Effects of Competition with Bromus Tectorum on Early Establishment of Poa Secunda Accessions: Can Seed Source Impact Restoration Success? Mollie E
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RESEARCH ARTICLE Effects of competition with Bromus tectorum on early establishment of Poa secunda accessions: can seed source impact restoration success? Mollie E. Herget1,2, Kristina M. Hufford1, Daniel L. Mummey3, Brian A. Mealor4, Lauren N. Shreading3 When landscapes are heavily impacted by biological invasion, local populations of native plant species may no longer be adapted to altered environmental conditions. In these cases, it is useful to investigate alternative sources of germplasm, such as cultivated varieties, for planting at restoration sites. This study compared cultivated and wild (local) varieties of the native perennial bunchgrass, Poa secunda J. Presl, grown with and without the exotic, invasive Bromus tectorum L. in a greenhouse setting. While P. secunda cultivars emerged and grew more rapidly than wild seed sources, this advantage declined in the presence of B. tectorum and cultivated germplasm did not outperform wild accessions in the presence of an invasive species. Given the novel genetic background of cultivars and their potential to alter patterns of dominance in native plant communities, we recommend the use of local or regional wild seed sources when possible to conserve regional patterns of genetic diversity and adaptation. Use of multiple seed sources may increase the potential for capturing vigorous genotypes in the restoration seed mix. In cases where sites are heavily impacted by exotic, invasive species, other control measures will be necessary to improve establishment of native species in grassland restoration programs. Key words: cheatgrass, cultivar, greenhouse, local genotype, Sandberg bluegrass, wild population widespread conversion of native perennial grasslands in the Implications for Practice western United States to annual-dominated systems (Mack • The source of seeds selected for restoration can infuence 1981; Stubbendieck et al. 2011). Introduced species such as seedling establishment and early performance at restora- B. tectorum spread rapidly with land disturbance, and are highly tion sites. competitive due to their rapid growth rate, early germination • Cultivated varieties of native species are on average more relative to native species, and abundant seed production (Sakai vigorous than wild populations, and yet some wild acces- sions may perform as well as certain cultivars. et al. 2001). Moreover, increased fre frequency associated with higher fuel loads in B. tectorum dominated grasslands pro- • Despite increased initial vigor, cultivars do not perform better on average than local plants when grown with motes continued invasion despite ongoing efforts to restore Bromus tectorum. native plant communities (D’Antonio & Vitousek 1992). When • Given that intra-specifc variation in performance among exotic species alter ecosystem properties, and landscapes are no seed sources may alter patterns of dominance in restored longer dominated by historical plant communities, restoration plant communities, we recommend using representative may require new strategies for reestablishment of native plants local or regional seed sources when feasible. (Brooks et al. 2004). A novel approach is the study of intraspe- • If competition with exotics is severe, a mix of seed sources cifc diversity of native species used in reseeding restoration encompassing more vigorous genotypes, as well as addi- sites, which may play an important role in susceptibility and tional control measures, may enhance reestablishment of native plants at restoration sites. Author contributions: MH, KH, BM conceived and designed the research; MH performed the experiments; MH, KH analyzed the data; KH, BM contributed workspace; DM, LS contributed funding and resources; MH, KH, DM, BM, LS wrote and edited the manuscript. Introduction 1Department of Ecosystem Science and Management, University of Wyoming, Invasive exotic species are one of the greatest obstacles to Laramie, WY 82071, U.S.A. 2Address correspondence to M. E. Herget, email [email protected] rangeland restoration in North America (Bakker et al. 2003; 3MPG Ranch, Florence, MT 59833, U.S.A. Stromberg et al. 2007). A well-documented example is the intro- 4Department of Plant Sciences, University of Wyoming, Laramie, WY 82071, U.S.A. duction of the annual grass Bromus tectorum L. (cheatgrass ©2015SocietyforEcologicalRestoration or downy brome) in the late 19th century, which resulted in doi: 10.1111/rec.12177 Restoration Ecology 1 Seed source impacts on restoration success tolerance to biological invasion (Hooper et al. 2005; Crutsinger Ranch, a 10,000 acre conservation property in the Bitterroot et al. 2008). Valley of western Montana, wild accessions were sourced from Guidelines for restoration of native species commonly three distinct sites representing local populations and varying recommend using local germplasm to maintain biodiversity, in elevation (1000–1300 m) and distance (3–6 km). Cultivated increase the likelihood that plants are adapted to site conditions, accessions included ‘High Plains’ sourced from Wyoming, and improve long-term sustainability of reintroduced popula- ‘Mountain Home’ originating from Idaho, and ‘Reliable’ with tions (Hufford & Mazer 2003; McKay et al. 2005; Jones 2013). origins in Washington (Majerus et al. 2009). During the summer However, local genotypes may no longer be adapted to altered of 2012, cultivar seeds were purchased from commercial seed environments in landscapes heavily impacted by biological growers, both wild P. s e c u n d a and B. tectorum seeds were col- invasion. In these cases, it is useful to investigate alternative lected at MPG Ranch, and all seeds were subsequently stored at sources of germplasm, such as cultivated, improved, or selected room temperature (15–25∘C) until planting in the greenhouse. varieties of native species (hereafter ‘cultivars’). Cultivars represent germplasm selected for improved vigor, seed ger- mination, and biomass production (Lesica & Allendorf 1999; Experimental Design Johnson et al. 2010). Many cultivars are offered for commercial We conducted our experiment at the Research and Extension sale and are commonly used in restoration due to their increased Center greenhouses in Laramie, Wyoming (elev. 2,184 m) in late availability and lower cost (Burton & Burton 2002). Vigorous, 2012. Seeds were sown into 164 mL Ray Leach ‘Cone-tainer’ cultivated genotypes may be able to outperform their wild pots (Stuewe & Sons, Inc., Corvallis, Oregon, U.S.A.). Six hun- counterparts in the presence of invasive species, but evidence dred pots were sown with combinations of P. s e c u n d a and B. also suggests cultivars can alter patterns of dominance among tectorum to evaluate the overall vigor and competitive response species in native plant communities (Gustafson et al. 2014). of cultivated and wild Poa seed sources. Control pots included Further research is needed to compare cultivated seed sources 40 replicates of each of the six Poa accessions grown alone, to wild populations and to determine whether cultivars are more and competition pots included another 40 replicates of each Poa likely to establish in the presence of biological invasion. accession sown with B. tectorum. An additional 120 pots were This study compared cultivated and wild (local) accessions of sown solely with seeds of B. tectorum to allow comparisons the native perennial bunchgrass, Poa secunda J. Presl (Sandberg of B. tectorum growth between pooled control and competition bluegrass), grown with and without B. tectorum. Poa secunda treatments. This experimental design resulted in 360 controls is an important native perennial bunchgrass of the sagebrush planted with either P. s e c u n d a or B. tectorum, and 240 competi- ecosystem in the western United States, and is commonly tar- tion pots. Each pot was lined with cheesecloth to prevent the loss geted in grassland restoration (Peterson 2002; Majerus et al. of planting medium while simultaneously allowing drainage, 2009). We selected P. s e c u n d a due to its early growth period and pots were subsequently flled with 1:1 fnely sieved sand and extensive root system, suggesting this species has the poten- and peat moss. Initially, four seeds of each species were sown tial to compete with invasive, exotic species such as B. tectorum into designated control or competition pots to ensure that at least (Perry et al. 2009). Our objectives were to: (1) study differences one plant established per replication. Experimental pots were in traits between cultivars and wild seed sources of P. s e c u n d a , placed under a bank of misting nozzles for 2 weeks to promote and (2) test the hypothesis that vigorous cultivars are more germination at the start of the study. likely to tolerate the presence of invasive species relative to wild Pots were monitored daily to record seedling emergence. seed sources. We also examined B. tectorum performance when Emergence was recorded as the number of days after planting grown with P. s e c u n d a to characterize effects of competition on (day 0) until a new seedling was observed. After 2 weeks, the invasive species, ultimately with the goal of understanding emerged seedlings were thinned to one plant per control pot, consequences of intraspecifc variation for restoration of native and one Poa and one Bromus plant per competition pot. Trays plant communities in sites heavily impacted by B. tectorum. containing up to 49 pots were then transferred to a standard greenhouse bench for the duration of the study. Thereafter, pots were randomized within trays