The concept of ecologically-based plantings is unfortunately a very slippery one, and one that is open to wide interpretation. The urban environment, characterised by altered climate and water relations, damaged soils, skeletal and man-made substrates, a specialised flora of native and non-native species, and a strong cultural context, means that taking a purist ecological line is untenable. Indeed, many core principles that have come to be associated with an ecological approach to designed vegetation can be seen to be full of contradictions when applied in the urban situation. For the remainder of this chapter we will consider how a number of ideas that are considered important to the application of the term ‘ecological’ to designed vegetation relate to the urban context. These include the origin of component species and issues revolving around native and non-native species, biodiversity, the use of chemicals in establishment, the structure and appearance of vegetation, and the promotion of ecological processes.
Biodiversity—native species, non-native species and provenance
Native species are typically seen as being inherently ecological, whereas exotic species are not, unless considered in the context of the country they hail from, in which case they immediately become ecological! Whilst superficially the idea that all urban space available for planting should be filled with communities of native species to counterbalance loss elsewhere is attractive, we argue that in an urban context this is just unworkable. Such vegetation simply would not meet the purpose of the inhabitants in many situations. In any case, irrespective of calamities elsewhere, cities and civilisation are not about remaking the world as it once was. Instead they are about transforming it, and shaping new realities. Given the mobility and cultural evolution of Homo sapiens, the dominant species in this habitat, it is inevitable that these landscapes will support spontaneous non-native species that exploit new ecological niches more effectively than the original native inhabitants. This idea of transformation has also been applied to the plant and animal communities of cities by Gilbert (1989) and others, leading to the development of the scientific discipline urban ecology. Philosophical interpretations of urban ecology have differed greatly. Some ecologists and conservationists persist in seeing urban ecology as dealing with native species that survive, plus alien species, and in doing so suggest that urban species essentially form a degenerate version of adjacent rural ecosystems. This view is unwittingly derived at least in part from romantic nineteenth-century views of industrialisation and urbanisation as being synonymous with social and moral corruption. Others, and particularly those able to construct a more culturally based perspective, for example McIntyre et al. (2000), see the anthropogenic jumble of urban plant assemblages as being of intrinsic worth. Why should, for example, nature-like plant communities brought into effect by intentional (or unintentional) human agency be ecologically and aesthetically intrinsically less valuable than those that result from random combinations of chance events? In biological terms they may be demonstrably less or more valuable, depending on their architecture and the species present, whilst in most cases being more aesthetically pleasing due to having been so designed. Why is human agency so bad when it was unconsciously or consciously employed in the past to help create semi-natural vegetation, such as meadows, steppe, prairie and various woodland communities that we now cherish as ‘nature’? To escape significant human agency one has to return to the Pleistocene.
Even within urban ecology circles that embrace these latter notions, researchers have shied away from including cultivated garden vegetation in the concept, on the basis that it does not arise spontaneously and that its composition is directly influenced by people, and therefore lies outside of ecology.
For both philosophical and pragmatic perspectives, this situation is difficult to defend. In many industrial and post-industrial countries, in excess of 75% of the population lives in towns and cities (90% in the UK and the Netherlands), and a greater percentage of urban areas are covered with gardens as opposed to spontaneously occurring plant communities. Thompson et al. (2003) have recently published a description of gardens and their plant communities as habitats; a significant step to correcting this historical bias. To return to the original question on the ethics of using non-native species in urban landscapes, for the combinations of factors referred to above, the authors of this chapter reject the notion that it is unethical to depart from a purely habitat restoration approach in developing urban vegetation.
Instead, we propose that there are opportunities in urban areas to make greater use of native and non-native species in naturalistic plant communities. In proposing this we recognise that these types of plant communities are not suitable for all planting environments, nor indeed are they necessarily intrinsically more worthy than more conventional, horticultural types of urban vegetation. Decisions on which is most appropriate need to be based on an understanding of the site and on the social, political and biological context. What this means in practice is that the outcome of the decisionmaking process will vary between practitioners in different countries in response to local conditions and issues. In Britain for example, and in some other European countries, there is a widespread, and highly intellectualised culture associated with the cultivation of non-native species. In parallel with this, Britain has a very small native flora, which may, in turn, be a factor that has encouraged interest in non-native plants in gardens. Given this combination, it is easy to see why, for example, the use of non-native plants in naturalistic plant communities might seem appropriate in some urban settings (Figure 1.7). The authors’ interest in anthropogenic plant communities is not driven by a preference for non-native over native species, but rather the desire to be able to effectively utilise visual and functional characteristics that are absent in the native flora (Figure 1.8).
In countries with very rich floras, and a relatively restricted tradition of sophisticated gardening, such as the USA, the impetus to use non-native species is likely to be greatly reduced. Countries such as Germany lie halfway between these two poles, and this is reflected in a clear split between the use of native and non-native species in practice, as discussed by Noel Kingsbury in Chapter 3. In most cases, the creation of completely anthropogenic plant communities will be a response to a particular set of needs, often associated with the users of a building or facility. As a result, most anthropogenic plant communities are likely to be less commonly used and to cover a smaller area of ground than ‘native’, nature-like communities, which will generally form the vegetative backcloth. These issues are discussed in greater detail by James Hitchmough for herbaceous vegetation in Chapter 6, and by Roland Gustavsson for woody vegetation in Chapter 7 (Figure 1.9).
We have previously argued that in urban situations it is often difficult to sustain the view that
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1.7 |
Naturalistic sown meadow in an urban park, Sheffield. By not including grasses in the meadow mix, the flowering impact is heightened dramatically
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1.8
The full zonation of wetland planting, from wet woodland through to submerged aquatics promotes both visual and biological diversity
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1.9
The interior quality of a woodland varies greatly according to tree species, arrangements, densities, composition and ground treatment.
How often are the many aesthetic possibilities considered in urban woodland design?
seed and other propagules of native plants used in these landscapes must be derived from a local population. A pragmatic reason for adopting this position is that in many situations it is difficult to locate local populations to act as a seed source. More important, however, is the fact that where extant populations of, say, common native species—for example, oak trees—are present, this does not mean they represent a ‘local population’ in a genetic sense. In many cases, they are likely to represent a combination of genes from planted oaks derived over the centuries from non-local and foreign seed, intermixed with genuinely local genotypes. Most urban sites are heavily transformed, particularly in terms of soil conditions and climate, and the assumption that original genotypes will be better fitted than non-local genotypes is inherently too unreliable to be a defining objective of practice. Natural selection results in local populations that are well enough fitted but no more than this, non-local genotypes may be equally well fitted (Gould 1997). Sackville- Hamilton (2001) has argued that we have a duty to use only local populations because our governments have signed up to the Bio-diversity
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1.10
Sown native-exotic annual meadow along an urban highway, Gloucester.
A very different approach to standard landscape treatments that, as well as providing visual interest, also supports biodiversity
Convention and this must be interpreted to mean every last bit of genetic variation must be conserved in toto. This is clearly impractical and indeed nonsensical in many urban contexts, and flies in the face of the reality of ongoing evolution of plant populations in response to environmental and cultural change. It is perhaps telling that some of the most vociferous supporters of using only local genotypes work in the least urbanised regions of the UK.
In the light of the biodiversity debate, can a cogent argument be made for using nonnative plants in naturalistic plantings in urban places? The rational arguments against using non-native plants are often contradictory. Non-native species are claimed to be inevitably poorly fitted, and need cosseting and therefore cannot be sustainable. Nonnative species are also however claimed to be invasive! Some non-native species are poorly fitted, and can only persist when competition with other plants and herbivores is carefully controlled, as in gardens. These species are of little value in naturalistic plant communities, however there are other species that are perfectly robust, even in the face of competition, but without being invasive, that are well suited to anthropogenic plant communities. Invasiveness is not a property of which geopolitical region a plant hails from, but is based on the possession of certain biological traits, such as high seed production, effective seed dispersal, capacity for vegetative spread, low palatability to herbivores and so on. There are invasive natives and there are invasive aliens. Most, if not all, of the non-native species that are likely to be used in nature-like plant communities will already have been cultivated in urban and rural gardens and parks for many years, in some cases for centuries, particularly in Europe. The widespread practice of dumping garden waste on roadsides and other places has provided abundant opportunity for the naturalisation of these species, yet only a very small percentage of the cultivated decorative flora has taken advantage of this. In Britain, for example, the commercially available cultivated decorative flora is in excess of 70,000 taxa, only a tiny percentage of which are extensively naturalised (Clement and Foster 1994) and fewer still have anywhere near the adverse ecological impact of invasive natives. The authors’ work on deliberately attempting to naturalise non-native herbaceous plants in purpose-sown native meadow vegetation in urban parks has demonstrated just how difficult it is to establish even well-fitted species in the grassy vegetation that dominates the British landscape (Hitchmough and Woudstra 1999; Hitchmough 2000). This is not, however, the situation in some other countries, and practice needs to reflect this.
Against the risks of naturalisation have to be set the cultural meaning and richness that many urban people derive from such plants. This is especially so in a country like Britain where the cultivation of decorative plants is one of the most widespread and important recreational activities.
It is also important to take into account the habitat that non-native species provide for fauna in towns and cities, and how these plants are important in developing a positive empathy with the natural world beyond the garden. As with native species, non-native species differ in their value as a habitat or foraging resource, but it is clear that they are very important for nature conservation in urban landscapes, as can be gauged for invertebrates by the work of Owen (1991). It is important to recognise that many users of non-native plant materials are equally passionate about native plants inside and outside their gardens. The relationship is not a mutually exclusive one, what is favoured at a point in time varies according to the urban-rural context. Issues of naturalisation and attitudes to non-native plants are heavily grounded in human culture and, once again, can only be discussed sensibly within the context of that culture, and therefore often the nation state.
Can the use of non-native species be considered to be sustainable? Sustainability is a difficult issue to address in terms of right and wrong because major elements within the sustainability model, for example, ecological, economic and social values, are sometimes at odds with one another. In addition, the component factors exist as a continuum of possible responses, and most importantly we read our own bias into sustainability. Within local authority planning departments there is anectodal evidence that sustainability is nearly always equated only with native plants. This is a convenient but often false assumption when applied as a universal truth. Plants that are likely to be most sustainable in a biological sense are those that are likely to be able to reproduce in situ, and thereby perpetuate themselves through subsequent sexual or clonal generations and undergo evolutionary change. Such plants may be native but they might also be exotics. In many cases, with both native and exotic species persistence will depend on human intervention to create the required conditions for regeneration and the growth of adults. Much of the native vegetation that the public regards as highly desirable, for example hay meadows and heathland, only persist in the longer term because of management. There are many situations in which we may, for a variety of ecological or cultural reasons, not want to use exotic species but this does not mean that they cannot be sustainable. Species that are too enthusiastically sustainable through profligate reproduction cross the line and are judged undesirable. Often we desire species that occupy the mid ground.
Within landscape practice our view of sustainability is often skewed to elevate biological above other categories of sustainability. Plant communities that are successful in terms of biological sustainability may score poorly in terms of social sustainability. The latter may, for example, be promoted more successfully with many lay people by plant communities that represent a compromise between what is ecological and what they are familiar with and value, for example the form and colour of a particular flower.
From a biological perspective, the degree of fitness for the environment as found is likely to be a better measure of likely sustainability than the origin of the species. Very often a key factor in this is the inherent productivity of a species (the capacity to produce vegetative growth to compete for resources with adjacent plants) in relation to the productivity of the environment it is expected to grow in. This is particularly marked for herbaceous plant communities which have fewer opportunities to distribute their canopies in space to avoid competition. On highly fertile productive sites, species that are not themselves highly productive are rapidly competitively displaced by other planted or spontaneously occurring species of higher productivity. This happens irrespective of where they originate from or what other cultural labels they bear. These ideas are explored further by Nigel Dunnett in Chapter 4.
As with nearly all of the ideas in this chapter, sustainability is subject to ongoing cultural reinterpretation in response to local conditions. In North America, relatively few people pursue the care of the vegetation in their ‘yard’ as an engrossing, intellectually rewarding recreation; it is maintenance and little more. As a result, there is a huge garden and lawn care industry with much routine garden-care work undertaken by contractors. Many of these employ a very prescriptive approach to plant care; pests and diseases are to be eliminated when found, irrespective of the damage they cause, and plants are to be watered and fertilised, irrespective of whether this is required or not. Against such a backcloth of resource consumption, it is not surprising that native plants should be seen as more sustainable by virtue of not requiring such care in the wild. The point is, however, cultivated non-natives do not necessarily require such care either, except when these are poorly fitted and inappropriately used in a particular location. Sustainability thus becomes an artefact of cultural perceptions rather than biological reality.
One of the most frequently made arguments concerning the use of native species is that they support a wider biodiversity. Native plant species generally appear to support a wider range of invertebrates than, for example, do non-native species, and in some cases planting that mimics the structure of naturally occurring arrangements may increase habitat value for some organisms, particularly birds. Exotic species will also, however, support some species, and in some cases more species than some native species. Many invertebrates in particular may not distinguish between vegetation that mimics natural arrangements and that which does not.
