Summary
Urban green spaces are a key element of sustainable urban development. Urban green spaces provide habitat for flora and fauna and has a substantial value as such. But the city's scenery and ecosystems also provide a variety of services to the city and its people - ecosystem services are partly composed of physical and biological functions such as purification of air, rainwater drainage and shelter from extreme weather and partly as social, recreational and aesthetic qualities that urban green spaces add to urban life.
This report is about the biological aspects of urban green areas. The urban green areas can accommodate a large number of species but not all species contribute equally to the quality of nature. This report describes a method to calculate the natural quality of urban nature and presents calculated natural quality of concrete, selected green areas.
The report is part of a larger interdisciplinary project at the National Environmental Research Institute (NERI) at Aarhus University on Lifestyle and Nature Quality in urban space (LiNaBy), where Copenhagen has been the subject of project studies. The study is based on quantitative analyses of the flora of two to three sites in each of the four districts of Copenhagen Vanløse, Ydre Nørrebro, Christians-havn/Holmen and Sundby/-Ørestaden. The botanical analysis is made both of the field layer and of the tree layer. A total of nine sites were further divided into from two to seven sub-areas. A total of 32 sub-areas were analysed. These sub-areas are all terrestrial including small ponds and water courses, but not neighbouring larger areas of water.
Nature quality is assessed from a total of eight variables; five variables based on species and three variables based on structural factors. Concerning the species, nature quality is based on species composition in the field layer, respectively, on species composition in the tree layer. Field layer is analysed both with a quantitative, total species list (inventory) and with a so-called documentation circle, ie. a circle with radius 5m and thus fixed area analysed in all sub-areas. The tree layer is described with a species list that contains the most important species of forest trees and of smaller tree species. The three variables describing the structure are based on height structure of the field layer, other physical conditions and hydrology.
In addition to classical variables like number of species and natural quality based on species scores on a nature quality scale (a scale adapted to Danish conditions by NERI National Focal Point) assessment of nature quality based on species present also includes a variable for recreational value of the field layer, which is described and included. This variable is derived from species that are suitable for flower bouquet and species used for children to play. Assessment of nature quality based on structure includes, besides the scores of vegetation height of the field layer, as developed by NERI National Focal Point, also assessment of nature quality including factors used by or inspired by NERI's methodology for assessing forest's natural quality. In addition, also included are other structures in particularly some relevant to a diverse fauna.
In this report the eight natural quality variables are described separately, how they are calculated and for aggregated variables, which sub-variables are included. The eight variables are: Variable no. 1) number of species in the documentation circle excl. problematic species, variable no. 2) number of problematic species in the documentation circle, variable no. 3) nature quality index for the field layer based on inventory, variable no. 4) recreational value of the field layer based on inventory, variable no. 5) natural quality index for species of forest trees and minor tree species, variable no. 6) structure index of the field layer, variable no. 7) quality index for other physical structure and variable no 8) number of hydrological types. Nature quality is produced partly as a ranking of all sub-areas based on all eight variables and partly as a natural quality index based on variables no. 3 and no. 5, respectively, no. 6 and no. 7.
Urban nature included in this report is primarily very planned and very human impacted, but there are also original nature areas with long continuity as natural and spontaneous migration of plants through the long span of time. All urban nature can not be readily suited into the usual nature habitats such as heath, dry grassland, moist grassland, marsh and natural forest. Urban nature includes many planted trees, some of which are of foreign origin, which therefore counts negatively in nature quality if they occurred in a natural forest. These trees, many of whom are old willow, poplar and common lime (Tilia x vulgaris), still play an important role for city residents and as a structural element for fauna, moss and lichen species of trees. Hence, these trees need a more positive score in present nature quality assessment of nature in urban areas than normally for natural forest. Also other variables are adapted to urban conditions.
The report examines how it is possible to describe the nature quality in urban nature by proxy variables from light open nature types and natural forest and partly with newly developed variables. Nature quality based on botanical analysis, as is done in this report, provides not only practical but also academic benefits.
The 32 sub-areas of the nine selected sites have a large range in terms of natural quality. Among the selected sites in the municipality of Copenhagen - chosen based on sociological arguments - there are sub-areas of natural quality as high as the national averages, but the average of the 32 sub-areas are lower than national averages.
Greatest natural quality is found on the site Amager Fælled, which is the authentic nature with a long continuity as nature (dry grassland with intrusions of salt pasture). The investigated part of Amager Fælled is planned for buildings (Ørestad). In view of the natural qualities of this area, the location of the remaining buildings in the Ørestad area should be reviewed. Among the nine sites included in the analysis, seven sites had sub-areas, which were among the top 15 ranking subareas. Only Nørrebro and Grøndals Park contained no subareas with high quality (among the 15 best ranked sub-areas). On Amager Fælled all seven sub-areas were among the top 15.
Nature quality is in addition to the presence of plant species related to physical structures. Old trees, cavities in trees, moss and lichen covered trees, woodpecker holes and dead wood - all these variables associated with long-term continuity and longevity are hard to come by in the Copenhagen parks. Even large stones are a rare sight.
Old trees will be a rare experience in the future if we do not change perceptions of risks and responsibilities associated with old trees. By wise management we can also do better than today and ensure the old trees in the future by placing lanes and rest areas with benches and tables in a distance from old trees.
The analysis of the data, gathered in this project, gives hints about what can be done to improve the nature quality here and now and on a longer time horizon. Changes in management operation can here and now improve certain natural quality variables based on species in the field and tree layer, recreational nature value, height structure of the field layer vegetation and presence of invasive species. In particular, the experience of the recreational nature values can be improved by changing mowing strategies. Thoughtful nature-enhancing interventions may eventually increase certain natural quality variables described under "other structure" including humidity gradient and terrain variation. For example old trees can get extended life span by management, old logs left in situ as a whole to decay when it becomes necessary to fell, cut tree stumps left as high stumps to decay, and large stones can be placed strategically. Establishment of humidity gradient and of terrain variations are a good basis for improved nature quality in monotone parks.
