Abstract
Elevation and latitude correlate with environmental factors that drive natural selection. Climate-change projections in the Great Basin suggest higher temperatures and increasingly lower annual winter snowfall, especially at low elevations. The transition from dormancy to germination represents a vulnerable stage of plant phenology, given the high susceptibility to death in newly germinated seedlings. Consequently, mechanisms involved with this phenomenon and its timing play an essential role in plant population fitness. This study investigates seed dormancy and germination for 2 co-occurring Asteraceae/Compositae species: arrowleaf balsamroot (Balsamorhiza sagittata (Pursh) Nutt.) and shiny mule’s ear (Wyethia amplexicaulis (Nutt.) Nutt.) from the same geographic area (near Provo Canyon, Utah) across an elevation gradient of 1584 m to 2438 m (5200–8000 ft). In laboratory experiments, seeds received increasing periods of cold stratification at 2 °C (35.6 °F) (from 4–20 wk) followed by incubation at 10 to 20 °C (50–68 °F). Shiny mule’s ear seeds from low (1584 m [5200 ft]) and mid (2011 m [6600 ft]) elevations required 8 wk of cold stratification while seeds from the highest elevation required 12 wk to reach 50% germination. Seeds of arrowleaf balsamroot from all elevations reached 50% germination after 16 wk of cold stratification although germination rate varied by elevation. These results suggest that selection pressure in contrasting environments results in variable dormancy-breaking durations, with implications for propagation, projected climate change, and seed sourcing in restoration efforts.
Brown A, Allen PS. 2023. Elevation impact on seed germination requirements for two Asteraceae species. Native Plants Journal 24(1):45–53.
Close-up of shiny mule’s ear (Wyethia amplexicaulis) taken in the study area near Provo Canyon, Utah.
In the Great Basin region of the western US, elevation and latitude strongly influence plant distribution. The plasticity and genetic variation of traits within a given species determine the range of conditions under which plants can successfully establish. Elevation and latitude correlate with several environmental factors and climatic conditions. Some of these include length of growing season, average temperature, solar radiation, and the frequency, form, and duration of precipitation. These factors can directly influence plant population distribution as they create conditions that may or may not surpass the limits of tolerance for a given species.
Successful germination and establishment of seedlings are important in determining whether a plant can survive in a given area (James and others 2011). This, in turn, influences plant population distribution and can lead to variation in plant traits depending on habitat (Larson and others 2015; Leger and Baughman 2015). Over the course of their growth and development, plants have the most vulnerability to death during seedling establishment (Baskin and Baskin 2014). Seeds that germinate too soon can die due to drought or frost, and those that stay dormant for too long may be negatively affected by disease, herbivory, or poor seedling growing conditions, all of which inhibit successful establishment (Clark and Wilson 2003). Seeds of wild plants often have dormancy requirements that reduce the likelihood that seeds will germinate under conditions that would otherwise allow germination but are not ideal for establishment (Vleeshouwers and others 1995; Baskin and Baskin 2014; Willis and others 2014). Given that germination cannot be reversed, the timing for the switch from dormancy to germination is under strong selection pressure (Meyer and others 1989, 1990, 1995; Beckstead and others 1996; Willis and others 2014; Jiménez-Alfaro and others 2016). Therefore, understanding factors that contribute to the development of seed dormancy traits provides insight into how populations may respond to a changing climate. In addition, it addresses questions of how well seed propagation methods can be generalized and the importance of seed sourcing for restoration seedings.
For ecosystems located in the Great Basin of the western US, winter cold and summer drought highly influence the successful timing of germination and establishment. Because climatic conditions strongly influence the development of dormancy requirements, projections regarding changes for these and other environmental factors make climate change relevant to our understanding of germination behavior and its implications for future population fitness. Past projections for the Great Basin indicated a future with overall warmer temperatures, decreased snowfall, and increasingly intense rain events (Logan and others 2007; Gillies and others 2012). Empirical evidence illustrates that decreases in snowpack across the western US are already taking place (Mote and others 2018). The literature suggests that the degree of change in these projections will vary by elevation, increasing in magnitude at lower elevations. Across the globe, climate change threatens biodiversity through novel interactions and altered environmental conditions within established ecosystems (Parmesan and Yohe 2003). As climate change leads to greater temperature and precipitation extremes, which will continually have an impact on the persistence of native plant communities, their distributional ranges appear to be shifting (Kelly and Goulden 2008; Chang and others 2015).
Climatic conditions for the site of a plant population create selection pressure that directly influences seed dormancy and germination (Kildisheva and others 2020). In fact, studies of multiple species have shown that seed dormancy requirements are often strongly correlated with habitat (Meyer and Monsen 1991; Meyer 1992; Meyer and Kitchen 1994; Meyer and others 1995). While elevation influences seed germination behavior, specific results vary by species (Weng and Hsu 2006; Salehani and others 2013; Ge and others 2020; Veselá and others 2020; Wang and others 2021). Multiple studies have found that elevation influences seed mass, a trait that can influence germination, but again, the results varied by species (Qi and others 2014; Ge and others 2020).
In this study, we investigated how elevation influences the dormancy and germination of two co-occurring Asteraceae/Compositae species, shiny mule’s ear (Wyethia amplexicaulis (Nutt.) Nutt.) (Figure 1) and arrowleaf balsamroot (Balsamorhiza sagittata (Pursh) Nutt.) (Figure 2). We selected these species as they are prevalent throughout the Great Basin and grow across a relatively large elevation gradient. Mule’s ear inhabits many plant communities throughout the Intermountain West in Colorado, Idaho, Montana, Nevada, Oregon, Utah, Washington, and Wyoming at elevations from 1360 to 3300 m (4500–11,000 ft) (Matthews 1993). Arrowleaf balsamroot inhabits the states of California, Colorado, Idaho, Montana, Nevada, Oregon, South Dakota, Utah, Washington, and Wyoming at elevations from approximately 305 to 2743 m (1000–9000 ft) (McWilliams 2002). Both species experience physiological seed dormancy that can be overcome through cold stratification (Baskin and Baskin 2014).
Shiny mule’s ear (Wyethia amplexicaulis) flower, June 2021.
Arrowleaf balsamroot (Balsamorhiza sagittata) with Buffalo Peak in the background, May 2020.
Our goal was to understand how germination behavior for these species varies across an elevational gradient, specifically as related to the duration of cold stratification. In the semi-arid climate of the Great Basin, many species have cold-stratification requirements as a mechanism to end seed dormancy, which is driven by selection pressure related to seed germination timing. We hypothesize that seeds from higher elevations require longer periods of cold stratification to break dormancy than seeds from lower elevations. This hypothesis is based on selection pressure associated with the greater amount and duration of snowpack and colder winter temperatures as elevation increases.
METHODS
Seed Collection Location
We collected fully ripe seeds of shiny mule’s ear and arrowleaf balsamroot from along the Squaw Peak Road (between Squaw Mountain and Cascade Mountain) leading up to Buffalo Peak near Provo, Utah. These species grow across the entire elevation gradient (1584–2438 m [5200–8000 ft]) from the base to the top of the mountain and co-occur in many locations. Collection occurred when seeds had reached full maturity (between late June and late July).
We collected seeds from 3 areas (high, middle, and low elevations) along a transect in the canyon (Figure 3). The high elevation seeds were collected from the top of Buffalo Peak, at 2438 m above sea level (asl) (8000 ft+). Mid-elevation seeds were collected midrange between the low and high elevation collection areas at around 2011 m asl (6600 ft). Low elevation seeds were collected from populations of sufficient size (large enough that seed collection would minimally disturb the population) at the base of the mountain (1584 m asl [5200 ft]). The distance between the peak of the mountain and the low elevation collection area is approximately 4.8 km (3 mi) in a straight line or 10.6 km (6.6 mi) by road and trail.
Aerial view of Squaw Peak Road showing collection areas at low (<1584 m [<5200 ft]), mid (2011 m [6600 ft]), and high (2438+ m [8000+ ft]) elevation. Image from Google Earth
Collection and Care of Samples
Collectors walked in random patterns through collection sites, clipping flower heads from random plants with mature seeds. Seed heads were collected into paper bags, returned to the laboratory, and placed in a freezer at −18 °C (−0.4 °F) overnight to kill any insects feeding on the seeds. Seeds were cleaned by hand or by using a series of sieves to separate chaff from seeds. Once cleaned, we stored seeds in sterile Petri dishes in ambient laboratory conditions (approximately 25 °C [77 °F] and 20% humidity) until the experiments were conducted. Because of a high percentage of seeds without embryos, we included a winnowing treatment to increase population viability. Specifically, we dropped seeds in front of a fan blowing at a constant velocity. Seeds that landed farthest from the fan were found to be unfilled (based on a cut test of 100 randomly selected seeds) and were not included in the experiment. Seeds were also removed if there were visible signs of granivory.
Experimental Design
For each experimental unit, 25 random seeds were counted and assigned to a Petri dish. Each combination of cold-stratification treatment and elevation was replicated 4 times. At the outset of the experiment, we applied a Captan anti-fungal powder treatment to the seeds and placed them on double blotter paper (Anchor Paper, St Paul, Minnesota), which had been saturated with distilled water pre-cooled to 2 °C (35.6 °F) in 86 mm (3.38 in) Petri dishes. Dishes were randomly arranged in stacks of 12 in sealed plastic bags; each bag contained a water-saturated paper towel in the bottom to prevent drying. A single blotter paper was also placed on the top dish in each bag to ensure uniform light exposure among all dishes.
For cold-stratification treatments, we placed Petri dishes in a dark incubator set to 2 °C (35.6 °F) to simulate conditions experienced by seeds under snowpack. Seeds of each species were subjected to cold stratification for 4, 8, and 12 wk, with arrowleaf balsamroot seeds also subjected to 16 and 20 wk. Following cold stratification, Petri dishes were transferred to an incubator alternating between 10 and 20 °C (50 and 68 °F) with a 12-h photoperiod during the warmer temperature.
Seed germination (radicle emerged to at least 1 mm in length) was counted after seeds were removed from cold stratification and on days 1, 2, 4, 7, 11, 14, 21, and 28 during incubation. Germinated seeds were removed from dishes. We added distilled water (kept in the incubator to match the temperature of the dishes) as needed during counting to prevent blotters from drying out. After 28 d, we determined viability of ungerminated seeds using a cut test. Firm, white embryos were scored as viable, while soft, discolored embryos were scored as nonviable (Association of Official Seed Analysts 1986). Prior to statistical analysis, germination data were converted to percent germination as a fraction of the viable seeds in each replicate.
Reporting Results and Statistical Analysis
We conducted this experiment using a completely randomized design. Collection site elevation and duration of cold stratification were considered independent variables, and 28-d germination percentage and days to 50% relative germination were considered dependent variables. To evaluate differences between elevation groups, the fit model function was used to characterize total germination after 28 d based on collection site elevation and length of cold stratification (JMP, Version 15, SAS Institute, Cary, North Carolina, 1989–2021). We found non-uniform variance in the seed germination data of arrowleaf balsamroot, so the germination data of this species were square-root transformed before analysis, although original data are reported. Statistical comparisons were not made between the 2 species.
RESULTS
Shiny Mule’s Ear
Seeds of shiny mule’s ear required at least 8 wk of cold treatment before 50% germination occurred (Figure 4, panel B). With this treatment, seeds collected from low and mid-elevations had significantly higher germination percentages than did those from the high elevation, which never reached 50% germination. Differences in germination rate between elevation levels were also evident. For example, low and mid-elevation seeds subjected to 12 wk of cold stratification reached approximately 85% germination after 2 d following transfer to 10 to 20 °C (50–68 ºF) whereas seeds from the high elevations required 11 d to reach this percentage (Figure 4, panel C). While the germination percentage between the elevation groups at 12 wk was closer than with shorter periods of stratification, seeds collected from the high elevation were clearly slower to germinate than were those from low and mid-elevation seeds.
Germination time course curves for shiny mule’s ear seeds subjected to increasing lengths of cold stratification (4 wk [A], 8 wk [B], and 12 wk [C]) followed by incubation at 10 to 20 °C (50–68 °F). Seeds were collected from low (1584 m [5200 ft]), mid (2011 m [6600 ft]), and high (2438+ m [8000+ ft]) elevations. Colored lines indicate elevation levels. Error bars represent ± one standard error from the mean.
Arrowleaf Balsamroot
Arrowleaf balsamroot required 16 wk of cold stratification to reach 50% germination for all elevations (Figure 5, panel D). No seeds in the 4-, 8-, or 12-wk cold-stratification treatments reached 50% germination following the 28-d warmer incubation period (Figure 5, panels A, B, C). However, seeds stratified for 16 or 20 wk germinated above 50% (Figure 5, panels D, E). Most of the germination (>50%) of seeds in the 16- and 20-wk treatments occurred during cold stratification (Figure 5). In both the 16- and 20-wk cold-stratification treatments, low and mid-elevation seeds reached higher germination percentages during cold stratification than did high elevation seed sources. Elevation differences were evident despite all elevation levels showing high percentages of germination (>80%) at 16 and 20 wk of cold stratification (Figure 5). Germination of low elevation seeds was significantly higher than those collected from higher elevations (P < 0.001). Germination during post-chill incubation was faster for low and mid-elevation seeds compared to those from the high elevation (Figure 5).
Germination time course curves for arrowleaf balsamroot seeds subjected to increasing lengths of cold stratification (4 wk [A], 8 wk [B], 12 wk [C], 16 wk [D], and 20 wk [E]) from low (1584 m [5200 ft]), mid (2011 m [6600 ft]), and high (2438+ m [8000+ ft]) elevations during a 28-d post-stratification incubation period at 10 to 20 °C (50–68 °F). Colored lines indicate elevation levels. Day 0 values indicate germination that occurred during cold stratification. Error bars indicate ± one standard error from the mean.
DISCUSSION
This study illustrates how shiny mule’s ear and arrowleaf balsamroot seeds showed variation in dormancy-breaking requirements, and those germination responses to cold stratification were associated with elevation. Results of this study support our hypothesis that seeds from higher elevations require a longer stratification period, even within the same geographic area. Findings are consistent with other studies reporting that contrasting environments are associated with differences in seed dormancy (Meyer and Monsen 1991; Meyer 1992; Meyer and Kitchen 1994; Meyer and others 1995). For example, Meyer and others (1995) characterized the germination behavior of several Penstemon species across a range of environments; in some cases, the same species showed differing patterns of dormancy and germination that were correlated to habitat and were ecologically relevant. Understanding how dormancy-breaking requirements differ with elevation is helpful for propagators who need to predict the best dormancy-breaking treatments for these species.
While both Asteraceae species in our study required longer periods of stratification when collected from higher elevations, they varied considerably in maximum germination rate. Shiny mule’s ear germinated almost immediately (≤2 days following stratification), which likely corresponds to seeds germinating in nature during the early spring shortly after snow melt. Arrowleaf balsamroot seeds primarily germinated during cold-stratification treatments longer than 12 wk in duration, which suggests that under field conditions seeds may germinate under snowpack and are prepared to emerge in early spring (Allen and Meyer 1998). Consequentially, the requirements for breaking dormancy varied greatly between the 2 species. Final germination percentages of shiny mule’s ear seeds increased steadily with lengthening stratification treatments. In contrast, arrowleaf balsamroot reached a threshold at 16 wk. Longer stratification (20 wk) did not result in statistically higher germination.
Findings of this study suggest that for shiny mule’s ear and arrowleaf balsamroot, collecting seeds at lower elevations and planting at higher elevations would likely allow for successful establishment since the stratification requirement would likely be met. Seeds from higher elevations, however, would have limited success if planted at lower elevations given their longer stratification requirement. In addition, seeds of both species collected from lower elevations showed earlier and more rapid rates of germination during cold stratification or the 28-d incubation period that followed. In the Great Basin region, rapid germination appears to promote successful establishment in many restoration projects for which competition from invasive weeds is a major challenge (Smith and others 2008; Peeler and others 2018). Technological advances, such as seedcoating technologies that allow for incorporating hormones such as gibberellins (Madsen and others 2014; Keefer and others 2021), may also improve seed-establishment success and may allow for seeds collected from higher elevations to germinate at lower elevation sites. However, other adaptations that serve plants from high elevations could be maladaptive traits for lower elevation plant communities (McKay and others 2005).
A 2008 study by Lenoir and others indicates that for some species, climate change is shifting the optimum elevation upward, especially grassy species and those limited to mountain habitats. As the climate warms, decreased snowfall, particularly at lower elevations, will likely lead to reduced germination and establishment of both species (Gillies and others 2012). Shiny mule’s ear may continue to have some level of germination and establishment success, even with low levels of snowfall as low and mid-elevations reached 50% germination with just 8 wk of cold stratification. However, arrowleaf balsamroot could potentially experience population declines if the cold-stratification requirement (at least 12 wk) is not met. Compounding this issue, populations of these species at all elevations may be adversely affected by the increased frequency of catastrophic disturbance, such as wildfires (Brown and others 2004; Dennison and others 2014; Westerling 2016). In other words, as variable establishment success due to changing climatic conditions shifts plant population distributions, wildfires may put existing populations at risk. Similarly, simulations of the Sierra Nevada forest found that interactions between climate change and wildfire regimes greatly influenced recruitment distribution of species and decreased species richness by preventing the establishment of less tolerant species (Liang and others 2017).
Young arrowleaf balsamroot (Balsamorhiza sagittata) plant at sunset on the Squaw Peak Road near Provo Canyon, Utah.
Understanding seed germination requirements aids efforts in successful plant community establishment. For example, these results give a clear indication that successful seeding of these species would need to take place in the fall in order for sufficient periods of cold stratification to occur. Our findings for shiny mule’s ear and arrowleaf balsamroot that show varied germination results between elevation groups support existing literature that suggests matching site conditions from the collection site to the intended site increases successful establishment (Kildisheva and others 2020). Our results also support the practice of sourcing seeds from comparable habitats to allow for seeds to be competitive at their intended site (Doherty and others 2017). Understanding the germination needs of these 2 key species and their specific adaptations to elevation through germination timing will allow restoration planners to make better decisions to address these concerns.
ACKNOWLEDGMENTS
We acknowledge Susan Meyer, a research ecologist with the US Forest Service, for her collaboration and support of this project. We express gratitude to the Plant and Wildlife Sciences Department at Brigham Young University for their support of this project. We appreciate individuals who reviewed this manuscript in preparation for submission: April Hulet, Brad Geary, Matt Madsen, and Ryan Stewart. In addition, we express gratitude to undergraduate research assistants Mckayla Sundberg, Abby Kjar, Spencer Mosely, and Ashlee Weight for assistance with seed collection, developing methodology, data collection, and other aspects of this project. Finally, we thank other volunteers who aided with seed collection for this project: Tami Allen, Joshua Kjar, Riley-Ann Pennington, and Tessa Brown.
Footnotes
Photos by Alyssa Brown
