Ξυλώδης χλωρίδα των αστικών βιοτόπων

Abstract

A city is formed by the interaction between natural, built and social environment. These three areas are fundamental for environmental development. Urbanization phenomenon around the world has been an important component of land use and land cover change and influences negatively the balance of those three areas. Management of urban environments must be based on urban planning, which takes into consideration the significant ecological components of an area, such as the woody plants. The present doctorate thesis, the first of its kind in Greece, is based on the knowledge and principles of the science of urban ecology and urban planning. The aim of the present study was to survey the woody species (trees, shrubs and woody climbers) of the city of Patras and to examine whether and in which way the urban structural units (defined as “areas with physiognomically homogenous character, which are marked in the built-up area by a characteristic formation of buildings and open spaces”) can be characterized by their planted woody stands. For that reason the spectra, combination, frequency and consistency of species were assessed, considering the relationships between plant ecology, biotope structure and land-use. The wider area of Patras‟ city was chosen as a study area, because there is a lack of knowledge on its cultivated flora and because it has the features of a typical Greek city concerning the formation of buildings and the open spaces. Eight urban structural units were identified within the city of Patras. These are Multi-storeyed attached housing, Multi–storeyed semi–detached housing, Low–rise attached housing (bungalows–two floors), Low–rise semi-detached housing (bungalows–two floors), Multi–storeyed and low–rise detached housing, Villas, Workers‟ housing, Parks (incl. squares and bocages). In total, 237,44 ha of urban land were studied thoroughly in the fifty-four study areas designated throughout the wider area of Patras city. 218 woody plants were recorded in the eight urban structural units studied. The number of woody species recorded in the study area of 237,44 ha is considered to be low when compared with that of central Europe, where similar studies have been undertaken. This low number is due to the very high building density found in the city centre and also to the limited number of green areas (e.g. parks). Concerning the origin of the woody species in the city of Patras non-native and hybrids dominate. The proportion accounts for 61,5% (125 taxa non-native and 9 taxa hybrids), of which the Asiatic (26,9%) and the American (17,2%) species are represented by high percentages in the woody flora of Patras. The native species represent the 38,5% (84 taxa) of the total woody flora. The life-form spectrum of the woody flora in the city of Patras differs from that recorded in other European cities. The proportion of trees (55,5%) is higher than that of shrubs in the city of Patras, whereas in central European cities shrubs dominate. The prolongation of the flowering period recorded in the study area reflects human attempt to assure blooming all year, so that monotony of the urban landscape can be avoided. The proportion of deciduous species is slightly higher than that of evergreen species. This is due to the presence of high proportions of deciduous species, which belong to the two richest in species families, to these of Rosaceae and Leguminoseae. Almost all woody species fruit, except from a small percentage (5,2%). A significant proportion (40,8%) of the total woody species form fleshy fruits, which are an important food source for the fauna of the study area, especially for birds. At the same time, the distribution of the woody species is assured. The proportion of cultivated woody plants (76,8%) is higher than that of woody plants forming edible fruits (22,6%). This fact reflects inhabitants‟ preference for ornamental plants non-native in origin, denoting social and financial standing. Statistical analysis proves the absence of a positive correlation between the species number and the size of the area. On the contrary, in central European cities the correlation is positive. This is explained by the „small island effect‟ theory, according to which, in small study areas the size of the area does not play the most important role, when species – area relationship is surveyed. Species appearing in all study areas, i.e. with 100% consistency, were not observed. Those species with high consistency, such as Morus alba, Nerium oleander and Ailanthus altissima, are also those with the highest frequency. This shows that these species benefited from them being planted. Floristic composition in the various urban structural units shows great heterogeneity as several species present high consistency in some structural units but low or zero consistency in others. Acer pseudoplatanus proved to be a characteristic species only within the urban structure of multi-storeyed semi-detached housing. Taking these findings into account, we conclude that the urban structural units of Patras can be differentiated based on the number of woody species recorded in each, but not on their floristic composition. Spontaneous diffusion could only be identified for 17,9% of the cultivated species. This proportion is comparable to that recorded in other central European cities, where similar studies have been undertaken. The proportion of non-native species (53,8%) is higher than that of native, and so is the proportion of trees in comparison to that of shrubs. Similar proportions have been also recorded for the total woody flora of the city of Patras. The most frequent non-native spontaneous woody species observed in Patras are Morus alba, Ailanthus altissima and Hibiscus syriacus. The commonest native spontaneous species in the city is Ligustrum vulgare. The urban structural units studied in Patras show relative homogeneity concerning the number of spontaneous woody species. This homogeneity is also observed in cities of central Europe. The correlation between the species number and the size of the area is negative. For the evaluation of the urban structural units in the city of Patras, the value of criteria that strengthen or weaken the character of the urban structural units was taken into consideration. Those criteria are mainly microclimate / wind (temperature, emissions), soil, water, natural environment (green spaces), traffic (heavy or not), noise (noise pollution), dangers for public health (quality of microclimate, emissions, traffic, noise pollution), degree of resources‟ exploitation (of energy, surface, buildings) as well as socio-economic criteria (land-use). For each of these criteria a 5 – degree scale of values was designated. This consideration of the values which affect the urban structural units enabled the definition of the advantages and the drawbacks of each urban structural unit. The urban structural units characterized by detached free building style (multi-storeyed and low-rise detached housing, villas, workers‟ housing) and parks are highly attractive and have high ecological properties. The urban structural units characterized by attached or semi-detached housing have low ecological properties but are socioeconomically benefited. Generally, high exploitation degree (of energy or soil) is related to social and economic integration. At the same time the quality of the natural environment is degraded. Urban structural units characterized by high exploitation in terms of soil, water and energy must upgrade not only their microclimatic conditions, but also the quality of air and soil. The contribution of the woody flora to the environmental development is significant. The management measures proposed, aim to upgrade the natural, built and social environment. They include management proposals that aim to the upgrade and conservation of specific parameters which affect the quality of the urban structural units. Such proposals could make a valuable contribution to the upgrade of the quality of city life.