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.