Vegetation dynamics in the urban ecosystem and applied aspects: the role of environmental and human-related factors in shaping urban and peri-urban plant communities

The present research is developed in the frame of the Earth and Environmental Science and Technology (STAT) Doctorate program - Curriculum in Biology applied to Agriculture and the Environment - of the University of Genoa, and is co-funded by the EU H2020 FET-OPEN project ECOLOPES (https://ecolopes.org/). The research investigates how plant communities respond to multiple environmental and anthropogenic factors acting at different spatial scales along an urban - peri-urban gradient. The taxonomic and functional response of various plant communities is assessed, ranging from highly anthropogenic green roof (GR) assemblages to the natural and semi-natural vegetation of peri-urban areas. Among the various nature-based solutions (NbS) implemented in urban areas to improve both human and environmental quality, GRs have emerged as a valuable tool for providing multiple ecosystem services (ES) while mitigating human impacts. GRs consist of vegetated building rooftops, often regarded as artificial ecosystems due to their high degree of human engineering. From a plant perspective, GRs present challenging environments, with limited substrate and direct exposure to sunlight and wind. Despite these harsh conditions, GRs have demonstrated the ability to support rich, dynamic plant communities, including rare and threatened species, and have thus earned recognition as novel urban habitats. However, more research is needed on the relationship between plant communities and GR site-specific factors, particularly in the European Mediterranean region where GR systems are still sparsely distributed. Research on Mediterranean GRs (MGRs) is relatively recent and has mainly focused on their potential to provide ES or on selecting suitable (native) plant species able to withstand environmental constraints. Studies on spontaneous plant colonisation, however, remain scarce. While GRs are widespread in Central and Northern Europe, where several studies have addressed ecological issues, few have simultaneously assessed the effect of multiple human and GR-related factors on plant communities in real, non-experimental GRs. Therefore, a clearer understanding of the key factors influencing GR plant communities is needed. Although the effects of human disturbance are more evident in urban areas, anthropogenic impacts extend beyond city boundaries into peri-urban regions. These areas still host natural and semi-natural plant communities that perform important ecological functions and services, making their proper management a key component of NbS. In the Mediterranean region, human disturbances typically acting on peri-urban plant communities can be classified as Chronic Anthropogenic Disturbances (CADs), due to their low intensity but prolonged nature. Despite this, the overall impact of CADs on Mediterranean vegetation remains underexplored, with existing studies primarily focusing on forest ecosystems. Starting from urban ecosystems, the first part of the dissertation (Chapter 2 and 3) assesses how the diversity and composition of spontaneous and planted communities are affected by site-specific factors and management practices on GR systems, across two climatic contexts: Mediterranean and temperate. Chapter 2 investigates plant communities on two adjacent Mediterranean green roofs (MGRs) in Genoa (NW Italy), which primarily differ in substrate depth and composition. One roof is a typical extensive GR with a shallower substrate (20 cm) composed of a growing mix, while the other, resembling an intensive GR, features a thicker substrate (35 cm), predominantly made of clay. The aim is to explore how spontaneous plant assemblages vary with substrate type over time by analysing both taxonomic and functional composition, as well as biomass distribution. The field activity, conducted between June 2022 and October 2023, began with an initial survey of the stable spontaneous plant communities across the entire surface of the two roofs. From this, 28 1m² plots were defined, within which the plant biomass was completely removed. The plant communities colonising the plots were surveyed repeatedly, with biomass being removed and weighed after each survey. The functional aspects of the communities were also explored using Grime's CSR model, which identifies competitor (C), stress-tolerant (S) and ruderal (R) plant ecological strategies, determined using the 'StrateFy' method. Chapter 3 takes into account the temperate climate context, focusing on 75 GR assemblages. Conducted at the urban scale in Ingolstadt (SE Germany), the study aims to simultaneously assess the response of plant communities to multiple site-specific and human management factors. In summer 2023, plant communities were surveyed within 1m² plots, and data on GR height, size, age, substrate depth, and human management were collected. Substrate samples were also collected from the plots to calculate water holding capacity (WHC) and soil organic matter (SOM). The species recorded were classified as planted/sown or spontaneous, and taxonomic diversity indices (alpha and beta diversity) were calculated. The last part of this dissertation (Chapter 4) moves to the peri-urban area to investigate how Mediterranean plant communities around the city of Genoa are affected by CAD intensity. To achieve this aim, plant surveys were carried out in 48 plots and CADs were identified using the EU list of pressures and threats. A range of environmental factors were also collected and used as predictors in the analyses, alongside CAD data. Additionally, values for five functional traits were obtained from the TRY database for each recorded species to examine their relationship with CAD intensity at the community level. The overall results of the research on GR systems (Chapters 2 and 3) highlight that plant community diversity and composition are significantly influenced by substrate factors and time. Functional strategies also vary notably, with stress-tolerant species dominating on extensive MGRs and ruderality prevailing on intensive MGRs. Human management emerges as a key driver of plant communities, particularly in Ingolstadt, where weeding and/or mowing practices influences the overall alpha and beta diversity. On the Mediterranean GRs the effect of human disturbance is associated with the dynamic changes in species assemblages and biomass distribution after the removal of plant cover. In the peri-urban Mediterranean plant communities (Chapter 4), environmental factors such as vegetation type, lithology, and aspect are the primary drivers of community composition. However, CAD intensity also plays a significant role, particularly in increasing the number of alien species. Functional responses to anthropogenic disturbance are more pronounced and include an increase in specific leaf area, leaf nitrogen content, and seed bank longevity with higher CAD intensity. The overall results of the research provide valuable insights into the factors that drive urban, in the form of GRs, and peri-urban plant communities, with a particular focus on human disturbance, which, although obvious, is often overlooked. The novelty of this work lies in its emphasis on the spontaneous colonisation process of MGRs, also explored from a functional perspective, as well as in the assessment of the effect of specific management practices on spontaneous and planted communities of GRs, an aspect that has been underestimated to date. Additionally, it is the first time that the response of peri-urban Mediterranean plant-communities to CADs is assessed, simultaneously considering different vegetation types. From a planning and management perspective, these findings could be valuable in promoting better governance of green spaces and natural remnants, aiming to make GRs more ecologically sensitive and reduce the impact on peri-urban habitats that are often underappreciated and threatened.