Season and plant life history stage in revegetation influence competition of foundation species, subordinate species, and weeds in a reclaimed grassland

Abstract

River embankments offer potential semi-natural habitats for grassland species, but their large-scale reconstruction leads to vast vegetation loss. To restore both ecosystem services and biodiversity on reconstructed river embankments, we tried to achieve rapid cover by native grasses and to enhance the richness of species typical of semi-natural grasslands. We conducted a 4-year experiment to clarify whether season and plant life history stage in revegetation determine competitive interactions among foundation species (Imperata cylindrica), subordinate species (eight species typical of semi-natural grasslands), and weeds (any other species). Square frames measuring 1 m × 1 m × 30 cm depth were filled with soil used for the construction of river embankments. Two I. cylindrica life history stage modalities (seeds, seedlings) and a control were established at three timings (March, May, July) with three replications. Fifty seeds of each subordinate species were sown in each plot. In the March and May timings, annual weeds were dominant in year 1. In the July timing, I. cylindrica became dominant because high temperatures inhibited the germination of weeds and several subordinate species. Planting seedlings in any season was successful in achieving dominance of I. cylindrica, but only in the absence of rhizomes of invasive Solidago altissima. Many individuals of subordinate species were recorded in the May timing. Subordinate species with higher competitive ability or with less niche overlap with I. cylindrica performed better. Even so, overgrowth by I. cylindrica or weeds generally emerged, leading to low survival rates of subordinate species in all treatments. Season or life history stage had no effect on richness. Since rapid coverage of river embankments by native grasses is essential, planting in July is desirable for restoration. A shortage of dominant weeds in year 1 and relatively slow dominance by sown I. cylindrica led to less severe competitive exclusion of subordinate species. Thus, sowing in July would enhance the dominance of I. cylindrica and the establishment of subordinate species. Since the germination rate in July sowing was the smallest, further studies of successful introduction methods in the warm season are needed.

Introduction

Across the globe, large areas of native vegetation have been removed for urban, industrial, and agricultural land uses. Destruction of vegetation can increase erosion and undermine soil stability. Revegetation can stabilize disturbed areas (Gyssels et al., 2005; Stokes et al., 2014). Ecosystem restoration was founded on recovering ecosystems using wildlands as reference sites (Swetnam et al., 1999; White and Walker, 1997). More recently, however, interest has shifted to the restoration of ecosystem services, i.e., the benefits that natural systems can provide to humans (Bullock et al., 2011; Palmer et al., 2014). This includes the development of new sustainable ecosystems (designed or engineered ecosystem; (Higgs, 2017)) that have both human and ecological values (Mitsch, 2012). Research focusing on both values at the same time should be favored (Bullock et al., 2011).

Artificially revegetated sites are traditionally fertilized, mulched, and seeded (or hydro-sown) with non-native grasses and legumes selected for rapid growth and effective erosion control (Andrés et al., 1996; Martínez-Ruiz et al., 2007). In recent years, native species have been preferred (Bochet et al., 2010; Hilvers et al., 2017; Matesanz and Valladares, 2007) for their higher establishment rate than non-native species in a wide range of conditions (Tinsley et al., 2006; Tormo et al., 2007). In addition, Berendse et al. (Berendse et al., 2015) and Gould et al. (Gould et al., 2016) demonstrated the positive influences of greater plant species richness and functional diversity on preventing soil erosion in grasslands. The effect of diversity can be driven by complementarity, resulting from vertical segregation of root systems and more efficient exploitation of soil resources (Gould et al., 2016; Zhu et al., 2015).

Overflow and overtopping caused by embankment breaches directly cause devastating consequences for surrounding areas. The large-scale construction of riverbanks leads to vast vegetation loss, making it vital to restore vegetation cover quickly. Riverbanks traditionally support typical species-rich plant communities (Bátori et al., 2016; Koyanagi et al., 2019; Liebrand and Sykora, 1996). But historical community compositions are difficult to achieve (Koyanagi et al., 2019). To restore both ecosystem services and biodiversity on reconstructed riverbanks, new floristic assemblages need to achieve both rapid restoration of vegetation cover and greater richness of riverbank species. However, despite reports of plant attributes that confer resistance to erosion (Helsen et al., 2016; Vannoppen et al., 2016), few revegetation techniques that both restore cover and increase species richness have been proposed.

Riverbanks are generally productive habitats (Koyanagi et al., 2019; Liebrand and Sykora, 1996). Grassland restoration of nutrient-rich sites is one of the most frequently used habitat restoration actions (Cramer and Hobbs, 2007; Török et al., 2011). Productive soils support rapid revegetation, and a closed and productive stand dominated by perennial generalists inhibits the establishment of target species (Janssens et al., 1998; Marrs, 1993; Vécrin et al., 2002; Walker et al., 2004). Therefore, for the restoration of species-rich semi-natural grasslands, a common method on productive soils is to remove soil nutrients or topsoil (Török et al., 2010; Walker et al., 2004). In cases where vegetation is managed without these soil treatments, sowing competitor species is recommended to prevent the dominance of invasive weeds (Critchley et al., 2006; Lepš et al., 2007). Vegetation dominated by perennial grasses, in which the establishment of weedy species is prevented in the long term, develops quickly, although at the expense of the suppression of other desirable species (Lepš et al., 2007; Török et al., 2010). However, regionally abundant non-target species may colonize the area before target species do, thereby possibly inhibiting establishment of the latter (Bakker and Berendse, 1999; Dobson et al., 1997). How to establish desirable perennial grasses and target species remains unclear.

Species composition diverges among communities even if they experience similar environmental conditions and share the same regional species pool, on account of their unique local histories (Drake, 1990; Law and Morton, 1993). One of the central underlying concepts of assembly is that the order of arrival of species (temporal priority) can influence long-term community structure through niche preemption; that is, one or more early arriving species affect the establishment, growth, reproduction, or abundance of later arriving species (Belyea and Lancaster, 1999; Fukami, 2015; Yang and Rudolf, 2010). Differences in the timing of growth in a single year could result from differences in phenology (e.g., early- and late-season forbs, cool- and warm-season grasses; (Page and Bork, 2005)). This process can occur within species also: an initial size advantage (e.g., sowing vs. planting, or size of plug-plants) can increase with time, because larger individuals get better access to limiting resources (Ellison and Rabinowitz, 1989; Harmon and Stamp, 2002; Rice and Dyer, 2001). Niche preemption is of high relevance in restoring native species and controlling invasive species (Young et al., 2001; Young et al., 2005) and could impede restoration if not explicitly managed (Suding et al., 2004). Reports suggest the importance of an initial size advantage and timing of introduction, but most studies were confined to harsh environmental conditions (e.g., roadside revegetation), where rapid development of vegetation cover is unlikely owing to poor soil fertility or water availability (Bochet and García-Fayos, 2004; García-Palacios et al., 2010). Niche preemption is expected to be stronger in less harsh, more productive environments that promote higher relative growth rates that in turn accentuate competitive interactions (Fukami, 2015; Kardol et al., 2013). Thus, niche preemption would be a solution for effective management of revegetation on productive soils.

In this study, we hypothesized that season and initial plant life history stage, both of which are major considerations in revegetation, markedly affect subsequent vegetation coverage. We asked: (i) Do the timing of introduction and life history stage of I. cylindrica determine competition with weeds at a reclaimed site? (ii) Can sown desirable subordinate species survive despite dense cover by grass or weeds? (iii) What is an appropriate method for the establishment of vegetation; i.e., achieving the rapid cover of native grasses and enhancing the richness of riverbank species on a reconstructed riverbank?