Do common assumptions about the wetland seed bank following invasive plant removal hold true? Divergent outcomes following multi‐year Phragmites australis management

Abstract

Question Limited funds and compressed timelines frequently translate into a reliance on seed banks for native plant recovery following invasive plant management. This approach assumes: (a) baseline seed bank communities are sufficient for native plant recovery regardless of site environmental conditions; (b) different management actions variably impact native and invader seed banks; (c) management actions reduce invader seed banks; while (d) native seed banks do not decline following management; and (e) returning above‐ground vegetation reflects the seed bank. Do these assumptions hold true in the context of invasive Phragmites australis management? Location Great Salt Lake wetlands, Utah, USA. Methods Across six sites, we applied six Phragmites management treatments (solarization, or glyphosate or imazapyr herbicide in the summer or fall with mowing) for three years, and monitored two more. Each year, we assessed the above‐ground vegetation and seed bank using the seedling emergence method. Results Site environmental conditions drove baseline seed bank communities: seed banks differed substantially across sites with varying anthropogenic degradation as less disturbed sites had greater seed bank richness. Different management actions had similar impacts on native and invader seed banks. Phragmites seed density was reduced after herbicide treatments while native seed bank communities were unaffected. Above‐ground vegetation generally reflected seed bank communities. However, native graminoids were present in seed banks but surprisingly rare above‐ground. Conclusion Commonly held assumptions about invasive plant management and native recovery from the seed bank did not all hold true. Native seed banks were particularly insufficient for recovery at sites with greater degradation, suggesting managers should prioritize management at less degraded sites or consider revegetation in highly degraded sites. Despite reductions in Phragmites seed bank density, Phragmites propagule pressure remained high, requiring a long‐term management commitment. Often seed banks are insufficient and revegetation of desired above‐ground species is necessary to meet restoration goals.

Landscape and site factors drive invasive Phragmites management and native plant recovery across Chesapeake Bay wetlands

“…Their repeated management efforts resulted in substantial reductions in Phragmites stem densities and, as importantly, significant declines in herbicide use and personnel time as the invasion was brought under control. Complete control of Phragmites requires addressing its abundant seed production (Kettenring et al, 2011), its large reservoir of stored seeds in seed banks (Hazelton et al, 2018; Rohal et al, 2021), and its extensive belowground rhizome network that may be only partially killed with a single or poorly timed herbicide application (Rohal, Cranney, Hazelton, et al, 2019). Therefore, it should not be surprising that lasting control of Phragmites will take many years of diligent management and long‐term monitoring (sensu La Peyre et al, 2001).…”

The effects of native seed mix composition and sowing density on plant community reassembly in wetlands

“…The results support the idea that native plant propagules in wetlands can be limited and relying on passive revegetation alone to establish native plant communities may not always result in desired restoration outcomes (Kettenring & Reinhardt Adams, 2011; Turnbull et al, 2000; Xiong et al, 2003). In the field experiment, multiple years of Phragmites treatment before the start of the experiment likely facilitated the reestablishment of native plants by reducing competitive pressure from the seed bank and surrounding areas (Rohal, Cranney, Hazelton, et al, 2019; Rohal, Cranney, & Kettenring, 2019; Rohal et al, 2021). Despite increases in Phragmites in the second growing season in the field experiment, Phragmites cover in seeded and control plots typically was <25%.…”

“…Each mesocosm received one of two Phragmites seed densities, low (538) or high (5382 PLS m −2 ), representing rates that Phragmites may be found in the seed bank (Rohal et al, 2021; Tarsa et al, 2022). Each treatment combination was replicated four times in a randomized complete block design for a total of 64 experimental units.…”

Long-term periodic management of Phragmites australis maintains native brackish wetland plant communities

“…The question of what happens following Phragmites removal has been addressed following non-chemical control (e.g., Warren et al 2002) and herbicide or mechanical management (Faison et al 2020), and two patterns emerge. First, if management ceases and Phragmites has not been completely removed, sites can become dominated by Phragmites again (Farnsworth and Meyerson 1999; Martin and Blossey 2013; Hazelton et al 2014;Rohal et al 2019bRohal et al , 2021Rohal et al , 2023. Second, if management is long-term and continuous or su ciently removes Phragmites, vegetation recovery occurs, even if small amounts of Phragmites remains (Lombard et al 2012), a potential example of functional eradication.…”