Urban greenspaces are extremely diverse, not only in their ecological contents and uses, but in their value to local residents. Vacant lots are a common type of informal urban greenspace [1
], especially in shrinking cities [3
], which are particularly controversial among city residents. Areas within a city are considered vacant after a demolition of a building and then its abandonment, never being utilized due to poor building conditions, or simply remaining as unoccupied areas of the city [4
]. Often considered a neighborhood eyesore, a place for crime and trash, or as dangerous, vacant lots are usually deemed a local problem for neighborhood residents. Even though vacant lots carry these negative social connotations, they also are locations that have enormous potential for increasing biodiversity and ecosystem services in cities [2
Over the past decade, vacant lots have increasingly been assessed for their ecological and social value across a number of the world’s cities [5
], however few studies have assessed these values together in the context of their characteristics and settings within the urban matrix. Each vacant lot within a city has unique features that either may constrain or promote ecological and social benefits for the surrounding community, and these features should all be evaluated together in order to maximize urban environmental potential [14
]. For example, vacant lots may be defined by their settings within a city block [15
], biological contents and habitat structure [12
], and social uses [12
], all of which are integral in determining how these spaces could be managed, their ecological integrity, and residents’ response to the lot.
Vacant lots are typically unmanaged lots; however, they are quite diverse depending on their location in the city and how the lot’s vegetation originated. Urban vegetation can be classified into three major groups: emergent, planted, and remnant vegetation [19
]. Depending on their origin, vacant lots can be characterized into one of these three vegetation groups. For example, vacant lots that are abandoned following a demolition often are dominated by weedy, dense shrubs, or have other forms of emergent, or ruderal, vegetation [19
]. These lots may have abundant and diverse plant life, which may provide increased primary production, additional sources of habitat for urban wildlife, air and water purification, and more regulating services [5
]. Plant communities in these sites are often dependent on the surrounding vegetation, as ruderal plant species are similar to species in adjacent habitats [21
], which may increase the potential for exotic invasive species colonization in emergent lots [19
]. Even though these emergent lots may be ecologically beneficial, these lots are often negatively viewed by residents as uninviting wastes of space [18
] due to their lack of cues to care [13
On the other end of the spectrum, vacant lots may be adopted by local residents and heavily managed through plantings and other landscaping efforts. Residents, especially those who live adjacent to vacant lots, may voluntarily manage the site as a way to create the impression of extending their own property lines, for recreational purposes, or as ways to beautify their neighborhood as an act of community pride. These planted vacant lots may be used as locations for community gardens or places for the community to gather. Even though these lots may provide areas of recreation, improve the community’s aesthetic value, and enhance community cohesion with positive resident associations [25
], they may not be beneficial in terms of urban biodiversity. Lots dominated by frequently mowed lawns or those that lack a shrub layer or other major sources of vegetation can reduce necessary food and cover resources for urban wildlife potential [8
]. Consequently, conflict may result due to this imbalance between urban biodiversity goals and residents’ needs and preferences without understanding how to simultaneously maximize both aspects.
Remnant vacant lots are patches within the urban matrix that have not been developed in recent history due to steep slopes, irregular shapes, or flooding hazards [18
] and remain unused or abandoned. Vacant lots with remnant vegetation tend to be rare and small in area, as the abundance of remnant vegetation tends to decrease along the urban-to-rural gradient [18
]. Due to their age, remnant vacant lots may have old, large trees and other forms of unmanaged vegetation, providing an abundance of habitat sources for wildlife [8
]. The biodiversity benefits of remnant vacant lots may juxtapose negative reactions by residents. Similar to emergent vacant lots, residents may not prefer remnant vacant lots in their neighborhoods due to their dense, unmanaged structure and composition. Conversely, residents may prefer remnant vacant lots for their natural features and species-rich plant and bird communities [29
], depending on the vegetation structure and residents’ attitudes and environmental knowledge [31
Beyond their vegetative characteristics, vacant lots also differ in their size, shape, and location within the city [15
]. Each vacant lot setting offers its own benefits and constraints regarding urban biodiversity and resident use. Vacant lots are often small in area and not conducive to development [32
], constrained by surrounding built features. For example, if a property becomes vacant within a city block, the size and contents of the vacant lot would be defined by the adjacent buildings. If the buildings are tall or surround the lot on three sides, vegetative growth may be hindered by a lack of direct sunlight [15
]. If a remnant vacant lot is large and takes up an entire city block, it may have high biodiversity potential yet low resident value if the site is not actively used or lacks indicators of management [33
]. Small, planted vacant lots may be ripe for community adoption as they are nearby residents’ homes, but may support little plant or animal diversity with large areas of lawn. Due to these conflicts between vacant lot origin, settings, and resident preferences, we ask if there is an achievable balance among multiple benefits for vacant lot management (Figure 1
The goal of our research was to synthesize multiple features of vacant lots to determine which combination is the most sustainable for future vacant lot management. There are many ways to manage a vacant lot for both biodiversity and people; however, the strategies that we use should maximize ecological goals and human values to ensure the longevity of the space and limit conflict [14
]. We may view the optimization of vacant lot settings, origin, and resident preferences as an urban sustainably goal, as these features promote biodiversity conservation, access to nearby nature for residents, and access to usable greenspaces, as defined by suiting residents’ preferences. Thus, in this paper, we define a vacant lot as sustainable if it can maximize both biodiversity and residents’ preferences within the constraints of a vacant lot’s setting and vegetation origin.
Our objective was to determine how vacant lot settings and origins constrain or enhance the biotic characteristics of the lot, and how these features influenced resident preference. If we define vacant lot sustainability by the maximization of biodiversity and resident preference, then vacant blocks and suburban yards containing a mixture of remnant and planted vegetation would be the most sustainable (Table 4
). These lot features and settings provided locations for biodiversity while also eliciting positive reactions from residents.
Even though vacant lots are transitional in nature, they are a major component of urban greenspaces in Baltimore. The majority of vacant lots were relatively small in size (<1 ha), yet they were abundant across the landscape. Even with limited interventions to maintain or improve these lots with further tree plantings and landscaping efforts, vacant lots can still have a major impact on urban biodiversity and sustainability goals.
We found that the major driver of lot sustainability was the origin of the lot’s vegetation. Remnant and planted vacant lots supported the most bird and plant life, while also being the most preferred lots as assessed by residents. The sustainability of remnant lots, planted lots, and suburban yards was primarily driven by their tree abundance, a positive feature for both urban biodiversity and residents (Table 4
The spatial distributions of vacant lots in Baltimore may impact their contents and structure. Lot origin may be dependent on the location of the vacant lot within the city, as remnant vacant lots are rare within more densely urbanized locations [18
]. This may also be seen in the lot settings, as suburban yard lots were found along Baltimore city’s periphery due to the location of neighborhoods dominated by single family homes. Even among lot settings evenly distributed across Baltimore (e.g., vacant blocks), tree abundance and canopy cover were highly variable (Table 1
). Tree cover is not evenly distributed across Baltimore, a consequence of urbanization, resident income [53
], social capital [54
], and pockets of resident resistance to tree plantings [55
]. Thus, the vacant lot’s location in the city may have a stronger impact on the lot’s vegetation rather than limitations imposed by the lot’s setting.
In previous studies, we found that trees were the most important feature for abundant native bird species [8
], while areas of dense shrubs best supported successful nesting efforts of common songbird species [26
]. Additional global studies highlight the importance of vegetation structure to increase bird species richness in a variety of urban greenspaces (e.g., [6
]). Thus, remnant vacant lots and lots with abundant trees should best support diverse and sustainable bird communities. Remnant vacant lots should be protected and, when possible, designated as park land to further enhance bird diversity [59
]. However, remnant lots are rare and do not occur throughout the city (Table 1
). To address citywide biodiversity needs, trees and small areas of dense shrubs could be incorporated into planted vacant lots. Through this approach, we may maximize wildlife benefits as well as providing features that residents prefer in more vacant spaces. Overall, conserving structurally complex, connected vacant lots should be a priority in urban ecosystems in order to support local biodiversity, provide ecosystem services, while also connecting people with nature [14
]. To further optimize remnant lots, walking paths or other cues to care may enhance the site regarding resident preference and their use.
Maintaining a mosaic of vacant lot settings and origins may be the best way to provide a variety of habitats in different successional stages, while increasing access to nearby nature for residents. Vacant blocks were the most abundant, and were often found throughout the city, potentially accessible to many residents. Their larger size is a benefit for urban biodiversity conservation, but may make the site seem daunting if vacant lot management efforts were left up to local community groups. Small missing tooth or corner lots may not have the greatest abundance of vegetation, but they can be easily landscaped by a small group of people and may foster more personal and private connections with residents [15
]. Thus, the vacant lot’s setting still needs to be considered prior to management as this feature can enhance or constrain the lot’s sustainability potential. Through simple modifications within the lot, such as picking up trash, planting a few flowers, mowing the grass, etc., many vacant lots have the potential to be positively viewed by residents and provide opportunities for residents to positively connect with nature.
Our resident survey results were indicative of Baltimore residents who were active in their communities, usually in a community group or nature-themed organization. We targeted these residents for our survey because they felt as though they could enact change within their neighborhoods. Overall, residents preferred lots that showed clear management efforts, contained trees and less artificial ground cover, and those that had potential for community or recreational use. The preference of large trees, vegetation complexity, and perceived naturalness is consistent with many landscape preference studies (e.g., [11
]). Our study continues to highlight the importance of a moderate level of human influence and management for these spaces, balancing the site’s natural features with enough management to encourage the sites’ recreational use [2
]. Our survey respondents overwhelmingly preferred for these sites to be used as a greenspace for community events, which is important in evaluating how a vacant lot’s setting may impact the potential for its use by residents.
Our resident survey results may be biased toward our sampled residents’ fondness for nature or those with some level of environmental education [31
], which may not be representative of all city residents. Some residents may consider wildlife as a nuisance and resist tree planting programs [38
]. Additionally, the public’s acceptance of these vacant lots may differ from their social acceptability, as acceptability is multidimensional, long-term management goal that incorporates the residents’ values, perceptions, and preferences for a habitat [66
]. Thus, short-term acceptance of transitional vacant spaces may differ from the social acceptability of neighborhood aesthetics and lot use. Input from more residents as to the future of a neighborhood vacant lot is vital, as residents are the most familiar with the lot and may have shared preferences for the lots’ future structure and use [47
In cities with an abundance of vacant lots, these spaces may not be temporary, as they are traditionally viewed. Especially for Baltimore, the transformation of vacant lots into useable spaces or forested parkland could be viewed as a long-term conservation management decision to increase neighborhood aesthetics, park land, and connect city-wide greenways [69
]. Planting trees in vacant lots may promote Baltimore government’s goal of doubling the city’s tree canopy to 40% by 2036 [70
]. Plant communities are slow to change in vacant lots, so management efforts are necessary to increase habitat connectivity and biodiversity across the city [22
]. Baltimore City officials currently manage vacant lots by periodically mowing accessible spaces, slowing the process of vegetative succession and limiting potential wildlife habitat. However, mowing efforts are often infrequent due to budget limitations and staffing, much to the dismay of residents who dislike the resulting tall grass. Grassroots efforts, either through adopt-a-lot programs or community organizations could be the key to managing vacant lots.
One major pillar of resource sustainability that we did not assess here is the equitable access and use of biodiverse vacant lots for all residents. Vacant lots tend to be clustered in low-income, population dense neighborhoods [12
]. Additionally, high quality greenspaces are often unequally distributed across cities by racial and economic lines [70
]. We found that many vacant lots were located in dense areas of Baltimore, and the sustainable suburban yards and remnant lots were often located away from the urban center (Figure 2
). Vacant lots in the center of Baltimore tended to be smaller inner blocks and corner vacant lots, which supported limited bird diversity and fewer plants, and often were emergent in origin. Even though inner-city neighborhoods would gain the most from transforming vacant lots into biodiverse, resident-friendly spaces, the resources to do so may be limited. This begs the question of whether vacant lots can be sustainable in these residential areas, or will they remain in poor environmental quality without substantial intervention? Monetary and other resource investments should be directed toward areas with abundant vacant lots, as these areas may offer the greatest in terms of overall greenspace size. These neighborhoods should be targeted by city sustainability managers and other stakeholders, as priority sites to manage for neighborhood enhancement.
Vacant lots can provide enormous resources to improve city-wide biodiversity and resident preferences as an urban sustainability goal. Even though sustainable management strategies for a vacant lot depends on the lot’s current features [5
], biodiversity needs can be balanced with resident’s preferences for vacant lot contents through simple management efforts. Particularly in shrinking cities where vacant land is often abundant, future research on vacant lots should incorporate these multiple levels of sustainability to assess impacts not only on people, but also on the urban ecosystem [25
]. Understanding how people perceive vacant lots and which aspects they prefer to have in their neighborhood is imperative in order to gain support from residents for any modifications [34