Effects of high- vs. low-intensity herbicide management on Phragmites australis propagule pressure in a brackish wetland of California

Abstract Brackish wetlands are vital habitats for migratory and endemic species in coastal areas, supporting nutrient cycling, flood management, and recreation. Phragmites australis, or common reed, is a relatively recent invader of brackish and freshwater ecosystems across North America. In California’s Suisun Marsh—a 46,950-hectare network of public and private wetlands—P. australis has dramatically expanded over the past two decades. This expansion reduces habitat quality for waterfowl and other species, as dense stands displace native wetland plants. Herbicide-based management, primarily using glyphosate, has been employed in some areas of the marsh for over a decade, while other areas remain untreated or were only recently treated. To investigate the effects of management, we collected inflorescences from 11 high-intensity treatment parcels (≥ 9 years of spraying) and 9 low-intensity parcels (0–3 years). Random Forest classification identified large patches of P. australis across satellite imagery, allowing us to estimate propagule pressure as seeds per square meter. High-intensity parcels produced fewer seeds on average than low-intensity parcels, due largely to differences in the area occupied by P. australis (t = − 5.307, df = 313.93, p < 0.0001). However, we found no significant differences in seed germination or herbicide resistance between the two groups (t = 0.449, df = 101.95, p > 0.05; t = − 1.52, df = 18, p > 0.05). These findings highlight the effectiveness of consistent herbicide use in reducing P. australis seed output and emphasize the need for coordinated, watershed-scale strategies to prevent the establishment of new patches.