The legacy of pipeline installation on the soil and vegetation of southeast Wisconsin wetlands

Installation of large-scale infrastructure, including pipelines, has the potential to damage soil and vegetation in wetlands within the path of construction by compacting soil, altering hydrology, decreasing plant diversity, and facilitating invasions of unwanted species. We sampled soil and vegetation in seven southeast Wisconsin wetlands eight years after they were crossed by a natural gas pipeline to compare areas inside and outside the pipeline corridor (land within 12 m of the pipe, where construction was permitted). Soils consistently showed evidence of compaction and hydrologic alteration. Across sites the pipeline corridor soil had 63% higher bulk density, 13% lower depth to refusal and 19% lower soil moisture (P < 0.0001 for all). In contrast, the diversity and quality of vegetation (species richness, Simpson's dominance, and mean coefficient of conservatism) inside and outside the pipeline corridor varied among sites. A site with relatively high native plant diversity outside the pipeline corridor showed lower richness and higher dominance inside the corridor (P < 0.05 for both); whereas, in sites that were strongly dominated by invasive grasses, Phalaris arundinacea and Glyceria maxima, vegetation either showed no change or positive changes in diversity and quality in the pipeline corridor. We suspect the net positive effects of pipeline installation on vegetation in some sites owes to (1) the presence of monotype-forming invasive species outside the pipeline corridor in those sites, (2) post-construction planting and subsequent maintenance-mowing of the pipeline corridor, and (3) colonization within the corridor by additional species (mostly upland species). However, our vegetation results do not undercut the overall negative impacts of pipeline installation on wetland soils, which were still observable eight years after construction. While soil might be the more consistent indicator of pipeline impacts in wetlands, the site-specific and sometimes-positive response of vegetation highlights (1) the need for better pre-construction surveys, so that impacts can be minimized at high-quality sites and (2) the potentially high return on post-construction planting and maintenance.

► We compared soil and vegetative qualities inside and outside a pipeline corridor. ► Soils inside the pipeline corridor were drier and more compact. ► Vegetation variables appeared to be more dependent on the pre-disturbance conditions. ► Glyceria maxima can form large monotypic stands.