Plant communities and soil chemistry of shrub-steppe and pine savannah grasslands in southern British Columbia, Canada

Habitat fragmentation alters the edges of remnant habitat patches. We examined changes in the plant community and soil in relation to distance from edge and edge type for shrub-steppe and pine savannah grasslands in southern British Columbia, Canada. Community composition showed significant nonlinear relationships with distance-to-edge more frequently at paved roads and fruit crops than at dirt roads or control sites (i.e., in the interior of grassland patches), with changes typically extending 25-30 m. More exotic species and fewer native species were found near edges, and edges showed decreased cryptogam cover and increased bare ground, especially near paved roads. The soil factors that best predicted compositional changes were soil pH and Cu/Mn at paved roads, soil pH and nitrogen at fruit crops, and soil resistance at dirt roads. Variation partitioning suggested that both direct (e.g., propagule pressure) and indirect (environmental change) factors mediated edge-related community changes, and provided evidence that nonlinear responses at developed edges were not due to natural gradients. Given the range of grassland patch sizes in this region (many patches 1-100 ha), the edge effects we observed represent a considerable loss of "core" habitat, which must be accounted for in conservation planning and site restoration.生境破碎化会改变剩余生境斑块的边缘区域。本研究针对加拿大不列颠哥伦比亚省南部的灌丛草原与松树稀树草原草地，探究了植物群落与土壤特征随距边缘距离及边缘类型的变化规律。相较于土路或对照样地（即草地斑块内部区域），铺装道路与果树种植区边缘的植物群落组成与距边缘距离间更常呈现显著的非线性相关关系，其群落变化的影响范围通常可达25至30米。边缘区域的外来物种数量更多、本土物种数量更少，且边缘处的隐花植物盖度降低、裸地面积增加，该现象在铺装道路附近尤为显著。能够最优预测群落组成变化的土壤因子分别为：铺装道路边缘的土壤pH值与铜锰比（Cu/Mn）、果树种植区边缘的土壤pH值与氮含量，以及土路边缘的土壤抗阻。变异分割分析表明，直接因子（如繁殖体压力）与间接因子（如环境变化）均介导了边缘相关的群落变化，同时证实人工边缘处的非线性响应并非由自然环境梯度所致。鉴于该区域草地斑块的面积范围（多数斑块面积为1至100公顷），本研究观测到的边缘效应会造成相当规模的"核心"生境损失，这一因素必须纳入保护规划与场地修复工作中。