Priority Treatment Leaves Grassland Restoration Vulnerable to Invasion

Abstract

Priority effects can be used to promote target species during restoration. Early planting can provide an advantage over later-arriving species, increasing abundance of these early-arrivers in restored communities. However, we have limited knowledge of the indirect impacts of priority effects in restoration. In particular, we do not understand how priority effects impact non-target species. Of particular conservation concern is how these priority effects influence establishment by non-native species. We use a field-based mesocosm experiment to explore the impacts of priority effects on both target and non-target species in California grasslands. Specifically, we seeded native grasses and forbs, manipulating order of arrival by planting them at the same time, planting forbs one year before grasses, planting grasses one year before forbs, or planting each functional group alone. While our study plots were tilled and weeded for the first year, the regional species pool was heavily invaded. We found that, while early-arrival of native grasses did not promote establishment of non-native species, giving priority to native forbs ultimately left our restoration mesocosms vulnerable to invasion by non-native species. This suggests that, in some cases, establishment of non-native species may be an unintended consequence of using priority treatments as a restoration tool.

Plant Community Assembly in Invaded Recipient Californian Grasslands and Putative Donor Grasslands in Spain

“…There is extensive evidence of the competitive dominance of these pest species in California grasslands, such as the grasses Bromus hordeaceus, B. madritensis, Brachypodium dystachyon, Avena barbata, and the forbs Hypochaeris spp. and Medicago polymorpha [24,[27][28][29]31,39,47].…”

“…The four grasslands within a region were at least 6 km apart, and all eight grasslands were located in sites with similar climatic characteristics along a coastal-inland gradient (Table 1, Figure S1). Because soil disturbance influences the establishment of invasive species [24,[27][28][29][30][31], we sampled sites with similar land-use histories for the last 30 years. Specifically, grasslands that had been continuously grazed by wild and domestic ungulates, and avoiding sites with any recent history of burning, ploughing or planting.…”

Using priority effects for grassland restoration: Sequential sowing can promote subordinate species

“…Species groups may be based on their origin (native vs exotic, Goodale & Wilsey, 2018; Grman & Suding, 2010; Hess et al., 2022; Ploughe et al., 2020; Yu et al., 2020), on their life cycle type (annual vs perennial, Schantz et al., 2015; Vaughn & Young, 2015), or on their functional group (grasses vs legumes vs non‐legume forbs; Delory et al., 2019; Stuble & Souza, 2016; Stuble et al., 2017; von Gillhaussen et al., 2014; Weidlich et al., 2017, 2018). Other studies examined priority effects by mixing these groups (Cleland et al., 2015; Lang et al., 2017; Mason et al., 2013; Schantz et al., 2018; Stuble & Young, 2020; Werner et al., 2016; Wilsey, 2020; Young et al., 2017). Alternatively, species may be assigned to groups according to their importance in reference plant communities, for example in distinguishing dominant (species frequently occurring in high abundance) and subordinate species (frequently occurring in low abundance; Grime, 1998).…”

Applying community assembly theory to restoration: overcoming dispersal and abiotic filters is key to diversifying California grassland

“…Restoration ecologists have explored the potential for priority effects to facilitate the establishment of less competitive native species (e.g. Porensky et al 2012; Werner et al 2016; Stuble & Young 2020) and to improve invasion resistance by giving native species temporal priority over non‐native species (e.g. Vaughn & Young 2015; Young et al 2015, 2017), thus, dampening the biotic filter for native species.…”