Study On Distribution Pattern Of Weed Communities Of Terrestrial Ecosystem And Forming Reason In Urban And Rural Of Shanghai

As one of the three components of urban vegetation, weed communities are essential to the maintenance and conservation of urban biodiversity. Weeds distributed in urban area widly without human intervention, and they had high sensitivity, strong plasticity and diversified adaptation strategies to the heterogeneity of habitats. Due to these factors, weeds can quickly respond to the formation of high degree variation of habitat attributes in urbanization. Therefore, study on the diversity of weed communities, spatial distribution pattern and its causes under urbanization will help to reveal the influence of urbanization on urban vagetation and the respondence of urban vagetation to urbanization.This study took the terrestrial ecosystem in urban and rural of Shanghai as the research areas. We investigated the composition and distribution of weed communities, surveyed the habitat factors in various zones of land formation, habitats, and urban-rural gradient, cataloged the species diversity of urban weeds, classified the weed communities and built a classification system. From the natrual factors(land formation and habitats) and human disturbance(urban-rural gradient) of urban environment mixed with the biological and ecological characteristics, we analysed the distribution characteristics of weed communities and their relationships with habitats, also determined the distribution types of weed communities, expounded the diversity of weed communities and revealed its causes. The study has important theoretical and practical significance for completing the comprehensive study on urban vegetation in Shanghai, for establishing research methodology on urban vegetation, for enriching the theory of urban ecology and guiding the management and conservation of urban biodiversity. The main results and conclusions are as follows:1. Floristic composition and community diversity226weeds belonging to42families and146genera were observed. From the geographical elements, the42families can be classified into5distribution types, the 146genera can be classified into13distribution types, and the226sepcies can be classified into14distribution types. Tropic elements dominated the distribution types of families, while temperate elements exceeded in the distribution types of genera and species. The proportion of cosmopolitan weeds was much higher on the levels of families, genera and species. In life forms,94species were perenial weeds and132species were annual weeds, the annual species were more than perenial weeds. In growth forms, weeds having a small growth form were more than those having a large growth form. In species source, only40species were alien species, while186species were native species, which took the dominant position.1868investigation plots of weed communities were classified into3life form types,22growth form types and152community types according to the vegetation classification system of "Vagetation of China":vegetation type, vegetation subtype, plant formation.There are194weed species, which have been recorded in Flora of Shanghai and have widly distributed in Shanghai area, were undetected in this investegation, and mainly are perenial and annual weeds(174species). The reason may be that the human disturbance with high intensity and high frequency under the rapid urbanization of Shanghai makes the urban habitats unstable, thus weed communities can not reach perennial. Due to the increase of buildings, asphalt pavements and urban drainage facilities, more and more original moist habitats disappeared, and urban habitats became dry gradually, which can’t provide living environments for the summer annual weeds that growing in moist habitats.Wedelia prostrata and Microstegium japonicum were two new records in Shanghai.2. Diversity of weeds community in various zones of land formationsShanghai were divided into various regions by the period of land formation. Weed communities in all of these regions showed a U-distribution pattern, that is to say the diversity of weed communities in Chongming Island and west of GangShen was higher than that in GangShen-XiaSha and east of XiaSha. This distribution pattern was positively correlated with the light condition and soil moisture content, while negatively correlated with the soil compaction and soil pH in all regions. The values of soil moisture content fluctuated violently in Chongming Island and west of GangShen. It caused high heterogeneity of habitats, which meant small habitats and microclimate conditions to meet the needs of plant growth. It may be the reason why the diversity of weed communities was higher in the two regions. The values of soil moisture content, soil compaction and soil pH had little fluctuation in GangShen-XiaSha, and it may be the reason for the low diversity of weed communities in this region.Based on the distribution of weed communities in all regions of land formation and adaptability to different landforms of their dominant species, the weed communities in urban and rural of Shanghai were divided into four groups. The first group were wide-adapted weeds. It included cosmopolitan and alien species having wide distributions in various zones of land formation. It also included some species having a small growth form, either over their whole growth period or during a short time. The second group were western limnetic plain weeds. These weeds mainly distribute in limnetic plain in the west of GangShen. They had a small growth form either over their whole growth period or during a short time. The third group were eastern coastal plain weeds. These weeds mainly distribute in coastal plain in the east of GangShen. They were primarily perennial and summer annual weeds having a large growth form. The fourth group were estuary delta weeds. These weeds mainly distribute in Chongming Island, and most of them were perennial and summer annual weeds that are common in farmland.3. Diversity of weeds community in various habitatsAmong the7types of habitat, the diversity of weed communities in shrub-grassland gap and forest gap was richer than other types of habitats. It mainly due to the values of the light condition and soil compaction fluctuated violently in these two types of habitats, the heterogeneity of habitats can provide living environment for the species with different life strategies. The types of weed communities in lawn were also rich, which was consistent with the good light condition and higher soil compaction in lawn. In road gap, human disturbance was frequent and habitats was poor. In soil-type and gravel-type abandoned land, the summer annual and penerial weeds having large growth form would hinder the distribution of weeds having small growth form from. The farmland mainly distributed in suburb and rural areas while absent in urban area. Due to the above-mentioned influencing factors, the richness of weed communities diversity in road gap, abandoned land and farmland was low.Based on the distribution of weed communities in each type of habitat and adaptability to different habitats of their dominant species, the weed communities in urban and rural of Shanghai were divided into four groups. The first group were wide-adapted weeds. It included cosmopolitan and alien species in5and more than5types of habitats. It also included some weeds having a small growth form and strong mechanical damage resistance. The second group were mesophyte weeds. These weeds mainly distribute in the shrub-grassland gap and forest gap. Most of them were hygrophilous or damp-resistant weeds. The third group were sunny-damp weeds. These weeds mostly were photophilic weeds which mainly distribute in soil-type abandoned land, gravel-type abandoned land and farmland. The fourth group were trampling and cutting resistance weeds. These weeds mainly distribute in road gap and lawn, and most of them had small growth form and strong anti-trample adaptability.4. Diversity of weeds community along urban-rural gradientIn all areas along the urban-rural gradient, the central area experienced a longer period of urbanisation. The urbanisation caused the loss and change of habitat, including size reduction, the high degree of fragmentation and deterioration of habitat conditions in central area. While the urban-rural fringe areas are now becoming the foci of the most intense urbanisation. Frequent disturbance created heterogeneity of habitat and resulted in more weed community types in the urban-rural fringe areas than in urban area. However, cement, mortar and other building materials used in urban construction intermingle with the soil, increase the value of soil pH, and thus affect the distribution of weed communities in these areas. These factors produced fewer types of weed communities in urban areas than those in suburb and rural areas. More types of weed communities occurred in suburb and rural areas for two reasons. First, the degree of human disturbance and damage to natural landscape in these two areas is lower. Second, habitats become more varied because of large fluctuations in soil moisture content, soil compaction and soil pH. The variety of habitat types in these two areas can furnish environments for growth of plants having different ecological habits.According to the distribution of weed communities in all areas along the urban-rural gradient and adaptability to different disturbance of their dominant species, the weed communities in urban and rural of Shanghai were divided into three groups. The first group were wide-adapted weeds. It included cosmopolitan and alien species having wide distributions in various areas along the urban-rural gradient. It also included some species having a small growth form, either over their whole growth period or during a short time. The second group were urban weeds. These weeds were mainly winter annual weeds which distribute in urban area only and perennial weeds having a small growth form. The third group were rural weeds. These weeds primarily distribute in suburb and rural areas. Most of them were perennial and summer annual weeds that are common in farmland.