Tipping the balance: The role of seed density, abiotic filters, and priority effects in seed‐based wetland restoration

Abstract

Sowing native seeds is a common approach to reintroduce native plants to degraded systems. However, this method is often overlooked in wetland restoration despite the immense global loss of diverse native wetland vegetation.Developing guiding principles for seed-based wetland restoration is critical to maximize native plant recovery, particularly in previously invaded wetlands. Doing so requires a comprehensive understanding of how restoration manipulations, and their interactions, influence wetland plant community assembly. With a focus on the invader Phragmites australis, we established a series of mesocosm experiments to assess how native sowing density, invader propagule pressure, abiotic filters (water and nutrients), and native sowing timing (i.e., priority effects) interact to influence plant community cover and biomass in wetland habitats. Increasing the density of native seeds yielded higher native cover and biomass, but P. australis suppression with increasing sowing densities was minimal. Rather, community outcomes were largely driven by invader propagule pressure: P. australis densities of ≤500 seeds/m 2 maintained high native cover and biomass. Low-water conditions increased the susceptibility of P. australis to dominance by native competitors. Early sowing of native seeds showed a large and significant benefit to native cover and biomass, regardless of native sowing density, suggesting that priority effects can be an effective restoration manipulation to enhance native plant establishment. Given the urgent wetland restoration need combined with the limited studies on seed-based wetland restoration, these findings provide guidance on restoration manipulations that are grounded in ecological theory to improve seed-based wetland restoration outcomes.

Role of priority effects in invasive plant species management: Early arrival of native seeds guarantees the containment of invasion by Giant ragweed

“…The biomass of resident communities has been documented as one of the best indicators of competitive ability of native species (Gaudet & Keddy, 1988 ) and their level of biotic resistance to invasive species (Byun et al, 2013 ; Byun, Choi, & Kang, 2020 ; Byun & Lee, 2017 ; Byun, Oh, et al, 2020 ; Lulow, 2006 ). Early sowing of native seeds significantly increased native canopy cover and biomass in this study, which is inconsistent with the results of a previous study (Tarsa et al, 2022 ). A canopy complexity, comprising the canopies of different species, can enhance canopy cover and suppress invaders (Lindig‐Cisneros & Zedler, 2002 ).…”

“…Native species arriving 6 weeks earlier than A. trifida were more effective in controlling invasion than those arriving 3 weeks earlier or at the same time as A. trifida . These results are consistent with observations made in previous studies on the priority effects in different plant systems, such as Festuca pratensis , Plantago lanceolata , Knautia arvensis , Trifolium pratense , Rhamnus cathartica , Phragmites australis , and California grasslands (Firn et al, 2010 ; Hess et al, 2019 ; Mwangi et al, 2007 ; Schuster et al, 2020 ; Tarsa et al, 2022 ; Weidlich & de Dechoum, 2021 ; Yannelli et al, 2020 ; Young et al, 2017 ). A study on priority effects, in which a native species was planted 4 weeks prior to the arrival of two invasive non‐native grasses ( Urochloa humidicola or U. decumbens ), reported poor development of invasive species (Evangelista et al, 2017 ).…”

“…Resident plants employ biotic resistance to control invasive species spread. Biotic resistance of native plants can also be used to design competitive plant communities to prevent the establishment and dominance of an alien invasive plant and/or to outcompete an already‐established alien plant species (Guo et al, 2019 ; Kettenring & Adams, 2011 ; Tarsa et al, 2022 ; Weidlich et al, 2020 ). Biotic resistance through competition to invasive species is determined by many factors, including limiting similarity (Hess et al, 2020 ; Price & Pärtel, 2013 ), functional traits or functional groups (Drenovsky et al, 2012 ; Hooper & Dukes, 2010 ), diversity effects (Byun, de Blois, & Brisson, 2020 ; Henriksson et al, 2016 ), seed sowing density (Byun, Oh, et al, 2020 ; Tarsa et al, 2022 ), and priority effects (Weidlich et al, 2021 ).…”

“…Biotic resistance of native plants can also be used to design competitive plant communities to prevent the establishment and dominance of an alien invasive plant and/or to outcompete an already‐established alien plant species (Guo et al, 2019 ; Kettenring & Adams, 2011 ; Tarsa et al, 2022 ; Weidlich et al, 2020 ). Biotic resistance through competition to invasive species is determined by many factors, including limiting similarity (Hess et al, 2020 ; Price & Pärtel, 2013 ), functional traits or functional groups (Drenovsky et al, 2012 ; Hooper & Dukes, 2010 ), diversity effects (Byun, de Blois, & Brisson, 2020 ; Henriksson et al, 2016 ), seed sowing density (Byun, Oh, et al, 2020 ; Tarsa et al, 2022 ), and priority effects (Weidlich et al, 2021 ). Among these, the role of priority effects (or niche preemption), use of resources by early‐arriving species and thereby affecting the performance of late arrivals (Fukami, 2015 ), to control invasion is gaining attention of scientific community to create new knowledge and its practical application.…”

Common, showy, and perennial species dominate a restoration species pool

“…From the remaining 1,684 taxa we removed all obligate wetland species (Missouri wetness rating of "À5"; n = 332). Here, we focus on upland plant species because emergent species are less common in the dominant habitats of this region, wetland plants are often introduced as plugs or in vegetation mats, and seed-based restoration techniques are less developed for wetlands (Tarsa et al 2022). We excluded species that occur within the state of Missouri, but not within the Ozark ecoregion, by consulting range maps and descriptions in the Flora of Missouri (Yatskievych 1999(Yatskievych , 2006(Yatskievych , 2013 and in the Biota of North America Program (BONAP) Taxonomic Data Center website (Kartesz 2015).…”