Management Implications
Many contemporary landscapes contain small patches of native species within a matrix of nonnative plants, necessitating decisions for how to most effectively deploy limited resources for invasive plant management. A potential way to prioritize treatments for maximum benefit on such landscapes is to strategically treat around the native patches to enlarge them over time, but this strategy has been little evaluated. We tested this strategy within a monotypic matrix of the nonnative annual forb Bassia scoparia (kochia) with scattered patches of native perennial grasses. Mechanically cutting B. scoparia within a 2-m buffer surrounding native grass patches rapidly enlarged native patches the first growing season. The positive response continued through the third and final year of the experiment, when treated native patches were seven times larger than before treatment. The expansion of native patches occurred despite dry conditions likely suboptimal for perennial grass growth. Results suggest that within landscapes dominated by nonnative plants with small native grass patches, prioritizing treatments around native patches can quickly enlarge them and be a minimal-input strategy benefiting native species.
Introduction
Questions of how and where to prioritize treating nonnative plants within landscapes are fundamental to invasive plant management and key research priorities (Davies and Sheley Reference Davies and Sheley2011; Ransom et al. Reference Ransom, Christensen, Edvarchuk and Naumann2012). Within landscapes containing a matrix of native species with small, discontinuous patches of nonnative plants, treatments for nonnative plants are often prioritized based on criteria such as size of nonnative patches and feasibility of removal, invasiveness of nonnative species, or resource values threatened by nonnative plant invasion (Rew et al. Reference Rew, Lehnhoff and Maxwell2007). However, many contemporary landscapes instead have the inverse patch structure, in which landscapes contain a matrix of nonnative plants and only small, discontinuous patches of native species (Gilbert and Levine Reference Gilbert and Levine2013). In these cases, treatments are usually prioritized among sites, which are then treated in their entirety or in systematically distributed sections (Abella Reference Abella2014).
Given limited resources for management, an alternative to treating entire sites could be a minimal-input approach that prioritizes treatments around the perimeters of native species patches to potentially enlarge them over time within the nonnative plant matrix. Little research exists directly examining the potential efficacy of this approach, and related literature suggests both possible hindrances to the approach and ideas as to why it could be effective. For example, hindrances could include limited native species propagules, legacy effects of nonnative plant presence, and environmental limitations to native plant recruitment such as dry conditions possibly exacerbated in small native patches (Corbin and D’Antonio Reference Corbin and D’Antonio2012; Cutway Reference Cutway2017; Pearson et al. Reference Pearson, Ortega, Runyon and Butler2016). In contrast, other ideas, such as the importance of gap-phase recruitment in competitive stands and opening regeneration niches, predict that creating growing space around native patches should enlarge them (Davies and Sheley Reference Davies and Sheley2011; Zimmerman et al. Reference Zimmerman, Shirer and Corbin2018). Additionally, for native species in patchy refugia, extinction risk can be lowered by increasing the size and connectivity of native species patches (Gilbert and Levine Reference Gilbert and Levine2013).
We assessed the efficacy of a minimal-input technique for increasing native species within a nonnative plant matrix by strategically treating around the perimeters of native grass patches in a dual context of archaeological and natural resource conservation. Managing for native vegetation compatible with archaeological resource preservation is challenging in diverse locations globally, such as around Iron Age earthworks in Europe (Jones Reference Jones1998). In the United States, native vegetation is frequently needed in national historical parks for maintaining authenticity of cultural resources such as battlefields, stabilizing earth- and stoneworks constructed by Native Americans, and protecting artifacts held in soil (Thorne Reference Thorne1990). Generally, for stabilizing soil-held archaeological features or material surrounding stone structures, relatively shallow- and laterally rooted, short-statured species are optimal for protecting soil while allowing access to archaeological features for maintenance and viewing (Jones Reference Jones2000). With their laterally extensive, fibrous root systems and low aboveground height, native perennial shortgrasses are often ideal for stabilizing archaeological sites (Jones Reference Jones2000). On an archaeological site containing small native grass patches in a matrix of nonnative forb monoculture (kochia [Bassia scoparia (L.) A.J. Scott]), we tested a hypothesis that treating nonnative forbs around perimeters of native grass patches would enlarge native patches.
Materials and Methods
Focal Nonnative Species
Native to Europe and western Asia, B. scoparia is an annual forb equipped with traits for invasiveness, especially in arid/semiarid regions outside the species’ native range. Since its introduction as an ornamental to North America during the 1800s, B. scoparia has become one of the most widespread nonnatives in western North America, invading agricultural fields, pastures, rangelands, roadsides, and a variety of disturbed sites in semiarid grasslands and woodlands (Friesen et al. Reference Friesen, Beckie, Warwick and Van Acker2009). Bassia scoparia employs C4 photosynthesis, is capable of deeply rooting to 5 m, and has high water-use efficiency, enabling competitive ability on dry, hot soil (Friesen et al. Reference Friesen, Beckie, Warwick and Van Acker2009; Phillips and Launchbaugh Reference Phillips and Launchbaugh1958). Bassia scoparia frequently forms monocultures, with one study recording 380,000 freshly germinated seedlings m−2 (Dille et al. Reference Dille, Stahlman, Du, Geier, Riffel, Currie, Wilson, Sbatella, Westra, Kniss, Moechnig and Cole2017). Most B. scoparia seeds exhibit minimal innate dormancy and readily germinate under suitable conditions, although the species can form soil seedbanks persisting across years (Zorner et al. Reference Zorner, Zimdahl and Schweizer1984). A seed burial experiment in the Great Plains reported that most B. scoparia seeds in soil were short-lived (1% remaining at 3 yr), but that some germination occurred 10 yr after burial (Burnside et al. Reference Burnside, Wilson, Weisberg and Hubbard1996). Bassia scoparia can be controlled with herbicides, but the species’ increasing resistance to herbicide can hinder effectiveness (Kumar et al. Reference Kumar, Jha, Jugulam, Yadav and Stahlman2019).
Study Area
We performed the experiment in the 2700-ha Pecos National Historical Park, 40 km southeast of the city of Santa Fe in north-central New Mexico, USA. The park, administered by the U.S. National Park Service since 1965, is managed for human visitation while conserving natural and cultural features, including Pecos Pueblo structures representative of 700-yr-old Native American architecture and Spanish Colonial mission churches constructed from 1625 to 1717. The park is a transitional area where three biogeographic regions meet: the southern Rocky Mountains to the north, the Great Plains to the east, and the Basin and Range Province to the southwest (Johnson Reference Johnson1969). The semiarid climate, recorded at an elevation of 2,116 m at the park headquarters, has averaged the following from 1916 through 2018: −9/8 C January daily low/high temperature, 12/30 C July daily low/high temperature, and 41 cm yr−1 of precipitation, including 69 cm of snowfall. Livestock grazing has been excluded since 1935 where we conducted the experiment.
The study site (35.54°N, 105.69°W) within the park was a 2-ha area encircled by the Pueblo and Mission Ruins Visitor Trail, encompassing the cultural resources of stone and rubble mound structural remains of Pueblo Native American cultures; a historical missionary church; and cultural material held in soil, including pottery fragments, faunal bone, and stone tools. Possibly related to a long history of anthropogenic use, the site was minimally vegetated by woody plants in the late 1800s and early 1900s, as shown in historical photographs and descriptions when initial archaeological investigations began (Kidder Reference Kidder1958). Based on historical descriptions and contemporary distributions of vegetation types at similar elevations in less-disturbed areas, Pinus–Juniperus woodland with openings of shortgrass grassland may represent the natural vegetation type at the site. While exactly how or when B. scoparia invaded the site is unclear, it is thought to have invaded the site before the mid-1900s. In 2015 when we began the experiment, the site contained a near monoculture of B. scoparia surrounding the archaeological features and often growing in or on top of them in the case of collapsed stone, adobe brick mounds, and soil-held features. Small patches (0.1 m2 to several square meters) of native perennial grasses were interspersed within the B. scoparia matrix.
Experimental Design, Treatments, and Data Collection
In October 2015, before treatment, we identified 40 native perennial grass patches to serve as experimental units distributed throughout the matrix of B. scoparia. We defined native perennial grass patches as containing one or more genets of the same species covering at least 0.1 m2 and separated by at least 4 m (with the intervening matrix area being B. scoparia monoculture) from another patch of the same or different grass species. There were 10 patches each (equally subdivided by treatment) of four focal native perennial grass species (their nomenclature, traits, and average cover are provided in Table 1). We randomly assigned a B. scoparia cutting treatment to half (n = 20) the patches, while the other half (n = 20) did not receive a B. scoparia cutting treatment. To implement the cutting treatment, we used mechanical, handheld brush cutters (Stihl, Waiblingen, Germany; Echo, Lake Zurich, IL, USA). For a 2-m distance into the B. scoparia matrix out from the perimeter of each assigned native grass patch and completely encircling the grass patch, we cut all B. scoparia plants at ground level (and laid cut stems on the ground in place) in June 2016 and again in July 2017. The cutting was timed to include nearly all B. scoparia germination that had occurred in spring, to maximize the number of plants treated, while taking place before B. scoparia had produced seed, to minimize seed production. Although cut B. scoparia individuals could resprout and seedlings could emerge in growing seasons after cutting, cover of B. scoparia within cut patches averaged 23% lower than in uncut patches at the end of the experiment in October 2018 (Table 1). Cutting B. scoparia also enabled native grasses to experience growing periods when height of B. scoparia was reduced to near ground level, potentially providing competitive advantages previously shown important for native grass species of our experiment to have for competitiveness with nonnative forbs (Allen Reference Allen1982).
Table 1. Names, traits, and average cover of native perennial grasses in 12.6-m2 circular plots (2-m radius) centered within native grass patches around which the nonnative annual forb Bassia scoparia was mechanically cut (n = 20) or not cut (n = 20) in Pecos National Historical Park, New Mexico, USA.a
a Grasses are divided into focal species present in patches when the experiment began in 2015 and additional species occurring within the patches later in the experiment. Average cover is for the final measurement in October 2018, 28 mo after the first B. scoparia cutting treatment.
b Mean ± standard error of the mean. For comparison, B. scoparia cover in October 2018 averaged 17.6 ± 4.4 (uncut) and 13.7 ± 4.2 (cut).
We measured native perennial grass patch size (area in m2) six times in a 3-yr period for cut and uncut patches. We measured patch size before (October 2015) and after treatment (July 2016, June and October 2017, and May and October 2018) up to 28 mo after the first B. scoparia cutting treatment. Grass patches approximated circles, so we recorded patch size as the area of a circle by measuring and averaging at least four radii from a patch’s center to outer perimeter. Throughout the experiment, we kept the original patch as measured at that location before treatment as the unit for analysis, which could subsequently change in size or species composition after treatment. In some cases, for example, an originally single-species native grass patch could subsequently contain intermingled native grass species after treatment (Table 1). Because of this turnover in species composition, we did not compare perennial grass species and instead measured change in size of perennial grass patches at the original pretreatment location regardless of species composition.
Statistical Analysis
The change in native perennial grass patch sizes (m2) for each of five posttreatment measurement times, relative to pretreatment sizes, was the response variable for statistical analysis. We were interested in differences between treatments for each measurement time independently, so we performed analyses within each measurement period. This approach kept focus on differences between treatments at each measurement time, rather than on exploring seasonal fluctuations. For each of the five posttreatment measurement times, we used two-sample Welch’s t-tests to compare mean change from the pretreatment baseline in native perennial grass patch size between B. scoparia-cut and uncut patches. Welch’s t-tests accommodate unequal variances between treatments, important in our experiment because the data set followed a common pattern wherein variances increased proportionally with means. We controlled for type I statistical errors across the five tests using the Benjamini-Hochberg procedure to optimize limiting the false discovery rate while maintaining power (Benjamini and Hochberg Reference Benjamini and Hochberg1995). We set a conservative critical value of 0.05 for the false-positive rate.
Native grass patches varied in size before treatment. To explore whether responses hinged on initial patch sizes, we used Pearson’s correlation to quantify relationships between pretreatment and October 2018 native grass patch size. We ran correlation analysis with and without the two largest patches (which were two to three times larger than the next largest patch) included. We excluded these two patches from the final correlation analysis to avoid disproportionate influence on relationships among the 38 other patches.
Results and Discussion
Change in Native Grass Patch Size
Mechanically cutting the nonnative forb B. scoparia around the perimeters of native perennial grass patches stimulated expansion of native grasses (Figure 1). Compared with patches where B. scoparia was not cut, native grass patches where surrounding B. scoparia was cut grew larger relative to pretreatment for all five posttreatment measurement times during the 3-yr experiment (Figure 2). By October 2018, native grass patches around which B. scoparia was cut averaged 28.8 ± 10.8 m2 (±SEM) in size. This was four times larger than the 6.6 ± 1.8 m2 for native patches around which B. scoparia was not cut. There was little relationship between the initial (October 2015) and posttreatment (October 2018) size of native grass patches (r2 = 0.01 and 0.09 for B. scoparia cut and uncut patches, respectively).
Figure 1. Example of a native perennial grass patch (in this example, Bouteloua gracilis with some Elymus elymoides intermixed), surrounded by near monoculture of the nonnative annual forb Bassia scoparia, in an experiment testing effects of cutting B. scoparia surrounding native grass patches, Pecos National Historical Park, New Mexico, USA. The photo was taken in October 2017, 16 mo after initial B. scoparia cutting. The native grass patch increased from a pretreatment size of 0.1 m2 (October 2015) to 50 m2 in October 2017 (photo by LPC).
Figure 2. Relative to pretreatment (October 2015), change in mean size of native perennial grass patches around which the nonnative annual forb Bassia scoparia was mechanically cut or was not cut in Pecos National Historical Park, New Mexico, USA. Error bars represent 1 standard error of the mean. Type 1 error–protected Welch’s t-tests compare cut and uncut means within each posttreatment time.
Overcoming Likely Unfavorable Weather Conditions
The experiment occurred during a generally dry period. A dry early summer (May to June, 47% of average precipitation) and July through October (67%) warm-season growth period occurred in 2016 during the first year of the experiment (Santa Fe, NM, weather station, 40 km northwest of the study site at a similar elevation of 2,049 m, 67 m lower than the study site). The second year, 2017, had a dry May to June (33%) and average July through October (100%). Most of the last year of the experiment was exceptionally dry, with only 26% of average winter/spring precipitation from November 2017 through April 2018 and only 40% in May to June 2018. The dry conditions could relate to why native grass patches contracted in May 2018 compared with October 2017 in both the B. scoparia cut and uncut treatments, although native grass patches were still larger where B. scoparia was cut. This dry period was followed by a wet July through October (144%) preceding the final measurement in October 2018, the only growing season during the experiment that was wetter than average.
Published literature suggests that the major increase in native grass patches after B. scoparia cutting might have been even greater had conditions been moister. Research spanning a century in semiarid grasslands, including with species in our study, revealed that native perennial grass cover is highly sensitive to precipitation within a growing season and across multiple years (Rondeau et al. Reference Rondeau, Pearson and Kelso2013; Weaver and Albertson Reference Weaver and Albertson1943; Witwicki et al. Reference Witwicki, Munson and Thoma2016). One experiment, for example, found that blue grama [Bouteloua gracilis (Willd. ex Kunth) Lag. ex Griffiths] cover declined by a third when moisture was experimentally reduced by 25% (Evans et al. Reference Evans, Byrne, Lauenroth and Burke2011). Ruderal forbs, including B. scoparia, increased under the experimental drying during the concomitant contraction in native perennial grasses (Evans et al. Reference Evans, Byrne, Lauenroth and Burke2011). These observations suggest that treatments in our experiment were able to overcome generally dry conditions that should have favored increases in B. scoparia and decreases in native grasses. The idea that alleviating competition from nonnative plants can increase the resistance and resilience of native plants to drought or other climatic events is an emerging research topic in invasive plant management (Abella Reference Abella2014; Monaco et al. Reference Monaco, Mangold, Mealor, Mealor and Brown2017).
Persistent Patch Increases under Transient Species Composition
Native grass patches expanded while experiencing high turnover in species composition, an ecological process likely explained in part by previous autecology research. In a study of mixed-species prairies, for example, B. gracilis genets had maximum life spans of 35 to 38 yr, average life spans of only 3 to 4 yr, and fewer than half survived more than a year (Fair et al. Reference Fair, Lauenroth and Coffin1999; Lauenroth and Adler Reference Lauenroth and Adler2008). Squirreltail [Elymus elymoides (Raf.) Swezey] was even shorter-lived, with an average life span of 1 yr (and maximum of 5 yr), and only 30% of genets persisted more than a year (Lauenroth and Adler Reference Lauenroth and Adler2008). In western wheatgrass [Pascopyrum smithii (Rydb.) Á. Löve], production of annual tillers (including modular units becoming new individuals) creates variability in the spatial configuration of genets and patches with multiple genets from one year to the next (Ott and Hartnett Reference Ott and Hartnett2015). Native grasses not among the initial focal species of patches in our experiment but that colonized one or more patches after treatment also were transient, such as buffalograss [Bouteloua dactyloides (Nutt.) J. T. Columbus], with genet longevity averaging only 2 yr (Lauenroth and Adler Reference Lauenroth and Adler2008). Transience in species in any given patch was why we analyzed native grass patches irrespective of their species composition. Further research tracking changes in individual species, perhaps using demographic techniques, may aid in identifying whether particular native grass species respond most vigorously to nonnative plant removal across a range of conditions.
Regeneration Components
Autecology literature for the native grass species offers insight into potential regeneration processes contributing to observed expansions of grass patches. Although all the grasses can reproduce vegetatively, recruitment could also have occurred via seed. In a long-term study in the Great Plains, B. gracilis seedling recruitment was frequent, occurring in 35 of 38 (91%) years (Fair et al. Reference Fair, Lauenroth and Coffin1999). All four focal native grass species can form persistent soil seedbanks (Abella et al. Reference Abella, Springer and Covington2007; Pérez et al. Reference Pérez, Waller, Moser, Stubbendieck and Steuter1998). While seeds are mainly dispersed near adults, at least some of the species on occasion can experience seed dispersal over tens of meters (Marlette and Anderson Reference Marlette and Anderson1986). This distance would suffice to have seed exchange among perimeters of many of the grass patches in our experiment. Patch expansion could also have resulted from rapid clonal growth. For example, Webb (Reference Webb1941) reported fast vegetative expansion of B. dactyloides: plants produced tillers at 1-mo-old, flowers appeared at age 2.5 mo, roots exceeded 1.4-m long before 4-mo-old, and stolons grew up to 2 cm day−1 in length. On a devegetated site where B. dactyloides was experimentally planted at a spacing of 0.7 m, the species formed a solid sod cover within 4 mo (Webb Reference Webb1941). In Wind Cave National Park, South Dakota, P. smithii maintained 18 m of rhizome length per square meter of soil and produced new tillers in both spring and autumn from these rhizomes, enabling the species to rapidly colonize open area (Ott and Hartnett Reference Ott and Hartnett2015). The rapid expansion of native grasses into growing space created by cutting B. scoparia could thus have resulted from a combination of the diverse colonization options the native grasses possess. While our goal was to test minimal-input strategies (i.e., limiting the area required for treatment and avoiding resource-intensive seeding or planting of natives), partitioning observed native grass expansions into regeneration components in future research might aid identifying whether strategic augmentation of propagules could enhance the grass response.
Time Frame and Treatment Sustainability
Although the 3-yr experiment encompassed multiple generations of the annual B. scoparia and exceeded the average genet life expectancy of focal native grass species (Lauenroth and Adler Reference Lauenroth and Adler2008), examining potential further trends in B. scoparia dynamics, native grass persistence, and competitive suppression of B. scoparia by expanding native grasses may help identify the longer-term sustainability and cost-effectiveness of the treatment strategy. Without seed inputs, the 2-yr cutting treatments should have depleted the B. scoparia seedbank around native grass patches by at least 95% (Dille et al. Reference Dille, Stahlman, Du, Geier, Riffel, Currie, Wilson, Sbatella, Westra, Kniss, Moechnig and Cole2017; Zorner et al. Reference Zorner, Zimdahl and Schweizer1984). However, some B. scoparia seed inputs likely occurred because of the inherent spatial arrangement of treatments around grass patches with intervening B. scoparia monoculture. Whether established native grasses could persistently competitively suppress B. scoparia is uncertain but possible based on theory of priority effects and invasion resistance of communities (Allen and Meyer Reference Allen and Meyer2014). Competitive dynamics might partly hinge on the fluctuating species composition of native grass patches, which could strengthen or reduce community invasibility. While limiting similarity theory proposes that communities composed of species most similar to B. scoparia should best resist invasion (i.e., not necessarily the perennial grasses), the fluctuating composition of grasses differing in growth seasonality and morphology that cycled among patches could provide continuous competition to B. scoparia (Abella et al. Reference Abella, Craig, Chiquoine, Prengaman, Schmid and Embrey2011; Allen and Meyer Reference Allen and Meyer2014).
The approach of treating nonnative plants around perimeters of native patches showed promise as a minimal-input strategy for increasing native species over time within landscapes dominated by a matrix of nonnative plants. While enlarged native grass patches persisted for at least 3 yr, including across dry years, further exploring how frequently or long the minimal-input treatments might be needed to sustain the native species increases is warranted.
Acknowledgments
The National Park Service (NPS; Southwest Exotic Plant Management Team and Pecos National Historical Park) funded the experiment through a cooperative agreement (initially organized in 2015 by CDS and administered from 2017 to 2019 by Jeffery Conn) with the University of Nevada, Las Vegas (UNLV). We thank Matt Rader (UNLV) for help with fieldwork; staff with American Conservation Experience, Southwest Conservation Corps (Ancestral Lands Program), and NPS for help with experimental setup and implementing treatments; and 16 traditionally associated tribal governments (including the Pueblo of Jemez, which claims Pecos Pueblo as an ancestral site) for consulting with NPS regarding the project. We also thank the associate editor and two anonymous reviewers for helpful comments on the article. No conflicts of interest have been declared.
