Roadside ecosystems mapped with UN accounting framework in first global study

Roadsides, the vegetated strips running alongside the world’s roughly 36 million kilometres of roads, have long been managed mainly for driver safety and operational efficiency. A new study argues they deserve to be treated as ecosystems in their own right, and it offers the first systematic framework for describing them as such. Published in the journal One Ecosystem, the research applies the United Nations’ System of Environmental-Economic Accounting and its Ecosystem Condition Typology to roadside environments for the first time, identifying 30 key biophysical variables and distilling them into a preliminary core set of 10 for monitoring purposes.

The research team, led by scientists working within the SEEA-EA framework, which has previously been applied to coastal, forest and freshwater ecosystems, combined a compartmental systems perspective with a systematic review of 97 peer-reviewed studies published between 1950 and 2024. The review followed the PRISMA protocol: from an initial 1515 records, 1079 remained after duplicates were removed, 516 passed a title and abstract screening, 302 survived a scan of introductions, methodologies and results, and 64 remained after full-text assessment. Reference tracking and cross-searching added 33 more studies, bringing the final corpus to 97 articles distributed across five ecosystem compartments: air (22 studies), soil (20), water (15), flora (30) and fauna (23).

The study frames roadsides as socio-ecosystems, dynamic systems in which biotic and abiotic components interact under continuous human influence from traffic, maintenance practices and infrastructure configuration. Rather than treating landscape context as an internal compartment, the researchers positioned it as a boundary condition, a contextual pressure shaping background conditions across all five compartments, including adjacent land-use effects on pollutant levels, species pools and hydrological inputs. The SEEA-EA framework was used not to produce monetary or physical ecosystem accounts, but as a conceptual and analytical reference to guide the selection, organisation and interpretation of ecosystem condition variables at the roadside scale.

The 30 identified variables span the physical, chemical, compositional, structural and functional state classes of the Ecosystem Condition Typology, organised across the air, soil, water, flora and fauna compartments. In the air compartment, particulate matter, including black carbon, ultrafine particles, PM2.5 and PM10, emerges as among the most frequently investigated variables. Black carbon and ultrafine particles typically show pronounced concentration decreases with distance from the road, whereas PM2.5 and PM10 show weaker or less consistent gradients because of the influence of background concentrations and other sources. Polycyclic aromatic hydrocarbons and traffic-related heavy metals, including nickel, cadmium, chromium, copper, zinc and lead, also decline with distance from the road. Nitrogen oxide concentrations in roadsides can reach up to 2.5 times background levels, decreasing with distance but increasing with road width. Ammonia deposition occurs predominantly within the first metres from the road. Studies of greenhouse gas concentrations along roadsides remain limited, but case studies report carbon dioxide and methane concentrations higher than worldwide averages reported by the Intergovernmental Panel on Climate Change, and roadside ditches have been suggested to generate methane, nitrous oxide and carbon dioxide emissions.

Roadside soils tell a parallel story of transformation. Studies consistently indicate that roadside soils are more compacted than background soils, with compaction generally decreasing with distance from the road, and subsurface soil generally more compacted than surface soils. Soils near the road tend to have higher proportions of sand and coarse particles, whereas clay content increases with distance. Organic matter content is generally low near the road and tends to increase with distance, a pattern relevant because organic matter can immobilise toxic elements by binding ions and reducing their mobility. De-icing salts applied in temperate zones produce decreasing gradients in soil salt content, electrical conductivity and pH with increasing distance from the road, with roadside soil pH generally reported within the neutral to slightly basic range. Nitrogen and phosphorus concentrations tend to increase with distance from the road, while potassium shows more variable patterns. Heavy metals are the most frequently investigated pollutants in roadside soils, with concentrations generally decreasing with distance and higher traffic intensity associated with increased concentrations.

The water compartment reveals how road ditches reshape hydrology. Ditches constructed parallel to roads are designed to evacuate water from road surfaces efficiently, but they also modify runoff patterns, increase erosion, facilitate sediment and pollutant transport and alter watershed hydrology by affecting both water levels and water quality. Ditches accelerate water drainage and intercept groundwater and shallow interflow, potentially increasing downstream flood risks while lowering local water tables and soil moisture levels. Mowing ditch vegetation increases hydraulic resistance and reduces water conveyance capacity in vegetated agricultural ditches. Yet roadside ditches can also contribute to sediment retention, potentially improving downstream water quality, and vegetated ditches can contribute to nutrient removal through microbial denitrification, with root systems enhancing microbial activity and facilitating nitrogen removal from water.

The flora and fauna compartments highlight roadsides’ double-edged ecological role. Roadside margins often support relatively high species richness, and several studies have reported higher plant species richness along roadsides than in adjacent agricultural areas. These areas act as important habitats for nature-value species, including those with protected status, and for meadow and grassland species whose natural habitats have become increasingly scarce. However, proximity to roads may negatively affect native plant populations while favouring invasive alien species, which tend to be more abundant near the road. High soil nitrogen content and disturbance levels near the road edge favour nutrient-demanding and ruderal species, whereas competitive species become more abundant with increasing distance. Road infrastructure and traffic pollution alter leaf traits, reducing photosynthetic activity. On the fauna side, wild bees benefited from linear ecosystems, showing higher abundance and species richness compared with urban forest areas, and roadsides provide nesting and foraging opportunities for birds, pollinators and decomposers. But proximity to roads also exposes fauna to vehicle collisions, and roadside disturbances may favour alien and invasive species, as reported for honeybees in Argentina, the brown marmorated stink bug in the United States and the isopod Armadillidium vulgare in Hungary.

The synthesis identifies cross-compartment interactions as structurally embedded in roadside functioning. Soil moisture acts as a key regulatory variable controlling water availability for vegetation; higher soil moisture was positively correlated with grass cover and species richness in calcareous roadsides in the United Kingdom, and soil moisture was identified as a dominant factor determining roadside plant community composition in Canada. Vegetation, in turn, modifies soil properties over time, with plant succession enhancing soil fertility by increasing nutrient content and organic matter as roadsides age. Vegetation also plays a dual regulatory role in pollutant exchanges, acting as a biological filter that captures airborne particulate matter: traffic-facing foliage of roadside hedges retained significantly more particulate matter than the leeward side, and maple and oak trees reduced black carbon and coarse particulate matter under favourable wind conditions, although finer particles were less affected. Rainfall and stormwater runoff drive the first flush effect, in which initial runoff carries disproportionately high pollutant loads of heavy metals, nitrogen and phosphorus mobilised during dry periods. Microtopography, comprising shoulders, slopes, ditches and back slopes, creates fine-scale variations in soil conditions that strongly influence vegetation, and compaction on shoulders limits root development and tree recruitment.

Building on these findings, the researchers propose a preliminary core set of 10 biophysical variables, combining literature-based prioritisation with considerations of ecological relevance and complementarity across ECT classes. Within the physical state, soil moisture and bulk density were retained. For the chemical state, soil nutrients, heavy metals in soil and organic matter were selected. The compositional state is represented by vegetation and fauna abundance, richness and diversity, together with vegetation species status. Fauna nesting sites were retained for the functional state, and vegetation form and stratification for the structural state. The prioritisation used a composite index based on the frequency of reported interactions between variables, the frequency of reported associations between variables and roadside pressures, and the number of distinct articles documenting each variable, refined according to SEEA-EA principles favouring stocks over flows and variables sensitive to human pressures.

The authors stress that the proposed core set provides a preliminary basis for roadside ecosystem condition monitoring rather than a validated assessment framework. Measurement feasibility, monitoring costs and the sensitivity and reliability of the selected variables have not been systematically evaluated, and future field-based studies across contrasting roadside and management contexts are needed to validate the selection, establish suitable measurement protocols and reference conditions and assess its applicability within the SEEA-EA framework. Given the predominance of studies conducted in temperate regions and industrialised countries, particularly Europe and North America, the reported patterns should be interpreted as general tendencies rather than universally applicable relationships. The results should be read as indicative gradients rather than universal benchmarks, since roadside functioning is strongly influenced by local factors such as road design, surrounding land use and landscape context. Still, the framework offers a transferable and internationally standardised foundation for evidence-based roadside management, ecosystem condition accounting and sustainable infrastructure planning, with zonation-based management, reducing intensity with distance from the road, proposed as a spatial basis for differentiating interventions while supporting roadsides’ potential to function as corridors and refuges for biodiversity conservation.

Subject of Research: Application of the SEEA Ecosystem Condition Typology to roadside ecosystems, identifying 30 biophysical variables and a preliminary core set of 10 for condition assessment

Article Title: Characterising roadside ecosystems using the SEEA Ecosystem Condition Typology: towards a core set of biophysical variables

Article References: Mayoral Solis, Z., Chaudron, C., Marche, B., & Camargo, M. (2026). Characterising roadside ecosystems using the SEEA Ecosystem Condition Typology: towards a core set of biophysical variables. One Ecosystem, 11, Article e200222. https://doi.org/10.3897/oneeco.11.e200222

Image Credits: AI Generated

DOI: 10.3897/oneeco.11.e200222

Keywords: roadside ecosystems, SEEA-EA, Ecosystem Condition Typology, systematic literature review, biophysical variables, ecosystem condition monitoring, soil compaction, heavy metals, road-effect zone, biodiversity, de-icing salts, microtopography