Research ArticleProceedings of the 2019 Native Plant Summit

Seedling growth and competition of a late-seral, native perennial grass and 2 early-seral, native forbs in the presence of 2 densities of the invasive annual grass Bromus tectorum L. (Poaceae)

Derek Tilley, Nathaniel Tilley, Adam Fund and Mary Wolf
Native Plants Journal, September 2020, 21 (3) 299-311; DOI: https://doi.org/10.3368/npj.21.3.299

Abstract

The invasive, annual cheatgrass (Bromus tectorum L. [Poaceae]) has invaded millions of hectares of the North American sagebrush steppe biome. Because of reduced forage of more desired species, cheatgrass-infested sites have significantly lower rangeland productivity resulting in significant lost revenue. Further, cheatgrass-invaded sites have far lower species richness and diminished ecosystem function. The scale of disturbance and the size of restoration projects in the region make seeding, often with little or no seedbed preparation, the only feasible restoration treatment. Seeding early-seral natives with characteristics advantageous for site colonization and stabilization has been seen as a possible means to transition between cheatgrass dominance and desired native plant diversity. We conducted greenhouse trials to compare competitive ability of 2 native, early-seral forbs, curlycup gumweed (Grindelia squarrosa (Pursh) Dunal [Asteraceae]) and hoary tansyaster (Machaeranthera canescens (Pursh) A. Gray [Asteraceae]), and also 1 late-seral perennial grass, bluebunch wheatgrass (Pseudoroegneria spicata (Pursh) Á. Löve [Poaceae]), when planted among low and high densities of cheatgrass during the critical seedling stage of growth. We also tested whether curlycup gumweed, a novel species under investigation for germplasm release and use in restoration seedings, had any negative effects on bluebunch wheatgrass. Over 12 wk, cheatgrass grew faster and produced more biomass than the natives. In addition, low and high densities of cheatgrass significantly reduced plant volume and aboveground biomass of all 3 natives. Of the natives, only bluebunch wheatgrass caused any notable reduction in cheatgrass growth. Curlycup gumweed did not affect bluebunch volume or biomass to any extent beyond the response it would have with a same-species competitor. Our results indicate that the 2 early-seral forbs tested were not effective at outcompeting cheatgrass, and their value in capturing cheatgrass-infested sites may be limited. Finally, we found no inhibitory effect from curlycup gumweed on bluebunch wheatgrass in the absence of other restoration practices during the first 12 wk of growth.

KEY WORDS

Cheatgrass (Bromus tectorum L. [Poaceae]) is a nonnative, annual grass introduced to North America in the late 19th century (Novak and Mack 2001) that has since proved to be highly invasive in western North American rangelands, especially in the Intermountain West. Cheatgrass infestation is now a major concern among landowners and land managers in western states because of its invasiveness and the negative impacts it creates on the landscape. Millions of hectares have already been invaded, and much of this land has been converted to cheatgrass-dominated rangeland (Stewart and Hull 1949; Mack 1981; Knapp 1996)—resulting in significant losses in forage for livestock and ungulates.

At the turn of the last century, approximately 5 million ha (12.4 million ac) of rangeland in Idaho and Utah were covered by almost pure stands of cheatgrass (FICMNEW 1997). Millions more are at risk of being invaded and have a high probability of converting to cheatgrass domination (Stewart and Hull 1949; Weltz and others 2014). Cheatgrass conversion provides copious amounts of fine fuel, which create perfect conditions for recurring wildfires (Brooks and others 2004). This takeover has led to a decrease in the natural fire cycle frequency interval in the sagebrush steppe ecosystem from every 50 to 100 y to every 1 to 5 y, in some cases (Whisenant 1990; Brooks and Pyke 2001). Cheatgrass invasion not only reduces forage but also degrades wildlife habitat by diminishing species richness, especially native forbs (Stewart and Hull 1949; Young and Evans 1978; Knapp 1996). Sagebrush obligates, such as Sage Grouse (Centrocercus urophasianus Bonaparte [Phasianidae]), are particularly at risk as cheatgrass has significant effects on multiple facets of their lives, for example, reduces nesting cover, decreases forb and sagebrush food sources, and reduces native insect populations (USDI FWS 2015). The total cost to land managers and taxpayers to stave off cheatgrass dominance is staggering (Knapp 1996).

Cheatgrass succeeds in invading Great Basin landscapes by exploiting key physiological and morphological traits. First, cheatgrass uses more water early in the season than natives use by initiating root and shoot growth at lower temperatures (Melgoza and Nowak 1991; Knapp 1996; Arredondo and others 1998; Norton and others 2007; James and others 2011). Second, cheatgrass is able to grow in extremely high densities ranging from approximately 10 plants/m2 to 10,000 plants/m2 (Young and Evans 1978). Once cheatgrass has become established in a location, it is exceedingly difficult to convert that site back to its original state without major inputs.

Despite cheatgrass’s advantages in disturbed sites, native plants have been shown to have the capacity to be effective at resisting invasive annual grass presence and at building ecological resilience to invasion under non-disturbed conditions (Row and Leger 2011; Abella and others 2012; Allen and Meyer 2014; Chambers and others 2014; Phillips and Leger 2015). Mature native perennials in areas exhibiting high ecological condition can be effective competitors against cheatgrass invasion; however, competition at the seedling stage is a crucial barrier to meaningful restoration (Booth and others 2003; Chambers and others 2007; Allen and Meyer 2014). Further investigation is needed to determine which native species, if any, can effectively compete against cheatgrass during the establishment stage. One group that shows potential are the ruderal, or early-seral natives.

Early-seral natives have evolved to capture a site following disturbance and to facilitate transition to a climax community, yet this important group is often overlooked by plant material developers and by restoration practitioners (Jones and Johnson 1998; Shaw and others 2005; Ogle and others 2014). Following disturbance, semi-arid western plant communities often respond initially with a flush of early successional annuals or short-lived perennials (Koniak and Everett 1982; Ott and others 2003) that colonize the site, altering soil biology and nutrient cycling in ways that ultimately favor later-successional species (Busby 2011; Stube 2012). One role of early-seral species that may be of significant importance is the plant’s ability to form associations with, and increase soil populations of, arbuscular mycorrhizal fungi (AMF), which are needed by the later-seral occupants. Over time, longer-lived, late-seral species re-establish from surviving root systems, soil seedbanks, or from seed dispersal from adjacent sites. Common restoration practices, however, tend to skip the early-seral step and attempt to establish mid- to late-successional communities directly after disturbance (Jones and Johnson 1998; Ogle and others 2014).

Ruderal and early successional species, especially annuals and short-lived perennials, occupy a similar niche to many of the region’s exotic grass invaders. Species with similar ecological functions and attributes should be better equipped to resist potential invaders (Emery 2007). Stube (2012) asserted that native early-seral species that have traits in common with cheatgrass could potentially compete with cheatgrass if used at competitive seeding rates in restoration efforts, which may alter site characteristics in ways that promote succession of the native plant community. For example, studies have indicated that early-seral natives can reduce red brome (Bromus rubens L. [Poaceae]) establishment and biomass more than late-seral natives can (Abella and others 2012). Likewise, Leger and others (2013) found the early-successional native forb bristly fiddleneck (Amsinckia tessellata A. Gray [Boraginaceae]) significantly reduced cheatgrass biomass and seed production, and Prasser and Hild (2016) showed that native annuals reduced halogeton (Halogeton glomeratus (M. Bieb.) C.A. Mey. [Chenopodiaceae]) biomass more than longer-lived perennials. Thus, it may be beneficial to include site-adapted, early-seral species that have evolved with disturbance in restoration seed mixes to promote full site conversion and resiliency (Leger 2008; Stube 2012; Herron and others 2013; Uselman and others 2015).

Including certain early-seral species in a seed mix may come with a cost. While in nature it is clear that plant succession from early- to late-seral species occurs over long spans of time, it is unknown if early-seral species might have an impact on initial establishment of plants typically associated with a climax community. There may be a threshold at which early-seral natives become “too weedy” and become a detriment to conservation objectives. Researchers need to determine if early-seral natives might have a negative effect on the establishment of late-seral species included in rangeland seed mixtures.

Most early-seral research to date for the western US has been conducted on native annuals, few of which are well-suited for commercial seed production. Many species are low-statured or prostrate in growth habit, making seed harvest difficult; for example, foothill deervetch (Lotus humistratus Greene [Fabaceae]), bigbract verbena (Verbena bracteata Cav. ex Lag. & Rodr. [Verbenaceae]), and sixweeks fescue (Vulpia octoflora (Walter) Rydb. [Poaceae]) (Herron and others 2013; Barak and others 2015). Still others may be limited in the quantity of seed produced, making them economically unsustainable and prohibitively costly for most restoration projects. Early-seral species that serve the same function, while having better seed production attributes, would be beneficial.

The NRCS Aberdeen Plant Materials Center (IDPMC) is currently working with 2 short-lived perennials that may fill this early-seral niche. Amethyst Germplasm hoary tansyaster (Machaeranthera canescens (Pursh) A. Gray [Asteraceae]) (Figure 1) was released by IDPMC in 2016 for inclusion in rangeland seedings in the Great Basin (Tilley 2016). Similarly, curlycup gumweed (Grindelia squarrosa (Pursh) Dunal [Asteraceae]) (Figure 2) is currently under investigation for potential selected class germplasm release (Tilley and Pickett 2019a). Both species are adapted to disturbed areas in semi-arid sites in the Great Basin (Whitson and others 1996; Welsh and others 2003) that receive 250 to 500 mm (10–20 in) mean annual precipitation (Cronquist and others 1994; Tilley and others 2010; Tilley and Pickett 2016). Both species appear to persist in disturbed sites, such as roadsides and gravel pits, but quickly disappear from established climax plant communities (authors’ personal observations). Hoary tansyaster is common in early successional plant communities but is less common in highly disturbed sites. Barak and others (2015) found a close relative, tansyleaf tansyaster (M. tanacetifolia (Kunth) Nees [Asteraceae]), to be a potentially valuable species for restoration of cheatgrass-invaded rangelands. Because it has no seed dormancy issues, hoary tansyaster has been observed to germinate after late summer and fall rains, a beneficial trait as it allows the species to compete directly with cheatgrass, another early germinator (Parkinson and others 2013; Barak and others 2015). Parkinson and others (2013) found biomass of hoary tansyaster was not reduced after 12 wk of growth in the presence of the native grasses, bottlebrush squirreltail, and Sandberg bluegrass, as compared to growing alone. Cheatgrass at a density of 100 plants/m2 significantly reduced total biomass and shoot relative growth rate of hoary tansyaster but not root growth rate. They attribute this response to the tap-rooted morphology of hoary tansyaster, which is different from and not overlapping with cheatgrass.

Figure 1.
Figure 1.

Hoary tansyaster (Machaeranthera canescens) is a common colonizer of disturbed sites in semi-arid rangelands of the Intermountain West.

Figure 2.
Figure 2.

Curlycup gumweed (Grindelia squarrosa) is being investigated by IDPMC for potential germplasm release for use in conservation and pollinator plantings in the Great Basin.

To increase understanding of the efficacy of using early-seral species to outcompete cheatgrass, we compared seedling growth of 2 early-seral forbs, curlycup gumweed and hoary tansyaster, and the late-seral perennial grass, bluebunch wheatgrass. Our objectives were to determine if the early-seral species had any suppressive effects on cheatgrass and if early-seral species were more effective at suppressing cheatgrass growth than a late-seral grass. We also wanted to determine if curlycup gumweed had any negative impacts on bluebunch wheatgrass germination or growth.

MATERIALS AND METHODS

We evaluated growth characteristics of curlycup gumweed, hoary tansyaster, and bluebunch wheatgrass in the presence of cheatgrass at 2 densities. Hull and Pechanec (1947) and Young and Evans (1978) found cheatgrass densities in highly degraded sites exceeding 10,000/m2 with an estimated average density of more than 6000 cheatgrass plants/m2. For our experiment we used modest densities of 350 plants/m2 (low density) and 1400 plants/m2 (high density). We also included low- and high-density treatments of native species (minus hoary tansyaster) to compare the degree of competition imposed by cheatgrass to the same level of intraspecific competition of the focus species, even though such densities are unlikely in nature nor currently recommended in restoration seedings.

Because some literature previously designated curlycup gumweed as a weed (Stubbendieck and others 1994; Whitson and others 1996), we decided to evaluate any potential ill effects its inclusion in restoration seed mixes might have on desired climax species. Bluebunch wheatgrass is a very common, late-seral species and is the most widely seeded native species in Great Basin restoration and conservation projects, making up 27% of the total seed purchased by the Bureau of Land Management (BLM) from 2010 to 2015 (Roller 2016). It is often the dominant grass species in 220 to 350 mm (8.8–14 in) precipitation areas of the sagebrush steppe ecosystem (Welsh and others 2003; Ogle and others 2010). Because it is often among the most desired climax species targeted for restoration, we need to better understand the impact of including early-seral “weedy” natives in seed mixes containing bluebunch wheatgrass (Parkinson and others 2013). To this end, we also evaluated the growth of bluebunch wheatgrass in low- and high-density combinations of curlycup gumweed and compared it to bluebunch growth in the same densities of cheatgrass and a bluebunch wheatgrass control.

We planted seed into 655 cc (40 ci) pots filled with Sunshine Mix #4 soil (Sungrow Horticulture, Agawam, Massachusetts) in a completely randomized experimental design. Eight to 12 pots were planted for each treatment in expectation of some losses. In total, we evaluated 18 treatments with 6 to 10 replications depending on the number of pots successfully established.

Bluebunch wheatgrass seed was of Anatone Germplasm produced in 2016, and hoary tansyaster was of Amethyst Germplasm produced in 2017. Both of these germplasm releases were made from single-population seed sources (Monsen and others 2003; Tilley 2016). Because of the release selection process, the seed used from these species may have limited genetic diversity, thus the seedling responses observed in this study should be viewed tentatively and may not be representative of all phenotypes. For curlycup gumweed, we used a blend of seed collected from 25 locations throughout the Great Basin in 2018. Cheatgrass seed was collected from multiple seed production fields at IDPMC.

Curlycup gumweed seed exhibits significant physiological dormancy and is typically put through a cold-moist stratification period of approximately 90 d to reach adequate germination (Nuzzo 1978; Baskin and Baskin 2002; Luna 2008). However, research has shown that submerging seed in oxygenated water effectively eliminates the need for stratification of some species, including curlycup gumweed (Tilley 2013; Tilley and Pickett 2019b). Hoary tansyaster exhibits no seed dormancy, but we have seen that it responds positively to soaking in an oxygenated water bath and quickens germination time (authors’ personal observations). Therefore, we germinated gumweed and tansyaster using an oxygenated water bath to bypass the stratification requirement and to produce uniform germinants. Seed was primed in oxygenated deionized water for 24 h in a Hoffman growth chamber (Hoffman Manufacturing Inc, Corvallis, Oregon) with a 12-h light/dark cycle and 22 °C (71 °F) day and 15 °C (59 °F) night temperatures. We then seeded with a large eye dropper at a rate of approximately 4 seeds for each desired plant. Cheatgrass and bluebunch wheatgrass were direct seeded with no pretreatment at a similar rate. All plants germinated within 7 d of one another. Additional germinants were thinned as necessary to meet the desired number of plants/pot for each species.

Pots were irrigated as needed with overhead sprinklers following daily visual inspection. Plants were fertilized once, 6 wk after planting, with Miracle Grow 24-8-16 All Purpose Plant Food (Scotts Miracle-Gro, Marysville, Ohio). Pots were spaced as necessary to avoid shading from one pot to another.

Evaluations began 2 wk after planting. We measured plant height (h) and 2 perpendicular diameters of each plant (d1 and d2) weekly to ascertain the canopy volume of each focus plant and the average canopy volume of the outer plants in each cone (Figure 3). Measurements were made of the plants as they stood without stretching the leaves or stems. Plant canopy volume calculations were adapted from Thorne and others (2002) using the formula for the volume of half of an ellipsoid (ellipsoid divided in half to represent the aboveground parts of the plant) (Figure 4):Embedded Image

Figure 3.
Figure 3.

Seedlings were grown for 12 wk under greenhouse conditions. Plant volume was measured weekly. Final volume and aboveground biomass were measured for statistical analysis at 12 wk after planting.

Figure 4.
Figure 4.

Plant volumes were calculated using the equation for one-half of an ellipsoid.

At 12 wk we also measured aboveground dry matter biomass of each focus species and average aboveground biomass for the competition species. Biomass samples were oven-dried at 60 ºC (140 °F) for 3 d prior to weighing.

Week 12 data were analyzed using Statistix 10 (Analytical Software, Tallahassee, Florida). Plant volume and biomass for curlycup gumweed and hoary tansyaster, as well as plant volume for bluebunch wheatgrass, were log-transformed to correct for abnormality. Bluebunch wheatgrass biomass data were similarly square-root transformed. Cheatgrass volume and biomass were normally distributed and required no transformation. Normalized data were analyzed using one-way analysis of variance, and means were separated using an LSD all-pairwise comparison with an alpha level of 0.05. Graphs were created using R (R Core Team 2017).

RESULTS

Plant Growth over 12 Wk of Growth

Treatments are hereafter designated by first letter of focus species followed by the first letter of the competitor. The number 4 preceding the competitor is used to describe plants per pot in high-density treatments (Table 1). Curlycup gumweed either in a pot alone (G) or with a single plant of an intraspecific competitor (GG) grew at essentially the same rate and showed little difference in the volume of the target plant over the first 12 wk of growth (Figure 5). Gumweed growing with high- and low-density cheatgrass (GB and G4B) grew at a significantly lower rate, and plant volume was greatly inhibited. Gumweed grown at high densities with members of the same species (G4G) likewise showed significant reductions in volume, but not to the extent caused by cheatgrass.

View this table:
TABLE 1

Competition treatments investigated.

Figure 5.
Figure 5.

Volume of curlycup gumweed (cm3) with same-species competitor and high and low densities of cheatgrass from 2 to 12 wk of growth. G = solitary curlycup gumweed; GB = curlycup gumweed grown in low-density cheatgrass; GG = low-density curlycup gumweed; G4B = curlycup gumweed grown in high-density cheatgrass; G4G = high-density curlycup gumweed. Error bars represent 1 standard deviation.

Hoary tansyaster plant volumes were greatly decreased in the presence of all competitors, including low densities of other tansyaster seedlings (MM) (Figure 6). After approximately 5 wk, competition became noticeable as hoary tansyaster plants grown without competition grew discernably larger and more quickly. Plants grown with competitors were noticeably stunted, occasionally resulting in mortality of the focus plant.

Figure 6.
Figure 6.

Volume of hoary tansyaster (cm3) with same-species competitor and high and low densities of cheatgrass from 2 to 12 wk of growth. M = solitary hoary tansyaster; MB = hoary tansyaster grown in low-density cheatgrass; MM = low-density hoary tansyaster; M4B = hoary tansyaster grown in high-density cheatgrass. Error bars represent 1 standard deviation.

Bluebunch wheatgrass seedlings’ response to competition was similar to that observed from curlycup gumweed. Volumes were visibly reduced with competition, even from low-density bluebunch competitors (PP) (Figure 7); however, the greatest reductions were seen in the high-density bluebunch wheatgrass (P4P) and both densities of cheatgrass (PB and P4B).

Figure 7.
Figure 7.

Volume of bluebunch wheatgrass (cm3) with same-species competitor and high and low densities of cheatgrass from 2 to 12 wk of growth. P = solitary bluebunch wheatgrass; PB = bluebunch wheatgrass grown in low-density cheatgrass; PP = low-density bluebunch wheatgrass; P4B = bluebunch wheatgrass grown in high-density cheatgrass; P4P = high-density bluebunch wheatgrass. Error bars represent 1 standard deviation.

Plant Volume and Aboveground Biomass at 12 Wk

Curlycup Gumweed

Curlycup gumweed volume was significantly reduced by the presence of low- and high-density cheatgrass (GB and G4B) as well as high densities of gumweed (G4G) (P < 0.001) (Figure 8). Low-density cheatgrass (GB) caused decreases in plant volume of 85%, statistically equivalent to those of high-density intraspecific competition of gumweed (G4G). High densities of cheatgrass (G4B) resulted in target plant volumes significantly lower than all treatments (Figure 9), with a 95% reduction compared to gumweed grown alone (G). We saw no significant reduction in target plant volume, however, from low-density gumweed (GG).

Figure 8.
Figure 8.

Curlycup gumweed target plant volume (left) and biomass (right) at 12 wk when grown solitarily (G), in the presence of an additional gumweed (GG), in high-density gumweed (G4G), in low-density cheatgrass (GB), and in high-density cheatgrass (G4B). Error bars represent 1 standard deviation.

Figure 9.
Figure 9.

Curlycup gumweed grown in high-density cheatgrass competition had significantly lower plant volume and biomass compared to a control grown alone.

Gumweed biomass results were significantly different (P < 0.001). Adding a second G to the pot (GG) did not reduce plant biomass; however, adding a single cheatgrass neighbor reduced gumweed biomass by 80%. High-density gumweed (G4G) reduced focus plant biomass by more than 50%, whereas high-density cheatgrass (G4B) caused a 90% reduction in the biomass of the target plant.

Hoary Tansyaster

Hoary tansyaster (Figure 10) focus plant volumes were significantly reduced in all treatments that added more competitors (P < 0.001) (Figure 11). Adding a single tansyaster competitor (MM) reduced the volume of the target plant by more than 80%, whereas adding a single cheatgrass reduced tansyaster volume by 95%. High-density cheatgrass reduced tansyaster volumes even further with average reductions of nearly 99%.

Figure 10.
Figure 10.

Hoary tansyaster grown in a low-density pot with a single cheatgrass competitor. Hoary tansyaster plant volumes and aboveground biomass were significantly reduced by all competitors including the native species evaluated.

Figure 11.
Figure 11.

Hoary tansyaster target plant volume (left) and biomass (right) at 12 wk when grown solitarily (M) in the presence of an additional tansyaster (MM), low-density cheatgrass (MB), and high-density cheatgrass (M4B). Volume and biomass were significantly reduced by all treatments. Error bars represent 1 standard deviation.

Tansyaster biomass at 12 wk followed the same trend as plant volume. Slight increases in plant density (MM and MB) resulted in significant biomass reductions of the target plants (P < 0.001). The addition of a tansyaster plant caused a 75% reduction, whereas adding a single cheatgrass plant resulted in 93% less biomass. Even greater reductions were seen from high-density competition of cheatgrass (M4B) with reductions of biomass of (95%). These data would appear to confirm the findings of Parkinson and others (2013).

Bluebunch Wheatgrass

Focus bluebunch wheatgrass plant volumes were greatly reduced by the addition of low- and high-density cheatgrass (PB and P4B), resulting in 90% and 95% reductions, respectively (Figure 12). Adding a single gumweed seedling to bluebunch wheatgrass did not reduce volume of the target plant; however, bluebunch wheatgrass volumes were reduced by a significant 50% by the addition of a single bluebunch competitor (PP). High-density gumweed competition (P4G) reduced target plant volume by 60%, yet those volumes did not differ significantly from the PG or PP treatments.

Figure 12.
Figure 12.

Bluebunch wheatgrass target plant volume (left) and biomass (right) at 12 wk when grown in the presence of an additional bluebunch (PP), low-density curlycup gumweed (PG), high-density gumweed (P4G), low-density cheatgrass (PB), and high-density cheatgrass (P4B). Error bars represent 1 standard deviation.

Bluebunch focus plant biomass trends were similar to trends observed from measurements of plant volume. A single cheatgrass seedling (PB) caused significant decreases in the focus species biomass, but low densities of gumweed (PG) did not reduce focus plant biomass significantly. Likewise, the low-density bluebunch (PP) treatment showed significant, though minor, reductions in aboveground biomass. High-density treatments (P4P, P4G, and P4B) resulted in dramatic reductions in target plant biomass with the greatest reduction resulting from the high-density cheatgrass treatment (90%).

Effect of Early- and Late-Seral Natives on Cheatgrass

None of the native species tested reduced cheatgrass volume when compared with being grown alone (P = 0.099) (Figure 13). A single cheatgrass competitor reduced the average volume of the target plant by 40% but was not significant. Cheatgrass biomass was not reduced significantly by the 2 early-seral forb species but was significantly reduced by bluebunch wheatgrass and an additional cheatgrass by 35% and 50%, respectively (P = 0.001).

Figure 13.
Figure 13.

Cheatgrass target plant volume (left) and biomass (right) at 12 wk when grown in the presence of an additional cheatgrass (BB), low-density curlycup gumweed (BG), low-density hoary tansyaster (BM), and low-density bluebunch wheatgrass (BP). Error bars represent 1 standard deviation.

DISCUSSION

After 12 wk, cheatgrass had produced more biomass when grown alone than any of the other species tested; 2.5 times more than bluebunch wheatgrass and curlycup gumweed, and 3 times more than hoary tansyaster. This ability to establish and grow quickly makes this species highly competitive. A single competitor of cheatgrass reduced biomass and volume of all 3 native species within the range of 75% to 95% regardless of successional niche. Higher densities of cheatgrass had an even greater effect on the target species. This finding is highly concerning when one considers we used 1400 plants/m2 for our high-density treatment when densities of 10,000 plants/m2 have been reported.

Differences in natural germination timing may further complicate the issues. In this trial, we essentially germinated all seed simultaneously; however, cheatgrass seedlings often have a competitive advantage in the field from germinating at lower temperatures and with less available moisture than many native species. Cheatgrass has the distinct advantage of producing significant root growth far earlier than most native perennials. Even with the benefit of uniform germination times, the natives we tested were unable to compete effectively and suppress cheatgrass growth. In natural conditions, cheatgrass would easily be able to establish and reproduce against the competitors we evaluated.

Cheatgrass aboveground biomass, but not plant volume, was reduced significantly in the presence of bluebunch wheatgrass. While bluebunch itself was greatly reduced under competition, it did have a moderate effect on cheatgrass production, at least more so than the forbs we tested. The 2 early-seral forbs tested were less effective in direct competition with cheatgrass than was the late-seral climax perennial grass. These findings would seem to be in line with those of Parkinson and others (2013) regarding hoary tansyaster. It is possible that curlycup gumweed and hoary tansyaster are better considered as advantageous occupants of disturbed sites rather than highly competitive colonizers.

Curlycup gumweed did not reduce the volume or biomass of bluebunch wheatgrass any more than the presence of more bluebunch seedlings. Despite being labeled as a weed by some, it does not seem to pose a greater threat than intraspecific competition at the seedling stage, and the concern of competition from gumweed may be unfounded. While curlycup gumweed and the plants evaluated are compatible at these early growth stages, the authors are currently investigating if mature curlycup gumweed causes any allelopathic effects on the germination and growth of bluebunch wheatgrass or other plants.

Bluebunch wheatgrass and hoary tansyaster seed used in this study originated from sources of limited genetic and phenotypic diversity. The seedling responses observed in this study, therefore, may not fully represent the range of response available to the species. In that light, there may be potential to select for traits that could improve performance relative to cheatgrass, such as earlier germination or increased seedling vigor among these species as well as for curlycup gumweed.

CONCLUSION

The 2 early-seral native forbs tested were not effective at reducing or suppressing cheatgrass growth when germinated concurrently with cheatgrass. It is possible that species with germination timing similar to cheatgrass would be more effective at direct competition; however, finding desirable natives with those traits remains a challenge. Bluebunch wheatgrass was the strongest competitor against cheatgrass in our trial. Finally, we saw no suppressive effect from curlycup gumweed on bluebunch wheatgrass at this growth stage.

Footnotes

  • Photos by Derek Tilley

  • This article was prepared by U.S. government employees as part of the employees’ official duties and is in the public domain in the United States.

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Seedling growth and competition of a late-seral, native perennial grass and 2 early-seral, native forbs in the presence of 2 densities of the invasive annual grass Bromus tectorum L. (Poaceae)
Derek Tilley, Nathaniel Tilley, Adam Fund, Mary Wolf
Native Plants Journal Sep 2020, 21 (3) 299-311; DOI: 10.3368/npj.21.3.299
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