| Urban ruderal is an important part of urban vegetation, which is essential for urban biodiversity conservation. Because of their high sensitivity, plasticity and resistance, ruderal species could respond to the urban habitat heterogeneity and rapidly adapt to changing habitats, ruderal species are ideal to facilitate studies on the impact of urbanization on urban ecosystems and subsequent ecosystems response. Supported by the National Natural Science Foundation of China "Response of ruderal to urban habitat heterogeneity and the adaptation strategies under urbanization", the present study took urban ruderal as the research object, and selected the study area in the Outer-Ring Road which is under the most dramatic urbanization during the nearly30years (i.e. the urban area of Harbin). To clarify the distribution pattern of ruderal communities and its causes in the context of urbanization, ruderal communities were investigated through the different habitat types along the urbanization gradient. To explore the adaptive strategy of ruderal species in response to high heterogeneous habitats, phenotypic plasticity of eurytopic ruderal species was analyzed. The present study filled the gap of ruderal research in middle latitude temperate zone cities in China, and also provided the theoretical basis for the study of influencing mechanism between urbanization and urban ecosystems, and the useful information for the local government to help them improve urban ecosystem conservation and management. The research results are as follows:(1) Rapid urbanization has resulted in obvious changes in ruderal species diversity.There were38families,128genera and175ruderal species in1763plots, with large proportion of monotypic-genus and rich geographical areal-types reflecting a substantially heterogeneous habitat condition, and the more completely of ruderal species evolution. Ruderal species in the urban area resided mainly in gaps constructed by each of forests, shrub-grasslands, abandoned fields and roadsides, indicating a progressively declined living space with increased heterogeneity and un-stability in species habitats. The widespread ruderal species with strong plasticity and short life cycle exhibited a relatively great tolerance with respect to intensive and frequent human disturbance. Based on its habitat properties and adaptive ability,175ruderal species could be classified into four ecological groups:eurytopic species, shady-and wet-tolerant species, mesophytic species, and trample-and mowing-tolerant species.There were436of ruderal species that recorded in1955had reduced. The pronounced changes indicated that the rapid urbanization had reduced the ruderal species richness.63species were new record,112species were remained,499species were failed to investigate over the past half century. More than a half of the remaining species were summer annual widespread ruderal species, although the urban habitat chaged obviously, they could remain because of the high resistance and plasticity.The unstable urban habitats under the obvious land use and land cover change maybe one of the reason of the disappeared of large number of perennial species. Moreover, large of hygrophytic species, farmland species and grassland species, which were suitable their relevant habitat, had a drought trend in urban habitats that consistent with the change of land use characterized. The urban re-naturalization was considered to relate with the emergence of plantations and ornamental species.Considered the high sensitivity of ruderal ruderal, ecological indicator value was added to the classification system of ruderal community on the basis of previous studies. A four-level classification system was established. Within this system, the identified1763ruderal communities could be categorized into two types of life form at the first level, five types of dormancy form at the second level,22community groups at the third level,119dominance communities at the lowest level. Urban habitat heterogenization, the commensalism of ruderal in the same habitat and species’ seasonal aspect niche differentiation resulted in the high diversity of ruderal community type growing in the urban area. The low species richness and high rates of uni-dominant species represented the rudral speciesdiversity was low; The high proportions of dominant mono-species and dominant annual species were the adaptive strategies in which ruderal community responses to unstable urban habitats under human disturbances with high intensity and frequency.(2) Habitat type and the direct human disturbance have affected the distribution pattern of ruderal communities.The number of ruderal community types showed a obvious trend along the urbanization gradient, followed by Z1(55)<Z2(80)<Z3(111). Based on the distribution of ruderal communities along the urbanization gradient, and adaptive ability to the habitat of their dominant and mainly companion species, ruderal communities in the study area could be classified into three groups:1) eurytopic ruderal community, which were widely distributed in three zones, they had shorter life cycle, strong fecundity and higher plasticit, could grow and reproduce normally in various human disturbance intensity through their plasticity adjust;2) dislike urban ruderal community, which were mainly distributed in Z2and Z3. They always distributed in artificial forests, wetlands and other relatively shady and wet habitats during the urban re-naturalization. The group had life form of annual dominant type and worse ability of disturbance tolerant, including lower plantations ruderal communities and escaped ruderal communities;3) the more dislike urban ruderal community, which were only distributed in Z3. They had life form of perennial dominant type and worst ability of disturbance tolerant. Most of them were tall with upright growth.The impervious surface with poor infiltration had the higher proportion in Z1with the highest urbanized degree, maybe made a declined living space of ruderal species. It also maybe the important reason of the low of the number of community types together with the heat island effect. Most of the ruderal communities in Z1were widely distributed ruderal communities with strong adaptation and high plasticity. The communities of the most highly adapt to urban habitat had not yet appeared. The land use types in Z2were similar to Z1, and consistent with the degree of urbanization in Z1. The ruderal community diversity index also had no significant difference in Z1and Z2. Dislike urban ruderal communities were appeared because of the forest gap.The urbanization degree of Z3was relatively lower, the various land use types maybe resulted in the high diversity of ruderal community type in a certain extent. Moreover, the suitable environment for ruderal growth and the lower strength of the human management and protection maybe the important reason of significantly higher ruderal community diversity in Z1and Z2. Non-significant in the distribution pattern of ruderal communities along the urbanization gradient (i.e. a comprehensive environmental factor), the mainly cause was the habitattype and the direct human disturbance type and intensity.There was more human disturbance in the process of urban land use changed, causing the more of chance and randomness. Therefore, the authors thought that when the habitat was unstable, the distribution of ruderal species was likely had no relation with natural selection, instead its distribution and coexistence was determined by the random factors, which means all the ruderal species had the same capacity of entering into new habitats theoretically, that is more easily explained by the neutral theory. When the habitat was relatively stable, the ruderal species distribution was determined by the habitat and itself, that is more easily explained by the niche theory. The ruderal community had high diversity of ruderal community type and richness under the high nutrition and less interference habitat conditions; The CR-strategy ruderal type was grown to be ruderal communities when the nutrition conditions had threatened; The SR-strategy ruderal type was grown to be ruderal communities in poor habitat conditions. If the ruderal habitat sustained stable, rudral communities will be succession from annual to perennial, culminating in woody communities. However, new disturbance always appeared before that in the urban habitat.(3) Phenotypic plasticity is one of the adaptive strategies of ruderal to heterogeneous habitat.Change patterns of the leaf traits of the six widely distributed ruderal species in different habitat types were the same. There was higher leaf area and lower specific leaf area in abandoned land and farmland; there was higher specific leaf area and lower leaf dry matter content in shrub and forest gap; and there was lower leaf area in lawn and there were lower specific leaf area and higher leaf dry matter content in wetland. Ruderal had the highest leaf area in farmland and abandoned land and lowest leaf area in lawn, mainly affected by light and soil moisture factors possibely; Ruderal had the higher specific leaf area in shrub and forest gap and lower leaf area in abandoned land and wetland, mainly affected by soil moisture and nutrient factors, however, interaction was occurred in most factors; Ruderal had the lowest leaf dry matter content in shrub gap and highest leaf dry matter content in wetland, mainly negatively correlated with soil moisture; no difference of Nmass and Pmass in different habitats type, and it was no significant correlation with soil total nitrogen and phosphorus content.The ruderal biomass in different organs in different habitat types had obvious change and strong plasticity. The biomass in each organ and total biomass were largest both in abandoned land and wetland and significant higher than the other four habitat types, mainly affected by light. However, interaction was occurred in most factors, which had different affected trend, led to no significant variation of ruderal biomass distribution in different habitats. Consistent with the individual level, ruderal community biomass in different habitat types had obvious change, with the largest in in abandoned land, and then in wetland, and significant higher than other four habitat types, mainly affected by light conditions. R/S of ruderal community had no obvious change? which it was more inclined to met the allometric relationship. |
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