Openbaar groen op ecologische grondslag

This thesis is concerned with ecologically managed public green spaces. Woody vegetation is an important component of such areas. The impact of woody vegetation on herbaceous plant species and wild bees is investigated. Then, guidelines are given based on the research for the design of woody plantings.Introduction (chapter 1)In the past, natural features were rather scarce in urban surroundings due to intensive green management which was aimed at neatness. Between 1960 and 1990, weed killers and other pesticides were repeatedly applied. As a result, herbs and wild bees were seldom found in urban amenity woodlands. Nowadays, however, many municipalities practice ecological management of their public green spaces. Such management takes into account natural processes: lifecycles of plants and animals are left basically undisturbed; biodiversity is increased as a variety of habitats is created; natural development is encouraged; and pesticides are no longer used. Ecological green management experienced a breakthrough after 1990.The city as an ecosystem (chapter 2)Green spaces, including woodlands, are part of the urban ecosystem. To guarantee the sustainability of that ecosystem, optimal coexistence must be sought among organisms within the system. Furthermore, other ecosystems and the global ecosystem must be left intact. 'Optimal' is interpreted here as 'well-being', which is defined as 'healthy physical, psychological and social functioning'. Numerous tasks performed in a city, such as dealing with resources, waste and energy, impact the ways of life and conditions of existence of people elsewhere in the world. One way to increase the well-being of urban dwellers is by improving the abiotic factors of the urban ecosystem. That means acting in an ecologically sound way. This applies to woodland vegetation because such vegetation helps regulate flows of water, energy and traffic. Woodland vegetation has a positive impact on people's health and well-being. It can contribute to the development of urban green spaces and is important for nature and environmental education.The research question and method (chapter 3)As chapter 2 described, woodland vegetation is a key feature of the urban ecosystem. Yet generally speaking, designers of such plantings have overlooked this aspect. This thesis provides guidelines and principles to improve the design of woodland vegetation. It emphasises the herb layer and wild bees, because herbaceous plants are the most visible components of ecological green management and bees are wholly dependent on pollen- and nectar-producing plants. Where flowers are lacking, there are no bees; and where many bees fly, flowery vegetation tends to be nearby. Flowers also affect the aesthetic quality of the environment, generally in favour of human well-being. These implications can be integrated into the design of woodland plantings.The study deals with the following aspects of urban woodland vegetation: spontaneous development (chapter 4); morphological typology, architecture and image (chapters 5 and 6); impact of ecological green management on the development of herbs in and around woody vegetation (chapter 6); the importance of these herbs for flower-visiting insects, especially for wild bees (chapter 7); and the contribution of designers to these aspects (chapter 8).In 1996-98, some 200 plots in ecologically managed woodlands were investigated (Braun-Blanquet method) at 40 sites in The Netherlands. In 1991, 30 plots in Arnhem were investigated using the Tansley method. The heterogeneity of the woodlands and plots made statistical computations infeasible. This was true for the occurrence of wild bees as well. This thesis therefore is restricted to a qualitative analysis. By means of interviews, data were recorded on layout and management of woodlands (amenity woodlands usually covering less than 0.5 ha and less than 30 m in breadth). Also inventoried were 100 places where woody vegetation had emerged spontaneously. This was to learn whether planting is indeed necessary. Other areas were visited for the inventory of wild bees. Bees were caught with an insect net and determined based on several keys.Naturally occurring woody vegetation (chapter 4)Woodland plantings are usually installed without much question. If habitat creation is the aim, a detailed design is not always required. As seen from the large number of woodland areas that emerges spontaneously, even planting is not always necessary. In some cases, depending on the context, partial planting might be sufficient, leaving space for a more natural development. Birds often make a large contribution in dispersing seeds of woody species.A typology of woody vegetation (chapter 5)Woodland development cannot always be left to nature as described in chapter 4. Purpose, social context and time constraints sometimes necessitate their design and planting. To meet the various quality requirements, a range of types of woody vegetation is available. In practice, however, confusion abounds as to species' names. Based on practical experience and an analysis of literature, this study initiates a typology of the woodland vegetation most useful in urban plantings.The woodlands described in this thesis are mostly called 'bosplantsoen', which means 'a vegetation having architectural characteristics of (more or less natural) woodlands and scrubs and chiefly composed of native species'. This term, according to this definition, is a general concept for different woodland types. The term 'landschappelijke beplantingen' (rural woodlands or plantings) refers primarily to woody plantings in the countryside. This too is a general term. Based on actual practice, one can say that 'bosplantsoen' is mainly used for amenity woodlands within the urban setting. 'Landschappelijke beplantingen' is used for those plantings in the country and on the fringes of a city. If the dividing line between the city and the country were clear, both terms could be used without objection. The problem, however, is that clear dividing lines have all but disappeared over the past few decades. Furthermore, both terms are intrinsically vague. More differentiation is needed in the typology of the various woody vegetation types. A first step towards such differentiation is taken in this thesis.Development of the herb layer (chapter 6)Which plant species become established in woodlands where ecological management has replaced a chemical weed-killing regime? In the city of Arnhem, where no chemical weed killers have been applied since 1983, 30 plots were inventoried. Records from other cities were also used to complete the picture of plant species' establishment. Five aspects were considered in some depth:species that can occur in the herb layer,places where the species can develop,architecture of the herb layer created by these plants,sources of the species that established in the woody vegetation,significance of the herbs for design and management.In the overall survey, 286 species were recorded on plots in 40 municipalities. Outside the plots, 28 species were found during the wild bee investigation, mainly on the edges of woodlands. With respect to design, several aspects of the plant species may be important:life forms (geophyts, therophyts, hemicryptophyts, which can strongly determine the image of the herb layer),habitats (the places wherein plant species complete their lifecycle),distribution (an indication of the species' prospects for establishment),way in which the species establishes (naturally, by planting or sowing or as garden escapes),morphology (structure, colour and smell).Bees in and around woody vegetation (chapter 7)It seems likely that ecological green management strongly stimulates the occurrence of wild bees. The following results refer to 26 municipalities where more than 5 species were caught.A total of 110 species were caught, bumble bees excluded (58 polylectical, 22 monolectical, 2 oligolectical and 28 parasitical (cuckoo) species).In 26 municipalities, an average of 23 species was recorded. In these municipalities, bees were caught at 290 sites resulting in 1471 records.At 141 sites, bees occurred frequently and at 27 sites more than 10 species were found. The bees were caught on 181 plant species, 38 of these with more than 10 records.Where pollen- and nectar-producing plants were found alongside nesting places, wild bees did occur in most urban habitats. In places where these plants were lacking, bees were absent. In vegetation mowed too early, bees seemed to disappear completely or their numbers were greatly diminished. In places where flowering was uninterrupted, wild bees tended to occur in the summer as well.It then seems plausible that wild bees are a sound indicator of the ecological quality of flowery vegetation. As a rule of thumb, the ecological quality of the habitat can be seen to improve with the increased occurrence of the following:polylectical, monolectical, oligolectical and parasitical species,parasitical species of monolectical and oligolectical species,rare species,abundant bee populations,nesting places and food resources in the same habitat.The abundance of flowers that is currently found in public green spaces is a result of ecological green management. Reduced use of chemical herbicides together with ecological green management has also been a key factor in the vigorous resurgence of bee populations in urban settings. For bees and other flower-visiting insects this management must be continued and refined when necessary. Both biodiversity and ecological quality will likely be greatly enhanced by appropriate design of woodland plantings.Guidelines for design (chapter 8)Most pollen- and nectar-producing plants in and around woody vegetation contribute to diversity of flower-visiting insects, especially wild bees. In fact, the occurrence of wild bees can be considered evidence of sound ecological quality. Maintenance of this quality should not depend on caprice or ad hoc decisions. Rather, it must result from resolute action. Landscape designers have an important role to play here. Appropriate choice of planting dimensions, together with well thought out selection of plant species, can result in thriving habitats for wild bees. Design must also be linked to guidelines for maintenance and plans for future development. Designers can make substantial contributions to biodiversity in woodlands and the associated ecological quality of urban ecosystems. Woody vegetation has been shown to attract flower-visiting insects and, moreover, it is highly appreciated by most people. Therefore, fostering biodiversity, as expressed in natural features that are appreciated by the public, can contribute to populations' general well-being. This latter should be the main purpose of the designer. For this, guidelines are provided in this thesis.