Nigel Dunnett

Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services

ABSTRACT Green roofs (roofs with a vegetated surface and substrate) provide ecosystem services in urban areas, including improved storm-water management, better regulation of building temperatures, reduced urban heat-island effects, and increased urban wildlife habitat. This article reviews the evidence for these benefits and examines the biotic and abiotic components that contribute to overall ecosystem services. We emphasize the potential for improving green-roof function by understanding the interactions between its ecosystem elements, especially the relationships among growing media, soil biota, and vegetation, and the interactions between community structure and ecosystem functioning. Further research into green-roof technology should assess the efficacy of green roofs compared to other technologies with similar ends, and ultimately focus on estimates of aggregate benefits at landscape scales and on more holistic cost-benefit analyses.

A new practical tool for deriving a functional signature for herbaceous vegetation

Abstract Hypothesis: For any one time and place a ‘functional signature’ can be derived for a sample of herbaceous vegetation in a way that concisely represents the balance between the different clusters of functional attributes that are present among component species. Methods: We developed a spreadsheet-based tool for calculating functional signatures within the context of the C-S-R system of plant functional types. We used the tool to calculate and compare signatures for specimen British vegetation samples which differed in management regime and location in time. Conclusion: The integrative power of the ‘C-S-R signature’ is useful in comparative studies involving widely differing samples. Movements in the signature can be used to indicate degree of resistance, resilience, eutrophication and dereliction. Systems of plant functional types other than C-S-R might also be approached in this way. Availability: The tool can be downloaded free of charge from the first authors web pages or from the journals electronic archive. Nomenclature: Stace (1997).

Vegetation composition of old extensive green roofs (from 1980s Germany)

IntroductionSince their development in the late 1970s in Germany, extensive green roofs (EGR) have become increasingly popular as mitigation tools for urban environmental issues around the world. EGRs are planted with select species, which ensure consistent cover and performance over time. This research presented herein is part of a systematic re-evaluation of EGR technology since the German industry began.MethodsGiven the opportunity to access a small sample of old EGRs installed over 20 years ago in south-west Germany, this research surveyed the vegetation and substrate with an interest in describing these parameters with time-through-space substitution.ResultsSimilar to previous studies, this preliminary work found correlations between roof age with vegetation (cover abundance and species diversity) and substrate properties (e.g., depth, organic content, pH, and nutrients). Roof age had positive relationship with soil organic content (Corg), and negative relationships with substrate depth and soil pH. These soil variables are inter-related, as shallow acidic substrates create unfavourable conditions for decomposition and thereby the accumulation of duff. Substrate variables correlated with EGR vegetation, suggesting a trend of simplified species composition over time. Indeed, Corg had a negative relationship with cover and species diversity of most life forms; only Sedum species had positive associations with Corg.ConclusionsConsidering the dynamics associated with shallow mineral substrates, and the greater floristic diversity of younger roofs, simple Sedum-based vegetation may represent a steady state for conventional EGRs.

Does Gulf Stream position affect vegetation dynamics in Western Europe

It is widely accepted that the climate enjoyed by maritime western Europe is strongly affected by the warming influence of the North Atlantic Gulf Stream. Until recently, however, little account has been taken of the fact that the latitudinal position of the Gulf Stream varies continuously. In theory, such displacements could generate year-toyear variation in crop yields and in the productivity and relative abundance of component species in natural vegetation. Here we test this hypothesis by examining the productivity of above-ground vegetation, monitored annually in permanent plots at Bibury (Gloucestershire, southern England) over the period 1966-1993, in relation to changes in latitudinal position of the Gulf Stream. The long-term field experiment on the road verges at Bibury (Yemm and Willis 1962, Willis 1972, 1988) has recently provided an unforeseen opportunity to examine year-to-year variations in species composition in permanent plots experiencing a standardized management regime (late autumn mowing). We examined inter-annual variation in mean total shoot biomass in each of the main Bibury species in relation to a Gulf Stream northerliness index (see legend to Table 1). Total productivity of above-ground vegetation was positively correlated with Gulf Stream northerliness (Fig. la). Annual species were negatively correlated with northerliness during certain periods, biennials showed no clear relations, and perennial species were positively correlated. Shoot biomass was positively correlated with northward displacement of the Gulf Stream in 10 species, showed a negative relation in three other species, and had no consistent effect on the remaining 10. Different species showed significant correlations with Gulf Stream movements at different times of the year (e.g. Fig. lb), and some species showed correlations with movements in the preceding year or in the year before that. Table 1 classifies the Bibury species according to response to Gulf Stream northerliness, life history and ecology (ecological data from Grime et al. 1988). Species favoured by northerly tracks include robust, perennial grasses (Bromopsis erect, Dactylis glomerata, Elytrigia repens and Festuca rubra). At Bibury, and across Western Europe, these grow rapidly in early spring and are major contributors to community biomass. The similarly responsive dicotyledons (Achillea millefolium, Gahum verum, Knautia arvensis, Plantago lanceolata and Vicia sativa) have relatively later phenologies and fairly deep root systems. Species declining in biomass with Gulf Stream northerliness (Anisantha sterilis, Galium aparine and Urtica dioica) are more heterogeneous ecologically, but are either annuals and/or have field distributions associated with damp or shaded locations where water relations may be important (Grime et al. 1988). The circumstantial evidence is that variation in Gulf Stream position is associated with changes in the relative abundance of components of the vegetation. Because these differ in phenology and rooting depth, moisture supply appears to be implicated, though the precise mechanism is unclear. Large-scale changes in atmospheric circulation are known to affect both the position of the Gulf Stream and the weather in the United Kingdom. However, it has been suggested by Taylor and Stephens (1980) that displacement of the Gulf Stream may directly influence the movement of cyclones across the North Atlantic and hence affect the frequency of anticyclones to the west of the UK (Taylor 1995a). Clearly, both of these actors may affect rainfall patterns. To test this assertion, we examined relations between Bibury weather and Gulf Stream position (see legend to Fig. 1). The relation for local atmospheric pressure appears in Fig. lc, showing that high pressure is clearly related to northward displacement. Analyses also confirm, as expected, that high pressure is associated with higher mean temperatures, more sunshine hours, and lower rainfall. We conclude (a) that the overall productivity of Bibury vegetation and the performance of certain individual spe-

The impact of Rhinanthus minor in newly established meadows on a productive site

ABSTRACT Question: What is the impact of the presence of Rhinanthus minor on forb abundance in newly established swards? Location: Wetherby, West Yorkshire, UK (53°55′ N, 1°22′ W). Method: A standard meadow mix containing six forbs and six grasses was sown on an ex-arable field and immediately over-sown using a randomised plot design with three densities of Rhinanthus minor (0, 600, and 1000 seeds per m2). Above-ground biomass was analysed over a period of three years, while detailed assessments of sward composition were performed during the first two years. Results: Values of grass biomass were reduced in the presence of Rhinanthus, especially at the higher sowing density. The ratio of grass:forb biomass was also lower in association with Rhinanthus, but only at the higher sowing density. The presence of Rhinanthus had no effect on species number or diversity, which decreased between years regardless of treatment. Conclusions: Although not tested in a multi-site experiment, the benefit of introducing Rhinanthus into newly established swards to promote forb abundance was determined. The efficacy of Rhinanthus presence is likely to depend on whether species not susceptible to the effects of parasitism are present. Nomenclature: Stace (1997).

Ruderal Green Roofs

An awareness of the ecological theory relating to the colonization, early successional stages and persistence of ruderal communities and their role within a matrix of other plant communities and plant types on green roofs provides an important basis for increased understanding of the long-term resilience of dynamic green roof vegetation assemblages. This chapter discusses the concept of the ruderal green roof, with its highly dynamic nature and inclusion of colonization, succession and change as core functioning elements. The theoretical background of a trait-based or functional type approach to working with green roof vegetation will be explored, and the wider role of ‘ruderal’ or disturbance-tolerant plant species in creating resilient and climate-adapted green roofs will be reviewed. Dynamic colonization processes have wide applications across typical extensive, semi-intensive and intensive green roof types where designers and users desire greater biodiversity, a more sustainable approach to long-term management, increased local distinctiveness, climate adaptation, and greater aesthetic and visual interest.

Dynamics of Chamerion angustifolium in grassland vegetation over a thirty-nine-year period

The dynamics of the tall perennial Chamerion angustifolium, as a component of neutral grassland vegetation of a roadside verge near Bibury, Gloucestershire, UK, is described for a 39-year period. Observations were made in July annually of its performance (height and above-ground biomass) and of its major co-dominant, the grass Arrhenatherum elatius. The expansion of a single colony of C. angustifolium by clonal growth from a small cluster of shoots to a substantial highly vigorous stand and its subsequent rapid decline, which was followed by some recovery, are considered especially with respect to weather data. Shoot biomass and flowering were found to be promoted by warm summers and mild winters. The switch of dominance between C. angustifolium and A. elatius is discussed and possible reasons for the decrease of the former are suggested.

The influence of vegetation on rain garden hydrological performance

Abstract Rain gardens are increasingly adopted in urban areas to mitigate urban stormwater impacts. They provide an opportunity to adopt taxonomically diverse plantings to enhance habitat and aesthetic value. However, few studies to date have quantified how rain garden hydrological performance is affected by vegetation type. In the present study, two vegetation types were considered: taxonomically diverse communities composed of forb-rich perennials; and mown grasses, as well as a bare soil control group. Detention effects were measured independently from retention. The forb-rich perennial mixes consistently provided the best hydrologic performance in terms of both stormwater retention and detention. The diverse perennial community showed up to 1.2 mm higher initial losses over the experimental catchment compared with mown grasses, and also offered 54% and 32% longer detention compared with bare soils and mown grasses, respectively. We therefore recommend prioritising taxonomically and structurally diverse planting for vegetated stormwater management facilities wherever possible.