Neighboring edges: interacting edge effects of linear disturbances on vegetation in treed fens

The influence of edges on forest biodiversity is an important environmental effect associated with habitat fragmentation, as edge effects can further reduce the remaining 'interior' habitat. However, extrapolating the influence of edges across the broader landscape has been difficult, especially regarding how to treat multiple edges in close proximity, where edge effects might interact. In this study, we examined the interaction of edge effects for multiple edges from a dense network of narrow (3-8 m wide) and low-severity linear disturbances called 'seismic lines'. Seismic lines are created during oil and gas exploration and are responsible for severe dissection of boreal forests in western Canada. Specifically, our objectives were to: (1) to compare the edge influence of "wide" (~8 m) and "narrow" (~3 m) seismic lines; and (2) to determine whether edges in close proximity show interaction of edge influences, i.e. do multiple narrow seismic lines have a stronger or weaker edge influence than a single narrow seismic line. We sampled vegetation along transects perpendicular to seismic lines in treed moderate-rich and poor fens. We used randomization tests of edge influence to calculate the magnitude and distance of edge effects. In moderate-rich fens, we found a positive edge influence on understory diversity from both wide and narrow seismic lines. We also found a weakening edge interaction on diversity, i.e., single narrow seismic lines had a stronger edge influence on diversity than multiple narrow seismic lines. In treed poor fens, multiple narrow seismic lines had a negative edge effect on tree density, understory abundance, richness, and composition. In addition, we found strengthening edge interactions in treed poor fens on tree density, graminoid cover, and understory composition. Our results show how assessing the edge influence of multiple disturbances can provide a better understanding of the cumulative effects of habitat fragmentation.

林缘对森林生物多样性的影响是与生境破碎化相关的重要环境效应，林缘效应会进一步缩减剩余的‘内部’生境。然而，在更大尺度的景观中推演林缘效应始终存在难点，尤其是如何处理彼此邻近、效应可能发生交互作用的多条林缘。本研究针对由狭窄（3~8米宽）、低烈度线性扰动形成的密集网络——即‘地震测线’（seismic lines）——所产生的多条林缘的效应交互作用展开分析。地震测线产生于油气勘探活动，是造成加拿大西部寒温带针叶林严重切割与破碎化的重要扰动源。本研究的具体目标包括：其一，对比宽度约8米的"宽幅"地震测线与宽度约3米的"窄幅"地震测线的林缘效应差异；其二，探明邻近林缘是否存在效应交互作用，即多条窄幅地震测线的林缘效应相较于单条窄幅地震测线是更强还是更弱。我们在木本型中富营养沼泽与贫营养沼泽中，沿垂直于地震测线的样带开展植被采样。我们采用林缘效应随机化检验方法，计算林缘效应的强度与作用距离。在中富营养沼泽中，我们发现宽幅与窄幅地震测线均对林下植物多样性产生正向林缘效应。同时我们还观测到多样性维度上的效应交互作用呈弱化趋势，即单条窄幅地震测线对林下植物多样性的林缘效应强于多条窄幅地震测线的叠加效应。在贫营养沼泽中，多条窄幅地震测线对林木密度、林下植物多度、物种丰富度及群落组成均产生负向林缘效应。此外，我们在贫营养沼泽中观测到林木密度、禾草盖度及林下群落组成维度上的效应交互作用呈增强趋势。本研究结果表明，针对多重扰动的林缘效应开展评估，能够帮助我们更深入地理解生境破碎化的累积效应。