| Road disturbance from maintenance and traffic frequently affect adjacent vegetation and environment, they have the potential to alter plant communities, and majority studies focused on relative mature ecosystems, such as forest and grassland. A majority of ecological studies of roads have usually focused on their deleterious effect, and it promotes more and more researchers to determine the exact relationship between road disturbances and plant community in diverse environments, and the universality of negative effect of road disturbance in different environments. However, these preconceptions have hampered a full evaluation of ecological functions of roads. Undeniable, road construction is inevitable because of economic benefit, how can we mitigate their negative effect? It is urgent to objectively and accurately assess the effect of road and discuss the possibility of changing road structure to mitigate their negative effect, for example, changing the structure of drainage system. However, there is little evidence documenting the effect of road ditch on plant communities. In this study, we examined the effect of road disturbance on plant communities in a protogenic and high salinization environment by investigating roadside vegetation in road ecosystem of the Yellow River Delta (YRD), China. Specifically, we examined the effect of road attribute (road width, pavement and density) on the distribution pattern of plant species richness, the effect of road age and distance from road verge on plant community, and the effect of road ditch on species richness, diversity and composition of plant community, and environment conditions. The investigation continued for 3 years (from July to August in 2007,2008 and 2009), we totally surveyed 2808 quadrats (quadrat method) and 352 line transects (line transects method), and collected 185 herbariums and 3269 soil samples. The investigation in 2007 was a previous trial, on the one hand, it was used to find a better investigation and analysis method, and on the other hand, it was a background survey for road and roadside vegetation in YRD, which is useful for the investigation in the next two years. The major findings are summarized as follows:(1) The results suggested that road attributions significantly affected species richness of roadside plant community. Asphalt roads retained higher species richness than terrene roads. Road with width of 20m retained more species than that in road with width of 5m. Both species richness and density were found to be significantly high in areas with high road density than that in areas with low density, even considering the effect of distance from road verge.(2) In spatial scale, roads retained higher species richness adjacent to the road verge than far away from the road verge (>200m), additionally, species richness and diversity of roadside plants significantly decreased as the distance from the road verge increased. In temporal scale, both positive and negative effects of roads on plant community were observed from road construction to the long-term operational phases. In road construction phase, road construction destructively removed all habitat and vegetation adjacent to it; therefore, the species richness in this phase was lower than that in area far from road verge. In this phase, the effect of road was negative. In short-term operational phase, plant species richness and diversity increased with road age in majority of prescribed distances, while species richness significantly increased with road age and peaked at a road age of 20 years, after which species richness plateaued. In this phase, the effect of road was positive. In long-term operational phase, the total species richness decreased which might attribute it to the decrease of the species richness of native plant. A correspondence analysis (CA) illuminated that the roadside vegetation primarily consists of non-halophytic native species. The percentage of halophytes increased with the distance from the road verge and decreased with road age. Furthermore, it is worth noting that roadside environments provide survival habitats for some threatened species, such as wild soybean.(3) In spatial scale, both the species richness of native and alien species decreased when the distance from road verge increased. In temporal scale, different performances of alien and native species in response to road disturbance were observed. Species richness of native species increased with road age in majority of prescribed distances, while their species richness significantly increased and peaked at a road age of 20 years, after which species richness decreased. While, species richness of alien species increased with road age. The frequency of majority alien species decreased when the distance from road verge increased, however, some species, such as Tamarix chinensis Lour., were found evenly distributed in road-effect areas. This tendency might be influenced by the biological properties of established alien species, including life habit, dispersal mechanism and morphology.(4) In this protogenic and high salinization environment, road disturbance promotes the change of roadside plant community from community dominated by halophyte to community dominated by non-halophyte. In the two age gradient, species richness in roadside areas was significantly higher than that in non-road areas. The high floristic dissimilarities values indicated that roadside and non-road areas differed greatly in community composition. In the two age gradient, the plant communities in roadside had lower percentage of halophytes than non-road communities. Correspondence Analysis and Two-Way Indicator Species Analysis showed that halophytes dominated in the non-road area, while a number of typical non-salt tolerant species dominated in roadside. Compared to non-road area, activities associated with roads significantly decreased soil moisture and salinity, and increased soil pH and nitrate concentration. Forward selection for the environmental variables in Canonical Correspondence Analysis showed that soil salinity was the most important factor related to the variation of species composition between roadside and non-road area. Our study demonstrated that road effects had a significant impact on the associated vegetation and soil, and these changes were consistent across roads of different ages in our system.(5) It's hard to say road ditch is good or bad for local biodiversity, and both positive and negative effects of roads ditch on plant community were observed in our system. Our result showed that the existence of road ditch could provide favorable environment for survival of more species, including decreased soil salinity. In road system without ditch, the refuge for halophyte was limited in road verge. In road system with ditch, this refuge was in the area from road verge to 30m from road verge. Therefore, road system with ditch retained abundant plant species with high level of species richness, including more non-halophyte vegetation. The width and structure of road ditch could affect the effect of road ditch on roadside plant community. Both road systems with large and small ditch decreased soil salinity, but the mechanism was different which may affect the distribution of halophyte. For the total species richness, road system with large ditch retained more species than road system without ditch, but the species richness in road system with small ditch was significantly lower than that in road system without ditch. The existence of ditch decreased moisture content along roadside where the species richness decreased. Additionally, road ditch did promote the invasion of exotic species compared to road system without ditch.
|
PDF Full Text Request