-native plants also experience less pressure from competing vegetation and natural enemies (i.e., pests, herbivores) in urban environments than natural areas [93]. These characteristics encourage the spread of introduced species from areas where they were originally planted. This spread is not limited to the neighboring urban surroundings; urban vegetation often escapes into outlying rural areas, where it can have negative impacts on natural vegetation and ecosystem processes.

2.3.4 Negative attributes

There are some concerns that urban green spaces may exacerbate urban residents’ allergies and rhinitis. There are several studies that have shown both improvements in allergies and negative impacts on allergies [78, 111]. In contrast to desired objectives, urban vegetation can act as a source of pollutants and chemicals into the urban environment [56]. Despite being highly pervious, vegetated areas may be sources of nutrients, herbicides, insecticides, sediments, and fecal bacteria. Fertilizers and pesticides applied to vegetation may runoff or leach out. Since these spaces often provide recreational opportunities and habitat, deposition of pet and wildlife feces commonly occurs. Vegetated sites may also serve as secondary sources of contaminants; resuspension or leaching may release pollutants originating from other sources [56, 112]. For this reason, it is essential to determine the pollutant loads and/or concentrations that the structure will receive and the percent reduction to be achieved prior to selecting the type of stormwater control measures or green infrastructure to be used and how it will be arranged in the urban drainage system [113].

Urban vegetation may intercept gaseous pollutants and particulate matter to a high degree, and vegetation grown on polluted sites may also take in contaminants through their roots. In turn, urban residents may be exposed to these pollutants when interacting with urban vegetation. Particulate matter may be deposited on edible plant parts, and/or the particulate matter deposited on foliage or taken up through the roots may be translocated to fruits and vegetables [114, 115]. Adverse health effects can occur through ingestion, inhalation, or skin contact with contaminated foods, soil, or non-food plant matter [78, 116]. Because we tend to focus on the health-positive aspects of vegetation in urban environments, people may not consider the unintended, potentially harmful effects on their health from urban vegetation.

3. Importance of vegetation for urban wildlife

Awareness of the conservation potential of urban wildlife goes back several decades [117, 118], with some mentions emerging even earlier (see [119], pp. xi–xii; 2005). Early work has given way to increasingly complex and nuanced research on urban ecology. Yet conservation-oriented studies of urban vegetation have often not touched on wildlife to any great extent [120, 121] and vice versa [122, 123]. A separate thread, reviewed elsewhere in this chapter, has focused on urban vegetation as a design component concentrating primarily on esthetics and human wellbeing [124] . Until recently, relatively little of this literature emphasized the importance of using native species. A third theme, the most recent, has emerged as awareness of the impacts of global climate change on urban areas has grown and concentrates on the value that urban vegetation can offer in increasing shade, taking up CO₂ and other pollutants, etc. [125]. This discussion, too, has mostly occurred independently of the other two, with ecosystem services being considered separately from biodiversity and from human health and esthetic pleasure. Instead, we advocate a One Health-based approach [126]; for an urban application see [127] that jointly addresses the wellbeing of wildlife, humans, and the environment (Figure 5).

Figure 5.

One Health-based approach to urban vegetation, wildlife, environment, and humans. Figure courtesy of Gad Perry.

Ever since the pioneering work of MacArthur and MacArthur [128], ecologists have been explicitly aware that animal diversity depends on the diversity of plant-created habitats. Wildlife biologists have adopted an “if you build it they will come” attitude toward supporting the populations of species they considered desirable via intentional manipulation of environmental vegetation structure. “In a landscape planting scheme for wildlife, it is also important to consider vegetation” ([119] p. 102). This is doubly true in urban environments, which by definition provide a structurally simplified habitat with limited and unfamiliar characteristics.

What do we know about the importance of vegetation structure for urban wildlife? In a series of studies on urban gardens in the United Kingdom, Smith et al. [129] documented urban vegetation and the variation among sites, found that most of the plant diversity was comprised of non-native species, and concluded that “garden floras may offer many more resources to wildlife than is implied by their species composition” [129]. In a separate study from the Central Arizona–Phoenix Long-Term Ecological Research site in the United States, Lerman and Warren [130] more explicitly connected urban plant and bird diversities. They found that that the abundance of native desert birds increased as native plant landscaping was emplaced and that racial and economic identity had important bearing on resident access to this biodiversity. The greater animal biodiversity associated with native vegetation appears to be a broad pattern, though non-native plant species also support substantial animal biodiversity [43].

Studying related issues in Baltimore, Maryland, USA, Rega-Brodsky et al., [131] surveyed the plant and bird communities in 150 vacant lots and found that the lots supported biodiversity but did not meet the esthetic preferences of residents, who preferred lots with more trees that they perceived as “well-maintained.” This type of vegetation, termed Urban Spontaneous Vegetation (USV) by Riley et al. [132], sustains urban wildlife and provides residents with a connection with nature and human physical and mental health benefits. Rega-Brodsky et al. [131] suggested that planting additional trees in vacant lots can simultaneously address the inclinations of neighbors while improving habitat for urban birds. However, some of the wildlife supported by USVs is considered objectionable, whether for esthetic reasons or their ability to vector human diseases [132].

Birds and pollinators remain the most studied taxa in this context [43], but there are some studies on other vertebrates. Although vegetation and its structural importance do not appear in the titles of any of the 40 chapters or 13 case studies presented in Mitchell et al. [133], there are some reptile and mammal studies [43]. For example, in a lizard study from Hobart, Tasmania, Australia, Jellinek et al. [134] found that reptile communities were influenced by vegetation type and structure. Similarly, while residential gardens in Australia support native mammals and have conservation value, the vegetative structure and diversity and their possible influences were not studied by Van Helden et al. [135, 136]. Likewise, although urban green spaces in Accra, Ghana, supported some mammal diversity, the importance of specific vegetative traits was not explored [137]. However, studies of mammals in forest patches in Durban, South Africa, showed that mammal diversity was positively affected by vegetative structure in urban forest remnants [138, 139]. The recent review by Brum et al. [140] did not indicate that the importance of vegetation structure has been of great interest in the herpetological literature, a definite research gap, and the situation appears to be similar in the urban mammal literature.

The relationship between vegetation structure and animal diversity is not limited to vertebrates and pollinators. In a multi-taxa study focusing on urban forest patch size in Basel, Switzerland, Melliger et al. [141] found that arthropod diversity depended on trophic rank and habitat specificity, with vegetative structural diversity predictive of ant and spider diversity. Plants species richness decreased with urbanization, but even small urban forests held substantial arthropod biodiversity [141]. More recently, Mills et al. [142] showed that more vegetatively diverse urban green spaces also had greater soil microbial diversity. Similarly, Francoeur et al. [143] reported that urban lawns with greater vertical complexity in Montreal, Canada, had greater arthropod biodiversity.

Finally, it is important to remember that the relationship between plants and animals is bi-directional. While urban plants provide essential physical structure and food for existence and sustenance of wildlife, the ecosystem services that wildlife provides for urban plants cannot be ignored either. Perhaps most obvious is the role of pollinators. Cities hold a greater diversity of pollinating insects than do nearby agricultural areas, especially in areas where hardscapes are uncommon [144, 145]. In return, those insects help fertilize urban plants and ensure their propagation, a service particularly important for urban farms. No less important, urban birds provide essential dispersal services for fruiting plants [146], just as adding artificial perches can attract birds and enhance plant dispersal in restoration projects [147].

4. Lubbock, Texas, USA: a case study

Lubbock, Texas, USA (Figure 6), is a medium-sized city, with a population of approximately 261,000 [148], that is located on the southern Great Plains of North America (33.5779 ^oN, 101.8552 ^oW). The climate of Lubbock is semi-arid, receiving an average annual rainfall of 48.6 cm, snowfall of 22.9 cm, and with a mean annual temperature of 15.8°C [149].

Figure 6.

Map of the USA, Texas, and Lubbock.

Lubbock provides a unique setting for evaluating the impacts of urban vegetation on wildlife species. Except for native riparian vegetation found on the eastern edge of town, most of the urbanization is occurring into areas of either native rangeland or agricultural fields, most of which are planted primarily in cotton (Gossypium hirsutum). Few trees are found in the native rangeland, with most being honey mesquite (Prosopis glandulosa). Scattered throughout Lubbock are a series of small, modified playa lakes, predominantly with permanent water and serving as part of a runoff control system that also has a role in public recreation. The dominant trees found naturally along playa edges are cottonwood (Populus deltoides) and black willow (Salix nigra). Thus, the majority of trees and shrubs found in town are a result of plantings as urbanization has proceeded over time. Table 1 lists the trees and shrubs commonly planted in parks and residential areas in the rapidly urbanizing portions of Lubbock (to the west, the south, and the southwest). Note that these contain a mix of native and introduced species. Many of the species listed as native to North America were not historically found in this region.

Common name	Scientific name	Native status
American elm	Ulmus americana	Native
Ash	Fraxinus sp.	Some species native, some introduced
Bald cypress	Taxodium distichum	Native
Black walnut	Juglans nigra	Native
Black willow	Salix nigra	Native
Cedar elm	Ulmus crassifolia	Native
Chinese Elm	Ulmus parvifolia	Introduced
Chinese pistache	Pistacia chinensis	Introduced
Cottonwood	Populus deltoides	Native
Crepemyrtle	Lagerstroemia sp.	Introduced
Desert willow	Chilopsis linearis	Native
Golden rain tree	Koelreuteria paniculata	Introduced
Honey locust	Gleditsia triacanthos	Native
Honey mesquite	Prosopis glandulosa	Native
Juniper	Juniperus sp.	Native
Lilac	Syringa sp.	Introduced
Live oak	Quercus virginiana	Native
Maple	Acer sp.	Native
Mulberry	Morus sp.	Some species native, some introduced
Pear	Pyrus sp.	Introduced
Pecan	Carya illinoinensis	Native
Persimmon	Diospyros sp.	Some species native, some introduced
Pine	Pinus sp.	Native
Red oak	Quercus sp.	Native
Sophora	Sophora sp.	Native
Sugar hackberry	Celtis laevigata	Native
Sumac	Rhus sp.	Native
Sweet gum	Liquidambar styraciflua	Native
Sycamore	Platanus occidentalis	Native
White oaks	Quercus sp.	Native
Yaupon	Ilex vomitoria	Native

Table 1.

Trees and shrubs commonly planted in parks and residential areas in Lubbock, Texas, USA.

Several studies have been completed in recent years on the wildlife in Lubbock, all of which included some aspect of the impact of vegetation on animal distribution, diversity, richness, or population density. Organismal groups included amphibians [150], birds (Coldren unpublished data, and Freeman [151]), and mammals [152]. Table 2 lists the major species evaluated in each group, which again include a mix of native and introduced taxa, as well as some that are native to North America but not originally found in this region.

Common name	Scientific name	Native status
Amphibians
American Bullfrog	Lithobates catesbeiana	Native to North America but not Texas
Great Plains Narrow-Mouthed Toad	Gastrophryne olivacea	Native
Plains Spadefoot	Spea bombifrons	Native
Spotted Chorus Frog	Pseudacris clarkii	Native
Texas Toad	Anaxyrus speciosus	Native
Birds
American Robin	Turdus migratorius	Native
Barn Swallow	Hirundo rustica	Native
Blue Jay	Cyanocitta cristata	Native
Chimney Swift	Chaetura pelagica	Native
Curve-billed Thrasher	Toxostoma curvirostre	Native
Eurasian Collared-Dove	Streptopelia decaocto	Introduced
European Starling	Sturnus vulgaris	Introduced
Great-tailed Grackle	Quiscalus mexicanus	Native
House Finch	Haemorhous mexicanus	Native
House Sparrow	Passer domesticus	Introduced
Killdeer	Charadrius vociferus	Native
Mississippi Kite	Ictinia mississippiensis	Native
Mourning Dove	Zenaida macroura	Native
Northern Cardinal	Cardinalis cardinalis	Native
Northern Mockingbird	Mimus polyglottos	Native
Red-winged Blackbird	Agelaius phoeniceus	Native
Rock Pigeon	Columba livia	Introduced
Western Kingbird	Tyrannus verticalis	Native
White-winged Dove	Zenaida asiatica	Native to Texas but not Lubbock
Mammals
Coyote	Canis latrans	Native
Domestic Dog	Canis familiaris	Introduced
Feral Cat	Felis catus	Introduced
Gray Fox	Urocyon cinereoargenteus	Native
Raccoon	Procyon lotor	Native
Red Fox	Vulpes vulpes	Native to North America but not Texas
Virginia Opossum	Didelphis virginiana	Native

Table 2.

Focal wildlife species studied in Lubbock, Texas by Carter [152], Coldren (unpublished data), Freeman [151], and Ramesh [150].

Ramesh [150] found that the most important features explaining the presence of amphibians at lakes in the urban zone were the type and structure of vegetation adjacent to lakes, as well as the land use matrix and age of development surrounding the lakes. Lakes with amphibian detections tended to be in areas of more recent development and supported greater cover of native emergent and fringing vegetation. This vegetation provided pollutant filtration, oviposition sites, and refuge from predators. Lakes in older parts of town were located in parks more heavily managed and manicured (Figure 7), and they supported fewer amphibians. In these parks, emergent and fringing vegetation were constantly being removed and the surrounding turfgrass was regularly mowed, reducing their suitability for amphibians. Additionally, the land use matrix surrounding lakes in older developed areas contained more roads, more impervious cover, and less vegetated areas. This resulted in reduced rates of movement between lakes, effectively eliminating the ability of amphibians to recolonize lakes from which they had disappeared.

Figure 7.

Typical vegetative structure in an older park in Lubbock, Texas, USA. Photo courtesy of Cade Coldren.

Two recent studies have been conducted on birds in Lubbock. Freeman [151] evaluated the effects of human population density, canopy cover, neighborhood age, and distance to the city center in relation to the distribution and richness of three groups of birds. Exploiter species such as rock pigeons and sparrows are often non-native and adept at exploiting human-controlled resources. Adapter species such as great-tailed grackles and American robins are native birds that have adapted to human-dominated landscapes. Uncommon species such as the curve-billed thrasher are native species that are not well adapted to urban or suburban settings. The best predictor of exploiter species presence was neighborhood age, which also served as a surrogate for a diverse vegetative structure, since plantings mature over time. Human population density and neighborhood age were found to be the best predictors for adapter species. Canopy cover was also found to be a good predictor of adapter species as it was generally accompanied by greater vertical stratification of vegetation, and therefore supported a greater diversity of birds. The uncommon species were found on the periphery of town, but also in areas where remnant native vegetation occurred. Additionally, socioeconomic and educational factors were found to play a role, as higher education and greater disposable income tended to result in greater vegetative diversity, and thus greater bird diversity and richness. Overall, it appears that decisions by individual homeowners regarding plantings in gardens and yards have a greater impact on birds than city planners do.

In unpublished data, Coldren investigated the distribution and density of native and non-native bird species in 20 parks across the urbanizing areas of Lubbock. Vegetative and land use data included in the analyses were tree diversity, mean tree canopy cover, herbaceous cover, development within a park, water, and several metrics for a 250 m buffer around each park, including impervious cover, green space, and extent of residential, commercial, and industrial areas. Cluster analysis revealed three distinct clusters based on differences in vegetative characteristics. The first cluster included the older parks with more mature trees and highest tree diversity and mean tree canopy cover (Figure 7). These parks tended to be the closest to the center of town. The second cluster was intermediate in terms of tree diversity and mean tree canopy cover, and these parks tended to be found at a greater distance from the center of town. The last cluster were the youngest parks, with the lowest tree diversity and mean tree canopy cover. These parks were the farthest from the center of town and were also in areas with the highest commercial development and greatest amount of impervious cover. Avian diversity declined from the first cluster to the second, and was lowest in the third, although these differences were not statistically significant. Interestingly, density of individual bird species tended to peak in the park cluster with intermediate tree diversity and mean canopy cover, while it was lowest in the cluster of youngest parks.

Carter [152] installed cameras throughout Lubbock to capture occurrences of mammals and evaluated various metrics of vegetation and land use within 200 m of each camera to assess which factors appeared more important in understanding mammalian distributions across the urban zone. Vegetation and land use metrics included canopy cover, green space cover, manicured grass, bare soil, anthropogenic cover, water, total road length, and the distance to the nearest 4-lane road. Carter [152] found that most of the species captured on camera were not associated with any of the habitat variables under consideration, although there were exceptions. Coyotes were seen only on the periphery of town, in areas with some intact native rangeland vegetation. Raccoons tended to occur in areas with greater canopy cover, as well as surface water and patches of wetlands. Gray foxes were associated with lower canopy cover in summer, but not the rest of the year. Feral cats had a positive association with green space cover in autumn, but negative associations with all vegetative variables in winter. Additionally, feral cat distribution and density appeared to be tied to food sources, whether supplied directly by humans or indirectly through the occurrence of prey species such as rodents and small birds, which tended to be found in areas with greater vegetative diversity and structure.

Considering the three organismal groups, it appears that wildlife use of urban areas of Lubbock is more strongly based on the age, structure, and diversity of vegetation, rather than the presence or absence of individual plant species (Table 1). The wildlife species found in these studies (Table 2) tend to be generalists and are not known to be reliant on one or a small suite of plant species. Vegetative age and structure tend to be a result of diverse plantings, including trees, shrubs, and herbaceous plants, and the time necessary for them to become mature plants. This appears to be the strongest requirement for the greatest diversity and use of urban vegetation by wildlife in this city.

5. Urban plants and their portrayal in the media

Traditionally, urban vegetation is not often covered by the media. Compared to animals, where more negative or unusual occurrences between wild animals and humans lead to media coverage spikes [153], urban plants do not seem that newsworthy. However, in the past years, especially with COVID-19 lockdowns, climate change and crisis focus and solution journalism, increasingly more news stories about plants emerged. These same topics also gained traction in public spheres and public online spaces. For example, on platforms like TikTok conservation experts are trying to educate the public, promote conservation and strategic gardening, and offer advice on what people can do to bring or save native vegetation in their own spaces.

While it is well documented that news media impact public opinions and can further impact actual policies [154, 155]. The related literature is also scarce—more recently a few studies looked at plant pest invasion coverage in the news and social media [156, 157]; or urban plant foraging attitudes [158].

Generally, plants covered in the media fall into the following categories:

1. Domestic—domestic plants, such as house plants, flower shops/flower markets, flowers in parks, gardening, edible plants etc.

2. Wild—wild plants such wild vegetation in the city, or wild parks, or forgotten poorly maintained areas.

3. Useful/utility—how and why plants can be used or help people (e.g., recipes, cures, prevention medicine, but also helping mental health, or general human health, etc.).

4. Dangerous/damaging—how and why plants can hurt humans or their environment (e.g., poisonous, allergies, financial damage such as trees falling over houses/cars, deaths, etc.).

5. Gray area—species such as cannabis that are somewhat controversial.

We conducted a non-exhaustive analysis of 100 news stories published in 2022. These news stories were mostly U.S. based and were extracted from repeated searches in news aggregators such as Google News, Yahoo, etc. Before analysis, we verified that each story indeed focused on urban related vegetation. Besides ensuring this criterion, stories were not further selected or checked for content until the actual analysis.

Our findings show some positive trends and some concerning ones. The majority of the analyzed news stories focused on wild urban vegetation and the usefulness or utility of vegetation (see Figure 8).

Figure 8.

A breakdown of 100 popular media news stories about wild urban vegetation. These categories were not mutually exclusive, and some news stories contained more than one angle, so they appear more than once.

Furthermore, compared to wildlife, where our previous analysis [153] showed media vilified animals and emphasized their dangerous traits, in the context of vegetation the majority of the news stories focused on the positive traits of plants and how they can be useful. These were diverse and solution- or how-to stories about topics such as:

• The need to integrate more native plants in cities, lawns, and gardens;

• The case for/need for controlling or sometimes even using wild invasive plant species that thrive in challenging environments;

• Urban revegetation and positive effects on humans; going foraging in the city; etc.

Often these stories were almost militant or activist about the high need to integrate wild plants and more general wildlife in urban areas in order to mitigate the negative effects of the climate crisis. We found fewer stories that were focused toward the negative or damaging attributes of wild plants, about topics such as invasive species displacing native plants, the need to pay attention when foraging as some plants are not edible or could even cause severe health damages or death, etc. Finally, compared to our previous wildlife analysis [153], in this current case, it is also encouraging that the majority of these news stories about wild urban vegetation used experts as sources. Experts were quoted, paraphrased, and usually given a good amount of news space. Taken together these findings suggest there is an increasing interest in positively covering such topics especially through solution journalism or how-to news stories and offer a platform for conservation and vegetation experts to share their knowledge and tips. Scientists should thus take advantage of these opportunities and continue to better educate the public in regard to how to best use, protect, and revive wild urban vegetation.

6. Conclusions

In conclusion, urban vegetation is important for humans and wildlife. Not only does urban vegetation provide ecosystem services such as wildlife habitats, climate mitigation, and more, but it also promotes human health, beautification, and psychological benefits. Plants are generally viewed as positive contributions to an environment, but many factors can affect this perception. Non-native species, water usage, and adverse associations with safety or health factors can negatively affect the functionality or services plants provide. There are also issues regarding accessibility, gentrification, and greenwashing that detract from beneficial aspects of plants. Yet, overall, the positive benefits and services plants provide in urban areas are widespread and affect humans and animal species. Conservation and education accomplishments have come a long way but continued efforts will ensure progress is made on these fronts.

Conflict of interest

The authors declare no conflict of interest.