Establishment gaps in species-poor grasslands: artificial biodiversity hotspots to support the colonization of target species

Abstract

Sowing of grass seed mixtures is a feasible and cost-effective method for landscape-scale grassland restoration. However, sowing only grasses usually leads to species-poor and dense grass sward, where the establishment of target forbs is hampered both by microsite and propagule limitation. To overcome these limitations and increase the diversity of species-poor sown grasslands we developed a novel method by creating 'establishment gaps'. We used tillage to open gaps of 1 m 2 , 4 m 2 and 16 m 2 size in the dense grass sward of six species-poor sown grasslands in the Great Hungarian Plain. We sowed high-diversity seed mixtures of 35 native species into all gaps. We analyzed vegetation development during the first five years after setting up the trial. We also studied the colonization dynamics of the sown species along four 20-m transects around each gap, resulting in a total of 1440 plots of 1 m 2 size that were studied. Our results indicated that most of the sown species were able to establish permanently in the establishment gaps. The total cover and the cover of perennial sown species increased independently of gap size. Meanwhile the cover of short-lived sown species 2 decreased during the five years. There was only a moderate level of weed abundance in the gaps, and weed cover decreased over the years. The sown target species started to colonize the species-poor grasslands surrounding the establishment gaps within five years. The highest number of species and individuals dispersed from the 4 m 2 -sized gaps, as they had a more stable development than small gaps and were exposed to lower grazing pressure than large ones. Implications for practice Establishment gaps are widely applicable tools to increase the diversity of speciespoor grasslands. Gaps of 4 m 2 represent a more feasible solution compared to larger openings also for the farmers, because there is only a moderate level of weed encroachment and smaller soil disturbance occurs during their creation.  We recommend sowing high-diversity seed mixtures containing both short-lived species that can establish in the first year and perennial species, which guarantee a high cover of target species later on.  Gaps sown with high-diversity seed mixture are highly resistant to unfavorable climatic conditions: increasing grass abundance in dry years does not hamper the recovery of target grassland species in the following years.

Seedling recruitment in mountain grassland restoration: Effects of soil preparation and grazing

“…The soil of disturbed sites generally contains high numbers of seeds of annuals and/or ruderal species (Bischoff, 2002; Donath et al, 2007). These species often dominate the vegetation of early successional stages before being replaced by typical grassland species that are usually perennials developing later in the succession (Kiehl et al, 2010; Valkó et al, 2016; Kiss et al, 2020). The first year after the restoration of Eastern European grasslands, Lengyel et al (2012) observed that annual weeds were dominant, but the cover of those weeds decreased dramatically after the third year due to the growth of perennial grasses.…”

“…Poor seed dispersal, as well as the lack of connectivity between source populations due to landscape fragmentation, often limit the seed rain (Münzbergová & Herben, 2005; Bischoff et al, 2009; Muller et al, 2014) and successful restoration may require seed addition to overcome seed limitation (Öster et al, 2009; Török et al, 2012; Valkó et al, 2016). Thus, seed addition methods, such as sowing local seed mixtures, hay transfer or soil transfer, are increasingly used in grassland restoration (Kiehl et al, 2010; Scotton et al, 2012; Kiss et al, 2020).…”

Seed use in the field: delivering seeds for restoration success

“…Site treatments prior to seeding may be required to remove or reduce competition with undesirable species, increase site stability, repair hydrologic function, or reduce nutrient levels (Morgan 2005). Seedbed preparation operations may then be required to produce physical disturbance of the germination substrate and provide establishment niches (Schmiede et al 2012; Kiss et al 2020). Tillage, herbicide application, prescribed fire, and other treatments can be used to create seedbed conditions appropriate to the species and seed delivery system (Whisenant 1999; Morgan 2005; Krautzer et al 2012; Armstrong et al 2017).…”

“…These operations can be conducted to increase species diversity, alter community structure across a site, or restore depleted or weed‐infested areas (Rayburn & Laca 2013; Silva et al 2019). Diversification of species‐poor native vegetation or seedings is often hampered by microsite limitations (Münzbergová & Herben 2005); disturbance to reduce competition or improve microsite availability is often necessary before species introduction occurs (Schmiede et al 2012; Baasch et al 2016; Kiss et al 2020).…”

Dynamics in vegetation and seed bank composition highlight the importance of post‐restoration management in sown grasslands

“…Therefore, sowing seeds of rare target species is a widely applied species introduction method in dry grassland restoration projects (Barr et al 2017). However, the availability of seed material of regional provenance is often a major limiting factor in restoration projects (de Vitis et al 2017), and especially in large‐scale projects only a limited number of target species can be included in the seed mixtures (Kiss et al 2020).…”