Monitoring the performance of urban green infrastructure: biodiversity challenges and assumptions in Greek Mediterranean cities

In Mediterranean cities, high population density inhibits urban sprawl and lowers transportation emissions. However, it may also amplify existing risks, such as extreme heat, short, heavy rainfall, and persistent summer water shortages. Recent studies in Greece highlight rising impacts from heatwaves (Galanaki et al., 2023) and increased attention to severe weather events, such as intense precipitation, drought, and combined extreme events (Kotrononi et al., 2025). In dense neighbourhoods, imperviousness and constrained drainage convert these hazards into exposure. Urban green areas-trees, shrub layers, green roofs, rain gardens-remain among the few interventions that can simultaneously cool, store stormwater, and deliver biodiversity and health co-benefits (Dodman et al., 2022;WHO Regional Office for Europe, 2017).Recent Greek studies on urban green spaces show that their contribution to both regulating the local microclimate and enhancing biodiversity is not "automatic". It depends on evidence-based planning and targeted management, as well as systematic, continuous monitoring, so that ecosystem services are maintained even in heatwave and drought conditions (Proutsos Leotta et al., 2023). Therefore, the Greek context is treated as a representative Mediterranean example and not as an outlier.Plant biodiversity matters because it increases reliability under compound stress. Evidence from vegetated roof systems indicates that higher species diversity can improve ecosystem multifunctionality and buffer performance through time, although outcomes are context-dependent (Cook-Patton and Bauerle, 2012). For Mediterranean-type roofs, species selection must explicitly address heat stress in shallow substrates as well as drought; temperature can be as limiting as water deficit for survival (Savi et al., 2016). Recent Greek experimental research indicates that arranging plants as communities instead of isolated specimens helps ensure survival and functional continuity in extensive green-roof setups (Varela-Stasinopoulou et al., 2023). Additionally, Mediterranean-focused syntheses highlight a wide variety of plant options beyond just Sedum-based templates (Leotta et al., 2023). In practice, biodiversity works through three levers. First, trait complementarity: mixing canopy architectures, phenologies, root depths, and leaf traits allows the same square meter to deliver shade, evapotranspiration, stormwater infiltration, and habitat simultaneously. Second, redundancy: mixed assemblages reduce the risk of systemic failure when a single taxon is hit by pests or heat extremes. Third, water realism: diverse Mediterranean assemblages can be selected for seasonal dynamics and lower irrigation demand, shifting expectations away from permanently lush, high-input lawns.A biodiversity strategy heavily reliant on exotic (alien) plants may increase the likelihood of invasive alien species, although not all exotic species are invasive and several may provide ecosystem services depending on context and management. In this paper, 'alien (exotic) species' refers to taxa introduced outside their native range, whereas 'invasive alien species' denotes the subset of alien species that spread and cause ecological, economic or social harm (Richardson et al., 2000;Blackburn et al., 2011). Greece's exotic plants have been widely reported (Arianoutsou et al., 2010;Solomou et al., 2023) and cities are areas where these can be introduced and dispersed by trade, disturbed soils and repeated planting (Gaertner et al., 2017;Hulme et al., 2008;Reichard and White, 2001). For dense-and-green strategies, a basic biosecurity package is needed: 'safe list' procurement (prioritizing native or regionally appropriate taxa), traceable nursery supply chains, and post-installation inspections, especially along transport corridors, riparian buffers, and construction interfaces. This is not an antihorticulture position. Biosecurity is used here as a preventive governance framework aimed at reducing ecological risks, long-term management costs and unintended ecosystem disservices (Lyytimäki and Sipilä, 2009;Veibiakkim et al., 2025). This approach is not restricted to exotic species alone, acknowledging that some alien taxa may deliver ecosystem services, while certain native species may also be associated with disservices depending on design and management (Tartaglia and Aronson, 2024;Russo et al., 2025).A concrete Greek example of turning a performance and biosecurity lens into routine municipal practice is the LIFE GrIn project (LIFE17GIC/GR/000029), which targets integrated management of Urban Green Areas (UGAs) and broader urban green infrastructure for climate adaptation through a policy framework, guidelines and a standardized indicator system (www.lifegrin.gr).In operational terms, project materials describe the creation of a cooperation platform and an Urban Green Infrastructure register as a prerequisite for comparable monitoring: municipalities inventory their green areas and populate a centralized database with standardized fields (e.g., green infrastructure quantity per type, woody vegetation quantity and quality -including health and biodiversity assessments -spatial distribution, use and function, management and maintenance, and climate value) (Greek LIFE Task Force, 2022). For more info, check the project's LIFE GrIn website www.lifegrin.gr. Together, these tools move cities from aspirational slogans ("increase greenery", "avoid invasives") to repeatable workflows of inventories, indicators, and routine monitoring, which is critical when space is scarce and benefits must be maximized per square meter.If the dense-and-green debate is to be actionable, municipalities need a small set of indicators that connect design choices to outcomes. Table 1 offers an illustrative package aligned with the infrastructure logic above: (a) target cooling and thermal comfort in heat hotspots, not uniform canopy everywhere; (b) treat roofs and façades as biodiversity-bearing surfaces with explicit heat and drought performance criteria; (c) couple stormwater functions (bioswales, rain gardens) with sediment and drought tolerance; and (d) mainstream alien-species surveillance into routine urban-green monitoring. To ensure operational consistency, the proposed monitoring indicators are defined in terms of measurable functional outcomes under stress conditions (e.g. heatwaves and intense rainfall), rather than the mere presence of green elements.Table 1. Illustrative indicators that make "dense and green" measurable (examples). Finally, dense-and-green is distributional. Because heat and flood exposure are spatially unequal, untargeted greening can underperform as risk reduction and may contribute to displacement through 'green gentrification' if benefits are captured by higher-income districts (Anguelovski et al., 2019). Conversely, urban green space interventions can deliver substantial health benefits when access, quality and long-term management are addressed (WHO Regional Office for Europe, 2017). A biodiversity-first approach should therefore be paired with equity-first siting (heat-and flood-risk prioritization), transparent maintenance budgets, and community stewardship arrangements that stabilize benefits where they are most needed.Mediterranean cities can be dense and green, but only if 'green' is treated as regulated infrastructure rather than decoration. For Greek Mediterranean cities, the key shift is from planting quantity to biodiversity-driven performance: specify trait-diverse, climate-ready assemblages; design roofs and streets as stacked ecological systems; and embed biosecurity so that adaptation does not seed future invasions. LIFE GrIn demonstrates that this can be operationalized through standardized indicatorsexplicitly including 'biodiversity and alien species occurrence' and through registry tools that make municipal green infrastructure comparable, monitorable and governable. Under these conditions, density and biodiversity reinforce rather than undermine each other. Although centred on the Greek context and the LIFE GrIn example, the governance, monitoring and biosecurity challenges discussed here are widely shared by Mediterranean cities facing similar climatic, ecological and spatial constraints.ADS: Conceptualization; Writing -original draft; Writing -review & editing.