Socio-Ecological Dimensions of Spontaneous Plants on Green Roofs

Abstract

Green roofs have the potential to provide socio-ecological services in urban settings that lack vegetation and open space. However, implementation of green roofs is limited by high construction and maintenance costs. Consequently, green roof projects often disproportionately benefit wealthy communities and can further marginalise disadvantaged communities by increasing property values and housing costs. Vegetation cover on green roofs is crucial to their provisioning of socio-ecological services. Evidence suggests that green roof plantings change over time, especially with limited maintenance, and are replaced with spontaneous “weedy” species. This is often perceived as a failure of the original green roof design intent and spontaneous species are usually removed. However, where good coverage is achieved, spontaneous vegetation could provide beneficial services such as stormwater mitigation, habitat provision, and climate regulation. While social norms about “weediness” may limit the desirability of some spontaneous species, research suggests that their acceptability on green roofs increases with coverage. As spontaneous species can establish on green roofs without irrigation and fertiliser, reduced input costs could help facilitate adoption particularly in markets without an established green roof industry. Construction costs may also be reduced in hot and dry climates where deeper substrates are necessary to ensure plant survival, as many spontaneous species are able to colonise shallow substrates and can regenerate from seed. If implemented based on socio-ecological need, green roofs with spontaneous vegetation coverage may apply less pressure to property values and housing costs than conventionally planted green roofs, increasing the resilience of urban communities while limiting gentrification.

Coexistence and Succession of Spontaneous and Planted Vegetation on Extensive Mediterranean Green Roofs: Impacts on Soil, Seed Banks, and Mesofauna

“…Species richness dynamics over time showed two different trends: an increase for spontaneous species and a decrease followed by a stabilization in planted species. Loss of planted species over time is consistent with several previous studies under other climates [27,45,55,56,69,70] and can be the result of competition with spontaneous species that established continuously in the roof by seed rain from the surrounding vegetation and to the impossibility to planted species to finish their life cycle and to produce new seeds due to early drought [23].…”

“…Lastly, our study emphasizes the importance of heterogeneity, which allows for higher species richness establishing in different niches [27,53], compensating for a planted species loss trend generally observed in other studies [55,70].…”

Architectural Multispecies Building Design: Concepts, Challenges, and Design Process

“…They are used for their ecological contribution and building performance contribution both at the building level (mainly reducing thermal loads and preservation of thermal insulation [54]) and the urban level (mainly increasing biodiversity and preserving habitats for plants and animals [55]). Extensive roofs can be based on initial planting or left alone for nature to evolve, according to natural and social demands and local biodiversity [56].…”

Preferences of Adults for Synanthropic Flora in the Sustainable Development of Polish Cities’ Green Areas

“…2 By intentionally introducing or accepting the presence of synanthropic flora (spontaneous plants) by the people in charge, the resilience of plantings is increased [5,6], and so does the biodiversity [7,8]. This kind of vegetation reduces ecological and financial costs, it increases the resilience of urban vegetation communities [9]. Accordingly, synanthropic species and spontaneous greenery, in general, can be beneficial in the design process of green infrastructure projects prepared on various scales.…”

“…Synanthropic flora appears on green roofs as a result of cessation of maintenance, causing wilderness by default [7,13], or it is accepted during the maintenance process [5]. It could be also intentionally introduced [6,9]. Spontaneous plant species are also implemented in more sustainable farming systems [14] and even in show gardens [15,16].…”