Morphological variables explain most of the variation in UTC across Fort Collins
Urban morphological characteristics consistently had stronger correlation than social-demographic or lifestyle characteristics, and also described more of the variability in UTC within Fort Collins based on their total impacts. Median building age and housing density had some of the strongest impacts on UTC variability. In Fort Collins, we expect that morphological characteristics have the strongest impact due to the city’s relatively early stage in urbanization, as the city has only recently experienced rapid growth in population and infrastructure. New neighborhoods are mostly being developed on the outskirts of the city, primarily replacing natural, semi-arid grassland. This is in contrast to the neighborhoods that have been long established in the central part of the city (e.g. Old Town), where much of the current UTC was planted decades ago.
Because Fort Collins’ rapid growth only started in the 1990s, UTC is best described by morphological characteristics that are consistent with conditions that existed when many trees were planted, rather than the current social-demographic or lifestyle characteristics that are only beginning to develop. The importance of temporal trends has been seen in other studies, where correlation between UTC and social-demographic or lifestyle characteristics depends on the amount of time since urbanization (Boone et al. 2010; Troy et al. 2007; Lowry et al. 2012). In many cases, the physical development of the city is innately connected to the underlying social-demographic patterns (Williams et al. 2000), but these patterns are extremely dynamic over time.
Divergence of Fort Collins UTC predictors from larger, temperate cities
We found a significant and negative relationship between grass cover and tree cover, as might be expected given the local biome. Nowak et al. (1996) assessed the distribution of UTC in 58 U.S. cities and found UTC to be lower in cities situated in grasslands, which also tend to have more agricultural land. Nowak and Greenfield (2020) stated that in drier grasslands, unmanaged land will also not naturally regenerate with trees and will have lower UTC unless tree planting and watering programs are established. These findings may explain the inverse relationship we see between grass and tree cover. Because Fort Collins is still in the process of urbanizing, areas on the outer edge of the city have yet to be transformed from grassland and agricultural land to a more urbanized landscape that can accommodate a larger population, and it may take years before trees are fully established. This differs from most cities in temperate regions where natural tree cover is often being removed to make way for new development (Heilman et al. 2002; Hostetler et al. 2013; Bonney and He 2019).
We found that Fort Collins block groups with older houses were associated with greater tree cover. This is supported by findings in Raleigh, NC, but in contrast to Baltimore, MD (Bigsby et al. 2014). Additionally, Conway (2009) analyzed vegetation in Toronto, Canada and found that older houses had less overall vegetation than newer houses. We speculate that these contrasting results may be explained by the climate or biome of Fort Collins; this semi-arid ecosystem has few native tree species that mostly occupy riparian areas, resulting in a planted and heavily-managed UTC. Much of the vibrant UTC has long been planted and maintained throughout the oldest parts of the city, whereas the UTC in newer developments has been recently planted as the city undergoes rapid population growth.
We also found higher UTC in areas with with higher housing density, while other studies have found that areas with higher house density contain less tree cover. In a study by Iverson and Cook (2000) that took place in Chicago, IL, the authors found tree cover to be strongly and inversely related to house density. In analyzing 29 different suburbs of Chicago, the authors evaluated a wider range of housing densities than those found in Fort Collins. However, it is interesting that Iverson and Cook (2000) found that the highest tree cover occurred in regions with 24.7–37.0 houses per hectare, which broadly matches the higher end of Fort Collins’ housing density of 0.6—38.8 houses per hectare. These similarities might point to common social preferences for tree cover within urbanizing areas. The nuanced finding within Fort Collins could be explained by the unique morphological development of the city, because housing density is highest in the oldest parts of the city where UTC has been created and maintained the longest. This contrasts with places like Chicago where the greatest density is no longer dominated by single-family housing.
The SAR model results revealed the only significant social-demographic characteristics for explaining the UTC distribution was the percentage of renter-occupied and 3 + person households. Previous work has suggested less UTC exists in areas with more disadvantaged populations, such as renters (see Riley and Gardiner 2020). The SAR model supports this in Fort Collins, suggesting that for every 10% more renter households, there was 1.1% less tree canopy.
However, we also found that the percent of 3 + person (non-family) households displayed a positive relationship to UTC, something we did not expect given its similarity to renters (see Fig. 2). The contrasting results between the rental population and 3 + person households may simply be explained by the fact that renters are not generally isolated to any particular region due to housing market constraints in recent years (Corona Insights 2019), whereas 3 + person households are often young, college students situated near Colorado State University and Old Town where there is a greater amount of tree cover. There has also been evidence of homeowners renting out portions of their home, such as a single room or a basement, resulting in more people per household. If this happens in neighborhoods with greater UTC, it could explain the positive relationship we are seeing with respect to 3 + person households.
Possible distributional inequities developing in Fort Collins
The SAR model indicated the percent of Hispanic / Latino population was associated with reduced UTC. Although the variable was not statistically significant (p = 0.202), removing the variable substantially increased the model AIC, supporting its potential importance in understanding the distribution of UTC. Examining the correlation coefficients associated of Fort Collins’ racial and ethnic groups reveals a positive relationship between UTC and the population percentage who self-identified as white / Caucasian and negative relationships for block group population percentages for those self-identifying as Hispanic / Latino or black / African American (Fig. 2). Although not significant, there was a difference in the coefficients for these three racial and ethnic communities in Fort Collins, which may indicate disparities developing within the city. Several studies have suggested that some disparities in UTC have reflected racial segregation (Flocks et al. 2011; Heynen 2003; Schwarz et al. 2015; Riley and Gardiner 2020). Fort Collins currently has low racial and ethnic diversity compared to larger, more mature cities in the US (see Table 1), yet based on our results, we still see potential distributional inequities that may be developing within the city.
Recommendations for future tree planting and maintenance
Studies on the distribution of UTC can have the power to inform future management decisions, indicating areas where we may want to prioritize tree planting and maintenance. We discuss planting and maintenance in the context of the “Three P’s” framework introduced by Grove et al. (2006). This framework can assist in maximizing the benefits of UTC while minimizing the potential disservices, as it considers areas for possible, preferable and potential UTC. Following previous research on planting guidelines (Locke et al. 2010), this analysis indicates areas where planting is both possible and preferable to maximize Fort Collins’ UTC.
Possible UTC includes areas where it is biophysically feasible to create and maintain UTC, which is shaped by the type of existing land cover (e.g. impervious vs. pervious cover). As Fort Collins continues to develop, more trees could be planted in recently urbanized areas dominated by grass, as these areas are currently associated with low UTC and high pervious cover, which is the most biophysically feasible land cover for tree planting. Conversely, impervious cover limits space for additional planting (see Nowak and Greenfield 2012; Coseo and Larsen 2019), and also limits the natural regeneration of trees (Nowak and Greenfield 2020), indicating maintenance is the preferred action to ensure adequate UTC is available in areas with high impervious cover. Maintenance will be especially important in impervious areas because of the offsetting effects tree shade has on the Urban Heat Island effect, a phenomenon that leads to higher temperatures in areas with more impervious cover (Zhou et al. 2017; Wang and Akbari 2016).
Extending beyond planting possibilities, cities considering preferable areas for increasing or maintaining UTC have the potential to reduce distributional inequities amongst communities (Flocks et al. 2011). We recommend areas with a greater percentage of minority-occupied block groups be a priority for planting to offset the identified early forming distributional inequity. Additionally, to prevent loss of existing UTC that could exacerbate possible inequities, maintenance should focus on areas vulnerable to future tree collapse from old age, or removal for pests and disease. For example, Fort Collins recently detected Emerald Ash Borer (EAB), an invasive pest that has decimated ash tree populations in other regions of the world (Herms and McCullough 2014), and it is expected that many public trees will be removed to help contain EAB spread. To prevent distributional inequities of UTC it is important to promote phased removal and replanting of public trees, otherwise these communities are at risk of becoming further underserved by the UTC.
There are many different contexts as to why distributional inequities appears in cities (Riley and Gardiner 2020; Nesbitt et al. 2019). We acknowledge that complex interactions take place between the social-demographic preferences of urban residents and the morphological development of cities that can influence UTC distribution. In Fort Collins, this is further complicated by the underlying semi-arid biome. These results should be framed within the context of local urban tree inventory data to understand the trajectory of UTC in newer neighborhoods. If these neighborhoods have the same number of planted trees as more mature neighborhoods, it is reasonable to assume that potential inequities are not attributable to social preferences or policy development, but rather to the slower process of ecosystem conversion; in which case, we simply need to allow more time for trees to establish.
The results of our study provide relevant information that can be used to guide future planning and development. The types of urban morphological patterns and social-demographic factors driving UTC in Fort Collins promotes ideas such as establishing zoning codes that require planting trees in new developments, targeting planting in areas with increased minority presence that are currently underserved by the UTC, and maintaining UTC in areas that will be susceptible to EAB.
Study limitations and future work
Most of our social-demographic variables were analyzed at the block-group scale. Due to Fort Collins’s relatively low population density, block groups cover a larger area, and do not necessarily aid in distinguishing local neighborhood dynamics or fine resolution changes in socio-demographics. The lifestyle variables we included in our analyses were not evenly distributed in the City, with some categories having much more representation in the city than others, and this clouds our understanding of the relationship between lifestyle and UTC, if indeed there is an important one. Further, Homeowners Associations are particularly popular in the west, and there is reason to believe that they are having an impact on neighborhood level management practices in Fort Collins (Rasmussen et al. 2021). Although, we did not separate out public versus private lands in our analyses, these are studies that we aim to do in the future. This kind of information is important for developing policies to support homeowners as they deal with EAB in Fort Collins, as it is a matter that will need strategic coordination between both public and private landowners.
Finally, since all of our land cover classes sum to 100% in each block group, there is an inherent correlation between each class and tree cover, and this needs to be considered when interpreting results. For the purpose of this study, we were mainly interested in the direction of these relationships, but further work can be done to understand their distribution and effects on one another. Future work can also expand on UTC relationships by comparing various spatiotemporal scales, as we expect different patterns to emerge under differing spatial and temporal conditions (see Locke et al. 2016). This work would greatly benefit by gathering unique household level data on social characteristics, which may require a more qualitative study using survey methods.
In Fort Collins, it will be especially valuable to consider legacy effects (see Troy et al. 2007; Bigsby et al. 2014), since the city has experienced rapid morphological and social change in a relatively short amount of time. We could also consider controlling for age in future analyses so we can isolate additional trends that may be temporally dependent. Additional research can be performed to compare Fort Collins to the neighboring city of Denver; since Denver is in a more mature urbanization stage, it may be valuable to investigate differing morphological, social-demographic and lifestyle influences on UTC between these cities with similar climates.