Urban plant diversity in Los Angeles, California: Species and functional type turnover in cultivated landscapes

Abstract

Societal Impact Statement People plant, remove, and manage urban vegetation in cities for varying purposes and to varying extents. The direct manipulation of plants affects the benefits people receive from plants. In synthesizing several studies of urban biodiversity in Los Angeles, we find that cultivated plants differ from those in remnant natural areas. This highlights the importance of studying cultivated plants in cities, which is crucial for the design and planning of sustainable cities. Residents have created a new urban biome in Los Angeles, and this has consequences for associated organisms, ultimately resulting in a responsibility for society to determine what type of biome we wish to create. Urbanization is a large driver of biodiversity globally. Within cities, urban trees, gardens, and residential yards contribute extensively to plant biodiversity, although the consequences and mechanisms of plant cultivation for biodiversity are uncertain. We used Los Angeles, California, USA as a case study for investigating plant diversity in cultivated areas. We synthesized datasets quantifying the diversity of urban trees, residential yards, and community gardens in Los Angeles, the availability of plants from nurseries, and residents’ attitudes about plant attributes. Cultivated plant diversity was drastically different from remnant natural areas; compared to remnant natural areas, cultivated areas contained more exotic species, more than double the number of plant species, and turnover in plant functional trait distributions. In cultivated areas, most plants were intentionally planted and dominated by exotic species planted for ornamental purposes. Most tree species sampled in Los Angeles were available for sale in local nurseries. Residents’ preferences for specific plant traits were correlated with the trait composition of the plant community, suggesting cultivated plant communities at least partially reflect resident preferences. Our findings demonstrate the importance of cultivated species in a diverse megacity that are driven in part through commercial distribution. The cultivation of plants in Los Angeles greatly increases regional plant biodiversity through changes in species composition and functional trait distributions. The pervasive presence of cultivated species likely has many consequences for residents and the ecosystem services they receive compared with unmanaged or remnant urban areas.

The importance of street trees to urban avifauna

“…In more mesic portions of the world, where native tree diversity is higher in locations adjacent to cities, relying more on native tree species that are suitable for urban environments (e.g., tolerance to air pollution; Grote et al 2016) may be an appropriate strategy when considering planting street trees (Jenerette et al 2016). However, this may not be optimal for a city such as LA, or other cities in arid regions of the world with relatively poor tree diversity in lowland areas outside the city boundaries (Avolio et al 2019). Thus, for many municipalities, nonnative street‐tree species likely need to be considered when thinking about a resilient urban forest canopy, which is a similar conclusion for cities elsewhere in the world (Sjöman et al 2016).…”

“…Vegetation in the urbanized areas experiences variable growing conditions throughout the year, depending on irrigation patterns, planting practices, and geographic position in the city. For example, there are over 1,000 species of nonnative plants throughout LA (Avolio et al 2019), and each likely has unique phenological patterns that may influence bird‐feeding behavior (Appendix ). Patterns of precipitation and temperature are also highly variable throughout the region (yearly averages: 19°C/13°C high and low temperatures and 379 mm precipitation).…”

Valuing the Role of Time in Urban Ecology

“…Complex urban legacies that arise from both the ecological system and the coupled social and technological system and their interactions deserve closer investigation. For example, human factors can influence horticulture and landscaping resulting in preferences of the past influencing current vegetation composition and structure (Ossola et al, 2019;Avolio et al, 2020). In addition, the spatial spread of tree diseases such as Dutch Elm disease through a population of urban trees is influences by the spatial distribution of that species.…”

Time Is Not Money: Income Is More Important Than Lifestage for Explaining Patterns of Residential Yard Plant Community Structure and Diversity in Baltimore

“…Given their high land cover, residential yards have been proposed to play a role in conservation of biodiversity in cities (Goddard et al, 2010;Lerman and Warren, 2011). Indeed, plant diversity in residential yards is often higher than in surrounding natural ecosystems (Pearse et al, 2018;Avolio et al, 2019b). While cultivated urban plant biodiversity is controlled, to an extent, by traditional ecological drivers such as climate (Jenerette et al, 2016;Padullés Cubino et al, 2019b;Pearse et al, 2018) and space availability (van Heezik et al, 2014;Padullés Cubino et al, 2019b), it is also shaped by residents' preferences and management activities, which may be influenced by their learned experience and social norms (Roy Chowdhury et al, 2011;Cook et al, 2012;Politi Bertoncini et al, 2012;Avolio et al, 2018).…”