Investigating Seed Longevity of Big Sagebrush 

(Artemisia tridentata)

Open-File Report 2009–1146

U.S. Department of the Interior

U.S. Geological Survey
Investigating Seed Longevity of Big Sagebrush 

(Artemisia tridentata

By Upekala C. Wijayratne and David A. Pyke 

) 

Open-File Report 2009–1146

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009 

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Suggested citation:

Wijayratne, U.C., and Pyke, D.A., 2009, Investigating seed longevity of big sagebrush (Artemisia tridentata

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

): U.S. 

Geological Survey Open-File Report 2009-1146, 26 p.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents

Executive Summary.................................................................................................................................................... 5

Introduction................................................................................................................................................................. 6

Objectives ............................................................................................................................................................... 7

Methods...................................................................................................................................................................... 8

Study Sites.............................................................................................................................................................. 8

Experimental Design............................................................................................................................................... 8

Seed Bag Construction ........................................................................................................................................... 9

Bag Effect ............................................................................................................................................................... 9

Statistical Analysis .................................................................................................................................................10

Results.......................................................................................................................................................................11

Seed Retrieval Experiment ....................................................................................................................................11

Bag Effect ..............................................................................................................................................................12

Seed Bank Sampling..............................................................................................................................................12

Plant Community....................................................................................................................................................13

Discussion .................................................................................................................................................................13

Seed Retrieval Experiment ....................................................................................................................................13

Seed Bank Sampling..............................................................................................................................................15

Implications ............................................................................................................................................................15

Conclusions ...............................................................................................................................................................16

Acknowledgments......................................................................................................................................................16

References Cited.......................................................................................................................................................16

Figures

Figure 1. Location of study sites. Within each study site, we located one Wyoming and one mountain big 

sagebrush plot ......................................................................................................................................................... 19

Figure 2. Median proportion of seeds remaining viable decreased over a period of 24 months for 

Wyoming and mountain big sagebrush, primarily in seeds beneath litter and on the surface.................................. 20

Figure 3. Viability of seeds in bags for Wyoming and mountain big sagebrush was higher than 

viability of free seeds, but only in the surface and litter treatments.......................................................................... 21

Figure 4. Median number of viable seeds/m2

22

in litter and soil samples of the Wyoming big sagebrush 

seed bank ................................................................................................................................................................

Figure 5. Median number of viable seeds/m2

23

in litter and soil samples of the mountain 

big sagebrush seed bank.........................................................................................................................................

Figure 6. Nonmetric multidimensional scaling (NMS) ordination of 12 Wyoming and mountain big 

sagebrush plots analyzed for plant species composition for a) Axis 1 and Axis 3, and b) Axis 1 and 

Axis 2....................................................................................................................................................................... 24

Figure 7. NMS ordination of twelve Wyoming and mountain big sagebrush plots analyzed for plant species 

composition for a) Axis 1 and Axis 2, and b) Axis 2 and Axis 3 ............................................................................... 25

Tables

Table 1. Study site information................................................................................................................................ 25

Table 2. Description of environmental and seed viability vectors in nonmetric multidimensional 

scaling (NMS) graphs .............................................................................................................................................. 26
iv

Conversion Factors, Datum, Acronyms, and Abbreviations

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

square millimeter (mm2) 0.00155 square inch (in2

square meter (m

)

2

) 0.0002471 acre 

cubic centimeter (cm3) 0.06102 cubic inch (in3

gram (g)

) 

0.03527 ounce, avoirdupois (oz)

meter per second (m/s) 3.281 foot per second (ft/s) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8. 

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). 

Acronyms and Abbreviations

ANOVA analysis of variance

h hour

min minute

NMS Nonparametric multidimensional scaling

s second

TZ tetrazolium
5 

Investigating Seed Longevity of Big Sagebrush

(Artemisia tridentata

By Upekala C. Wijayratne and David A. Pyke

) 

Executive Summary

The Intermountain West is dominated by big sagebrush communities (Artemisia tridentata

subspecies) that provide habitat and forage for wildlife, prevent erosion, and are economically important 

to recreation and livestock industries. The two most prominent subspecies of big sagebrush in this 

region are Wyoming big sagebrush (A. t. ssp. wyomingensis) and mountain big sagebrush (A. t. ssp. 

vaseyana). Increased understanding of seed bank dynamics will assist with sustainable management and 

persistence of sagebrush communities. For example, mountain big sagebrush may be subjected to 

shorter fire return intervals and prescribed fire is a tool used often to rejuvenate stands and reduce tree 

(Juniperus sp. or Pinus

Laboratory germination trials indicate that seed dormancy in big sagebrush may be habitatspecific, with collections from colder sites being more dormant. Our objective was to investigate seed 

longevity of both subspecies by evaluating viability of seeds in the field with a seed retrieval experiment 

and sampling for seeds 

sp.) encroachment into these communities. A persistent seed bank for mountain 

big sagebrush would be advantageous under these circumstances. 

in situ

Initial viability of seeds used in the seed retrieval experiment was 81 and 92 percent for

mountain and Wyoming big sagebrush, respectively. After remaining in the field for 24 months, buried 

Wyoming big sagebrush seeds retained 28−58 percent viability,11−23 percent of seeds under litter 

remained viable, and no seeds remained viable on the surface (estimates are 95-percent confidence 

intervals). The odds of remaining viable did not change from 12 to 24 months. However, after 24 

months the odds of seeds beneath litter being viable decreased to 75 percent of the odds of viability at 

12 months. Similar to Wyoming big sagebrush, buried seeds of mountain big sagebrush were 31−68

percent viable, seeds under litter retained 10−22 percent of their viability, and no surface seeds were 

viable after 24 months. 

. We chose six study sites for each subspecies. These sites were dispersed 

across eastern Oregon, southern Idaho, northwestern Utah, and eastern Nevada. Ninety-six polyester 

mesh bags, each containing 100 seeds of a subspecies, were placed at each site during November 

2006. Seed bags were placed in three locations: (1) at the soil surface above litter, (2) on the soil surface 

beneath litter, and (3) 3 cm below the soil surface to determine whether dormancy is affected by 

continued darkness or environmental conditions. Subsets of seeds were examined in April and 

November in both 2007 and 2008 to determine seed viability dynamics. Seed bank samples were taken 

at each site, separated into litter and soil fractions, and assessed for number of germinable seeds in a 

greenhouse. Community composition data from each site, as well as several environmental variables, 

were used to evaluate seed viability within the context of habitat variation.
6 

Both subspecies of big sagebrush had some portion of seed that remained viable for more than 

one growing season provided they were buried or under litter. Although seeds beneath litter may remain 

viable in intact communities, seeds are susceptible to incineration during fires. Nine months after seed 

dispersal, seed bank estimates for Wyoming big sagebrush ranged from 19 to 49 viable seeds/m2 in litter 

samples and 19−57 viable seeds/m2 in soil samples (95-percent confidence interval). For mountain big 

sagebrush, estimates were 27−75 viable seeds/m2 in litter samples and 54−139 viable seeds/m2

Seed longevity of big sagebrush depends on vertical position of seeds in the soil, indicating that 

seeds have higher odds of persisting in the seed bank if they are buried. Naturally occurring soil 

disturbances such as frost-heaving or rodent burrowing may enhance seed bank persistence by 

incorporating a portion of seeds beneath the soil surface. Restoration methods that ensure seed contact 

with soil and provide a soil surface disturbance that aids in seed burial may increase the probability of 

having big sagebrush seeds present when conditions for germination are favorable. 

in soil 

(95-percent confidence interval). The number of viable seeds present in the seed bank 9 months after 

seed dispersal was not significantly different from the number present immediately after seed dispersal.

Seed viability was highest in mountain big sagebrush sites for seeds on the surface and beneath litter, 

but decreased after one season. Buried seeds of both subspecies were in equal abundances and may be 

insulated from the effects of the environment. 

Introduction

Big sagebrush (Artemisia tridentata

Several subspecies of big sagebrush occur in this region, but two currently dominate, Wyoming 

big sagebrush (

) communities in the Intermountain Region provide habitat 

and forage for wildlife (Sands and others, 1999), prevent erosion (Goff and others, 1993), and are 

economically important to recreation and livestock industries (Maher, 2007; Evans and Rollins, 2008). 

Land managers are responsible for supervising lands for multiple, often competing, uses (Federal Land 

Policy and Management Act). Sagebrush subspecies are integral to their specific communities, and land 

managers would benefit from a sound understanding of the population biology of these subspecies, 

particularly seed bank dynamics.

A. t. ssp. wyomingensis) and mountain big sagebrush (A. t. ssp. vaseyana). Both 

communities have been declining due to expansion of invasive weeds such as cheatgrass (Bromus 

tectorum) in the Wyoming big sagebrush community and encroachment of native conifers like juniper 

(Juniperus

Thorough knowledge of seed longevity and dormancy for key big sagebrush subspecies in the 

Intermountain West will aid land managers in restoring and conserving these native landscapes. For 

example, prescribed burning in montane regions is a tool commonly used to rejuvenate native stands 

and reduce tree (

 sp.) in both communities (Miller and Rose, 1999; Miller and Tausch, 2001). Cheatgrass 

invasion has shortened fire return intervals in lowland habitat (Whisenant, 1990), which provides

favorable conditions for further habitat conversion (D’Antonio and Vitousek, 1992), and conifers have 

shifted fire regimes from relatively low to high intensity fires (Miller and Tausch, 2001). All big 

sagebrush subspecies lack the ability to sprout after burning and must regenerate from seed (McArthur 

and Stevens, 2004). A persistent seed bank for both subspecies of sagebrush would be advantageous to 

reestablishing populations after disturbance. 

Juniperus sp. or Pinus sp.) encroachment. If viable seeds of big sagebrush were 

available in the soil, then funds spent on reseeding or transplanting could be allocated elsewhere. In 

contrast, if seeds of big sagebrush do not persist beyond a year, then restoration of sagebrush may be 

necessary and likely would involve active reseeding or transplanting of this species. Exploring the link 

between seed location and seed longevity is important for appropriate placement of seeds, which may 

improve seed survival, germination and seedling establishment. 
7 

Big sagebrush seeds exhibit characteristics of nondeep physiological dormancy (Baskin and 

Baskin, 2001), which is caused by a physiological inhibiting mechanism of the embryo. This kind of 

dormancy can be broken by relatively short periods of cold stratification. Seeds of both subspecies show 

increased germination after being cold-stratified (McDonough and Harniss, 1974; Meyer and Monsen, 

1991). Seeds also may undergo afterripening in dry storage or come out of dormancy over time. 

Afterripening decreases the length of cold stratification necessary for breaking dormancy. Seeds in 

warehouse storage (6−8 percent relative humidity and less than 10 °C) typically hold full viability for 

2−3 years (Stevens and others, 1981; Hillary Parkinson, unpub. data, Boise State University, 2004), and 

germination rates were documented to be higher than rates immediately after harvest. A higher 

percentage of A. t. ssp. vaseyana

Unexplained Wyoming big sagebrush seedling emergence taking place years after reseeding 

efforts (Schuman and others, 1998) and mountain big sagebrush recolonization occurring considerable 

distances from potential maternal plants after a large fire (Ziegenhagen, 2003) indicate the possibility of 

acquired seed dormancy or seed persistence under a particular set of environmental conditions (Young 

and Evans, 1989). Although laboratory studies suggest dormancy in big sagebrush is habitat-specific, to 

our knowledge, there have been no direct field experiments or observations to confirm this hypothesis. 

There has been no empirical verification whether seeds of either subspecies could experience extended 

viability in the field beyond one winter season. 

seed collected from cold winter sites with mean January temperatures 

below -5 °C were nearly 100 percent light-requiring (Meyer and others, 1990), another manifestation of 

nondeep physiological dormancy. Germination trials indicate that dormancy may differ across 

subspecies and population (Meyer and Monsen, 1992). Wyoming big sagebrush was largely nondormant whereas mountain big sagebrush exhibited site-dependent dormancy. All Wyoming big 

sagebrush collections in the study had seeds that germinated quickly and supported previous field

studies that concluded little or no seed carry-over from one year to the next (Hassan and West, 1986; 

Young and Evans, 1989). 

Objectives

We investigated seed longevity of Wyoming and mountain big sagebrush in the field with a seed 

retrieval experiment and seed bank sampling. Very little old seed has been found in the soil after initial 

seed dispersal; thus it is assumed that most big sagebrush seeds either germinate quickly or die, and do 

not persist in the soil as viable seeds. Meyer and others (1990) argue that the small amount of seed that 

does carry over is likely buried, receiving inadequate light for germination. Soil disturbances like 

burrowing by subterranean mammals or frost-heaving could expose the seed to surface light and allow 

germination. We asked three questions to clarify seed viability and longevity in these subspecies: 

1.

2.

Do viable seeds of either Wyoming or mountain big sagebrush survive in the soil for more 

than one growing season? 

3.

Are seed longevity and persistence similar within a subspecies regardless of the seed’s 

location in the soil and its age? 

Does seed longevity and persistence change with environmental conditions (or habitats)?
8 

Methods

Study Sites

In August 2006, we chose study sites in Oregon and Idaho, located in the Western Intermountain 

Sagebrush Steppe, and in Utah and Nevada, in the Great Basin-Colorado Plateau Sagebrush SemiDesert (West, 1983) (fig. 1). We located three sites in an east-west gradient, and three sites in a northsouth gradient. At each site, we established one plot each in a Wyoming big sagebrush and mountain big 

sagebrush stand. Placement of plots was based on ocular estimation of good sagebrush and bunchgrass 

cover, and low cover of cheatgrass in the interspaces. Environmental differences between Wyoming big 

sagebrush plots and mountain big sagebrush plots (table 1) were not a concern because each subspecies 

was evaluated separately. 

Experimental Design

We used a seed bag retrieval experiment to examine fates of viable seeds in three soil positions 

and four collection times over about a 24-month period. Seed bags were positioned at three soil levels:

(1) buried 3 cm below soil surface, (2) at the soil surface but buried beneath 2 cm of sagebrush leaf 

litter, and (3) above soil and litter. In the collection time treatment, one-fourth of the seed bags from 

each soil level were collected at each of four time intervals: (1) late spring after normal germination

during the first season – April 2007, (2) autumn around the time of seed dispersal and one year after 

placement – November 2007, (3) after germination during the second field season – April 2008, and (4) 

autumn at seed dispersal at the end of 2 years – November 2008. 

Plots were established at least 20 m from the nearest road or obvious disturbance. Eight transects 

in a 35 × 40 m area were used as guides to place and locate seed bags and to sample for seeds in the soil

(seed bank sampling). Every 4 m along each transect, we located the nearest big sagebrush shrub with a 

minimum canopy size of 0.25 m2

. Number of paces and general cardinal direction from the point were 

recorded, along with shrub canopy area (long axis × short axis). In this manner, we located n

After seed bags were retrieved from the field, seeds were examined and placed into categories: 

(1) germinated − having evidence of either cotyledons or a radicle protruding from the seed (achene), 

(2) intact and possibly viable, and (3) not intact and/or obviously decomposing. Viability of intact seeds 

was tested by a tetrazolium (TZ) test using standard Association of Official Seed Analysts techniques 

(2000) and conducted by the Utah Department of Agriculture (State Seed Laboratory, Salt Lake City, 

Utah). We used the number of seeds determined to be viable by the TZ test plus the number of seeds 

recently germinated as the response variable to indicate total number of seeds that retained viability to 

that moment. 

= 96

shrubs (3 soil positions × 4 collection times × 8 replicates) for each plot. Each seed bag was randomly 

assigned a soil position and collection time, and placed beneath a specified shrub 10 cm from the outer 

edge of the foliage and directly beneath an inflorescence. In addition, a set of eight replicates was set 

aside as a pre-treatment control to determine initial viability of the seeds placed in the experiment. 

These seed control replicates were randomly selected during seed bag construction. 

We also sampled for seeds in the seed bank by taking soil cores beneath a big sagebrush shrub 

every 8 m along the same series of transects used for placement of seed bags, yielding a total of 48 

samples per plot. Samples were taken by pounding a length of sharpened PVC pipe (about 5 cm 

diameter) to a depth of 3 cm. We sampled the seed bank twice during the growing season for two

seasons (2006−07 and 2007−08): once immediately after seed dispersal (November) and again 9 months 

later (August), before the next season’s seed dispersal. Location of the first season’s soil samples could 
9 

easily be detected, thereby avoiding the problem of resampling during the second season. Soil cores 

were divided into the litter fraction and soil fraction. Litter fractions were mixed with about 300 g of 

sterile sand. Both fractions then were wetted to field capacity before being stored in a cooler for 3−4 

months at 4 oC for cold stratification. After stratification, samples were placed in a greenhouse in the 

spring (February−April) and monitored for big sagebrush seedling emergence. Each big sagebrush 

seedling was counted and removed. Daytime temperatures ranged from 20.8 to 41.3 oC and night 

temperatures ranged from -1.2 to 11.8 oC. Samples received ambient light and were kept continuously 

wetted until emergence of new seedlings stopped. 

Plant community data were recorded at each site by the line-point intercept method (similar to 

Herrick and others, 2005) with points every 1 m along four transects in May 2008. Individual species 

foliar cover, overall foliar cover, bare ground, and moss/lichen crust cover calculated from these data 

were used as variables for statistical analyses.

Seed Bag Construction

To keep the experiment manageable, we did not collect seeds from study sites, but purchased 

seeds of both subspecies (from Granite Seed Co., Lehi, Utah) certified by the Association of Seed 

Certifying Agencies. The purchased seed was collected in autumn 2005 and was source-identified. The 

mountain big sagebrush seed was collected from Wyoming, and the Wyoming big sagebrush seed was 

collected from Utah. Although this experiment does not allow comparisons of genetic differences 

(genotypic differences) among populations, it does allow for comparisons among sites (environmental 

differences) for a site-identified population because all seeds come from a single population. The 

Wyoming big sagebrush was 17.45 percent pure, and mountain big sagebrush was 26.98 percent pure. 

We used the facilities at the USDA Forage and Seed Research Center (Corvallis, OR) to purify 

seed lots further to 95.16 and 95.37 percent purity for Wyoming and mountain big sagebrush, 

respectively. Seed lots of both species were sieved through 20 × 20 (0.94 mm

2

 opening) and 40 × 40 

(0.43 mm

2

Bag Effect

 opening) mesh screens on a custom built autoshaker (Vibration Systems, Inc., Aurora, OH) 

at 10 Hz and 10 mm amplitude. This separates the seed lot into chaff, seed, and fine dust fractions. The 

chaff fraction was sieved again to separate out any residual seed. The seed fraction was run through an 

indent cylinder with a pocket diameter of 2.25 mm (Westrup Inc., Plano, TX) to divide heavy and light 

fractions of seed. The heavy seed fraction then was passed through a round-hole sieve (1.22 mm 

circumference) that segregates fine vegetative matter from seeds. The seeds were then placed in a seed 

blower (E. L. Erickson Products, Brookings, SD) with air velocity set to about 1.4 m/s for 45 seconds to 

blow away any remaining pieces of chaff. The fraction used for this experiment contained the heaviest 

seeds because they have the greatest germination potential (Busso and others, 2005). We used a seed 

counter to count out sets of 100 seeds for Wyoming (± 0.52) and mountain (± 0.90) big sagebrush to put 

into the seed bags. Seed bags, measuring 5 × 5 cm, were constructed from fine mesh (less than 0.5-mm 

opening), white polyester (chiffon) that allowed 85 percent ambient light penetration. 

We investigated how seed bags would affect seed viability by replicating the field seed retrieval 

experiment in the greenhouse in June 2008. Topsoil and litter from each field site were collected and 

bulked by subspecies. Soil was sieved to remove large debris and rocks and to homogenize across sites. 

An equal amount of sterile sand was mixed with the soil and used as a potting medium. The experiment 

was a factorial design with three levels of the bag treatment (bag, free seeds, control) and three levels of 

the depth treatment (surface, litter, buried). Seed bags were constructed in the same manner as for the 

field experiment. Seeds in seed bags and free seeds were subjected to a 21-day, cold stratification (4 oC) 
10

before being placed in treatment pots. Each 5 × 5 cm pot was randomly assigned to one of nine 

treatment combinations, which were replicated six times. Seed bags and sets of 100 free seeds were 

placed 3 cm below the soil surface, on the soil surface, or atop 2 cm of litter. Controls were filled with 

soil and topped with litter but contained no added seeds. After 1 month we assessed viability of intact 

seeds. Free seeds from control and treatment pots were extracted from the soil medium using a 

modification of the method by Malone (1967). Soil was first passed through sieves (2 mm, 1 mm, 425 

μm opening) to obtain the seed-containing soil fraction. The fraction then was mixed with a highdensity 1 M sugar solution and centrifuged for 10 minutes at 2,600 rpm. Seeds from all treatments were 

sent to the Utah Department of Agriculture (State Seed Laboratory, Salt Lake City, UT) to test for 

viability.

Statistical Analysis

Data were analyzed separately using SAS 9.13 statistical software (SAS Institute Inc., Cary, NC)

and PC-ORD 5.19 (MjM Software Design). A mixed-model analysis of variance (ANOVA) was carried 

out on seed bag data (number of viable seeds) with site as a random effect. Modeling the response as a 

proportion was appropriate in this case because seed bags initially contained 100 seeds and after logtransformation the data approached a normal distribution (Manuela Huso, oral commun., Oregon State 

University, 2008). We averaged replicates of each treatment combination (extraction time and seed 

position) within each site and removed any treatment combinations for which seed viability responses 

yielded zero values (no variance). Autumn 2008 surface treatments were removed from the Wyoming 

big sagebrush dataset, and spring 2008 and autumn 2008 surface treatments were removed for the 

mountain big sagebrush dataset. This technique, along with log-transformation, enabled responses to 

remaining treatment combinations to better meet assumptions of the statistical test (McEvoy and others, 

1993). Analysis of the greenhouse data was carried out with a mixed-model ANOVA after logittransformation. Bonferroni adjustments for multiple comparisons were applied where appropriate. 

Seed bank data (number of germinable seeds) were modeled using a generalized linear model 

with a Poisson distribution. Overdispersion was included in the model with site as a fixed effect. As 

with the seed bag data, we averaged replicates of seed bank samples for each treatment combination 

within each site (sampling month by site) and removed those that had all zero values before analysis.

Nonparametric multidimensional scaling (NMS) with the Sørensen distance measure was used to 

analyze plant community data at the site level. NMS is an iterative ordination technique that seeks a 

stable solution by minimizing stress or departure from monotonicity in the reduced ordination space 

(Kruskal, 1964; Mather 1976). NMS avoids assumptions of linearity and normality that are prevalent in 

community ecological data (McCune and Grace, 2002). The ‘slow and thorough’ setting of the autopilot 

mode in PC-ORD version 5 was used with random starting configurations and 250 runs of real data. 

Monte Carlo tests were used to test for significance of the k-dimensional solution against 250 runs of 

randomized data. 
11

Individual species’ foliar cover was used to ordinate study sites. Environmental variables and 

seed viability responses (number of viable seeds retrieved in bags and sampled in seed bank) associated 

with each site were added to the resulting ordination to create a joint plot. Environmental variables 

included total precipitation for study duration, elevation, heat load, potential incident radiation, soil 

texture, overall foliar cover, percent of bare ground, and moss/lichen crust cover. Heat load and 

percentage of incident radiation were estimated using latitude, slope, and aspect of each site in 

HyperNiche (version, 2.60 Beta), after the method described in McCune (2007). Overall foliar cover, 

moss/lichen crust cover, and percentage of cover of bare ground were calculated from line-point 

intercept data by dividing the number of intercepts for each cover response and dividing by the number 

of total intercepts (180) for each plot (Herrick and others, 2005). Seed viability responses were averaged 

over each treatment combination. This method describes the variation in plant community among sites

(habitats) and allows the evaluation of the seed viability responses within that variation. 

Results

Seed Retrieval Experiment

The median proportion of seeds remaining viable was significantly affected by length of 

exposure in the field for Wyoming (p < 0.01, F3,50 = 39.41) and mountain (p < 0.01, F3,45 = 23.16) big 

sagebrush. In autumn 2007, after one growing season, we determined that some proportion of Wyoming 

and mountain big sagebrush seeds were still viable across all depth treatments (fig. 2). Both subspecies 

experienced a reduction in median seed viability over time, but magnitude of the reductions depended

on the location of seeds in soil (Wyoming big sagebrush p < 0.01, F5,50 = 6.14; mountain big sagebrush 

p < 0.01, F4,45 = 5.29). After 12 months, there was a significant difference in median seed viability 

between buried seeds and seeds beneath litter or on the surface for Wyoming big sagebrush (p < 0.01 

both differences) and mountain big sagebrush (p

Initial viability of seeds used in the seed retrieval experiment was 81 and 92 percent for 

mountain and Wyoming big sagebrush, respectively. At 12 months, buried Wyoming big sagebrush 

seeds were 27−57 percent viable, seeds under litter were 14−29 percent viable, and surface seeds were 

9−19 percent viable. After remaining in the field for 24 months, buried seeds and seeds beneath litter 

retained their viability at 28−58 and 11−23 percent, respectively (estimates are 95-percent confidence 

intervals). However, no seeds remained viable on the surface at the end of 2 years in the field. After one 

year, buried seeds of mountain big sagebrush were 22−49 percent viable, seeds beneath litter were 

12−26 percent viable, and surface seeds were 11−23 percent viable. Buried mountain big sagebrush 

seeds retained 31−68 percent viability, whereas seeds beneath the litter retained 10−22 percent of their 

viability, and no surface seeds retained viability after 24 months. Both subspecies of big sagebrush had 

some portion of seed that stayed viable for more than one growing season provided they were buried or 

under litter. 

 < 0.01 litter difference; surface value is zero). 

 Proportions of seeds remaining viable in different seed location treatments are similar between 

subspecies during the first year of the study. Mountain big sagebrush seeds in the surface treatment did 

not remain viable beyond 12 months, but one Wyoming big sagebrush site had seeds that survived for 

18 months at the surface. The proportion of seeds remaining viable at the study’s conclusion was lower 

than when the study began. Wyoming big sagebrush seeds at the beginning of the study were about 13 

times more likely to be viable than seeds in the field after 6 months and 40 times more likely to be 

viable than seeds after 12 months. After one growing season, the location of the seed above or below the 

soil or litter affected the odds of the seed remaining viable. Buried seeds of Wyoming big sagebrush 
12

were 2.5 times more likely to be viable than seeds beneath litter after one growing season, and were 3.5 

times more likely after two growing seasons. Mountain big sagebrush also experienced a loss of viable 

seeds over time. At the beginning of the experiment, mountain big sagebrush seeds were 4.5 times more 

likely to be viable than seeds in the field after 6 months and 16 times more likely to be viable after 12 

months. Buried seeds were twice as likely to be viable than seeds beneath litter after one growing season 

and 5 times more likely to be viable after 12 months. 

Bag Effect

Seeds in bags on the surface had higher viability than seeds without bags for both subspecies 

(Wyoming p < 0.01, t = 5.87 and mountain p = 0.04, t = 2.13; fig. 3). Wyoming and mountain big 

sagebrush seeds beneath litter in bags were more viable than free seeds (p = 0.02, t = 2.39 and p = 0.02, 

t = 2.55, respectively). However, there was no effect of bags on seed viability of buried seeds for either 

Wyoming or mountain big sagebrush (p = 0.17, t = -1.39 and p = 0.89, t

Seed Bank Sampling

= -0.14, respectively). There 

were no viable seeds found in mountain big sagebrush control treatments, while a few were found in the 

Wyoming big sagebrush controls. 

The median number of viable seeds in the seed bank for Wyoming big sagebrush was highly 

variable among sites and within sites (fig. 4) for the first year of sampling. Sampling time had a 

marginal effect on the median number of seeds in the litter (p = 0.07, Χ1,470 = 3.36) and no effect on the 

median number of seeds in the soil (p = 0.21, Χ1,376 = 1.55). For mountain big sagebrush, the median 

number of viable seeds in the seed bank was not as variable, with only one or two sites increasing the 

overall median (fig. 5). Sampling time had no effect on the median number of seeds in litter (p = 0.11, 

Χ1,470 = 2.59) or in soil (p = 0.31, Χ1,470 = 1.03). After 9 months in the field, estimates for Wyoming big 

sagebrush ranged from 19 to 49 viable seeds/m2 in litter samples and 19−57 viable seeds/m2 in soil 

samples (95-percent confidence interval). For mountain big sagebrush, estimates were 27−75 viable 

seeds/m2 in litter samples and 54−139 viable seeds/m2

The second year of sampling the seed bank yielded no germinable seeds in litter or soil samples 

immediately following seed dispersal for Wyoming big sagebrush. Litter samples from one mountain 

big sagebrush site had 3-36 viable seeds/m

 in soil (95-percent confidence interval). 

2 (95-percent confidence interval) following seed dispersal. 

After 9 months, only a couple Wyoming big sagebrush sites had viable seed, ranging from 5 to 24 

viable seeds/m2 in litter and soil samples (95-percent confidence interval). The two mountain big 

sagebrush sites that had viable seeds after 9 months varied considerably. One site had 3-36 viable 

seeds/m2

, whereas the other site had 31–91 viable seeds/m2 (95-percent confidence interval). 
13

Plant Community

The NMS ordination of plots in species space (fig. 6) showed incomplete grouping of plots by 

sagebrush subspecies (3-dimensional solution: final stress = 5.04, final instability = 0.00, cumulative R2

= 0.92). Environmental variables and seed viability responses were overlaid on the ordination to 

examine trends (table 2). Axis 1 was most highly correlated with elevation (r = 0.74, Fig 6a), heat load 

(r = 0.45, Fig. 6b), and the percentage of bare ground (r = 0.50, Fig. 6b) in the direction of sites located 

geographically farthest east and south. The other end of Axis 1 was strongly correlated with overall 

canopy cover (r = -0.72, Fig 6a), moss and lichen crust cover (r = -0.44, Fig. 6a), and associated 

primarily with low-elevation Wyoming big sagebrush sites. The environmental variable that had the 

highest correlation with Axis 2 was elevation (r = 0.66, fig. 6b). Axis 3 (fig. 6a) was equally correlated 

with elevation (r = 0.33) and overall canopy (r 

Seed viability vectors indicate treatments (seed position and collection time) with the highest 

number of viable seeds and strongest correlation with plots (fig. 6). Viable seeds in surface and litter 

(not shown) treatments sampled in April 2007 were more abundant in sites with high canopy cover and 

plots higher in elevation, respectively. In November 2007, the preponderance of viable seeds for both 

surface and litter (not shown) treatments occurred at plots higher in elevation. By April 2008, seeds 

beneath litter are still abundant at plots higher in elevation; however, there were more viable surface 

seeds in plots that were high in moss and lichen crust cover. By November 2008, no seeds on the 

surface remain and the only seeds viable beneath litter are primarily at one Wyoming big sagebrush 

plot. However, seed viability of buried seeds is similar across plots of both subspecies over the duration 

of the study. 

= 0.35). 

The ordination also was overlaid with the second environmental matrix containing seed bank 

responses (fig. 7; table 2). Axis 1 (fig. 7a) strongly correlated with elevation (r = 0.48) and precipitation 

(r = 0.30). Axis 2 was highly correlated with canopy cover (r = 0.54, fig. 7a) and moss and lichen cover 

(r = 0.43, fig. 7a), and negatively associated with elevation (r

Discussion

= -0.64, fig. 7a). Overall seed bank counts 

were highest in two mountain big sagebrush plots so all the seed bank vectors were correlated with Axis 

2 (fig. 7b).

Seed Retrieval Experiment

The results of our study indicate that, after 2 seasons, the median proportion of buried viable, 

non-germinated seeds ranged from 40 to 60 percent for both subspecies. Our findings are similar to 

those of Bouwmeester and Karssen (1992) who demonstrated that buried seeds of Polygonum persicaria

are secondarily induced into dormancy by lack of light. Seeds also have the potential to persist in soil 

seed banks in a non-dormant state for many years (Ooi and others, 2007); therefore, buried big 

sagebrush seeds of these two subspecies may only be inhibited from germinating as opposed to being 

dormant. The mechanism for prolonged persistence of big sagebrush seeds in the seed bank may either

be a function of induced secondary dormancy or lack of proper germination cues. 
14

Big sagebrush seeds exhibit characteristics of physiological dormancy that can be overcome by 

cold stratification. Meyer and Monsen (1991, 1992) established that a 20-week, cold stratification at 1°C 

was effective in removing dormancy in mountain big sagebrush, and cold stratification at 15°C was 

enough to break dormancy in most Wyoming big sagebrush seeds. Buried seeds of Wyoming and 

mountain big sagebrush in our study experienced two seasons of winter cold, where soil temperatures 3 

cm below the surface can range from 0 to 10 °C in autumn (Evans and Young, 1972). Though 

conditions were favorable for breaking dormancy, neither subspecies showed a decrease in the quantity 

of buried seeds that remained viable, indicating that seeds were potentially inhibited from germinating. 

Dark germination does occur in both subspecies but varies greatly by collection site (Meyer and others, 

1990), signifying that a portion of seeds require light to cue full germination. If suitable conditions are 

not present for germination when physiological dormancy is broken, and germination does not occur, 

secondary dormancy may be induced (Crocker, 1916; Vleeshouwers and others, 1995; Baskin and 

Baskin, 2001). In April and November 2007, the highest abundance of surface seeds occurred on sites 

with high overall canopy cover and with high elevation. These were principally mountain big sagebrush 

sites that likely would have reduced light due to higher cover than Wyoming big sagebrush sites, 

potentially inhibiting germination and increasing the portion of viable, intact seeds in the seed bank. 

Crist and Friese (1993) determined that decomposition and attack by fungi were responsible for 

the greatest decrease in seed viability among shrub-steppe species during winter, and that seed viability 

of big sagebrush decreased most rapidly. Mountain big sagebrush seeds had much lower seed viability 

overall than Wyoming big sagebrush. We observed that mountain big sagebrush seeds experienced a 

higher proportion of moldy seeds than Wyoming big sagebrush after cold-stratification. By April 2008, 

the greatest quantities of viable seeds on the surface were primarily at Wyoming big sagebrush sites. 

This indicates that mountain big sagebrush seeds on the surface experienced a greater decrease in 

viability than seeds of Wyoming big sagebrush. We also determined that more than 75 percent of 

surface and litter samples had evidence of fungal infection whereas less than 50 percent of buried seeds 

showed signs of infection. Mountain big sagebrush seeds on the surface and beneath litter in wetter, 

colder sites may have succumbed to fungal infection and lost viability after one growing season. 

Wyoming big sagebrush surface seeds in drier sites may have experienced less or no fungal infection, 

and therefore maintained a lower, but still viable, portion of seeds after 24 months in the field. 

Seed bags used in our study reduced ambient light by about 15 percent and potentially inhibited 

seed germination, and thus increased the proportion of viable seed. Seeds in bags for the surface and 

litter treatments tended to have higher estimates of viability than seeds without bags, which indicate that 

our estimates of seed viability for these positions in the seed bank are high for both subspecies. 

However, buried seeds were unaffected by this design artifact and may represent the potential maximum 

viability of seeds buried beneath the soil surface. There is the possibility that the mesh bags maintained 

a higher level of moisture and potentially increased the risk of fungal infection. If seed bags increase 

moisture levels and rate of fungal infection compared to conditions outside seed bags, then estimates of 

seed viability in the seed retrieval experiment likely are conservative.
15

Seed Bank Sampling

Estimates of germinable seeds in the seed bank for the 2006−07 growing season were variable 

among and within Wyoming and mountain big sagebrush sites. Wyoming big sagebrush estimates were 

an order of magnitude lower than viable seed estimates by Young and Evans (1989) and an order of 

magnitude higher than estimates by Hassan and West (1986). Each sample was about 48 cm3

Implications

 in volume 

and the total surface area sampled per plot amounted to less than one-tenth of a square meter. Despite 

taking samples directly beneath the inflorescence of a sagebrush shrub, less than 25 percent of the 

samples at all sites yielded germinable seeds. Young and Evans (1989) determined that the dispersal 

pattern of big sagebrush seeds is skewed to the east due to prevailing winds from the west. We did not 

take dispersal direction into account when taking seed bank samples. Future studies of the seed bank in 

this species may warrant sampling at a finer scale than was used in this study and using a more stratified 

random approach. The low number of viable seeds found in seed bank samples from the second season 

may be a result of lower precipitation. On average, precipitation during the 2005-06 growing season was 

65 percent of that received during the 2006-07 growing season (climate data from PRISM Climate 

Group, Oregon State University). 

Ours is the first study to document that seed longevity of big sagebrush depends on vertical 

position of seeds in the soil, and clarifies observations of seedling emergence years after a dispersal or 

seeding event (Schuman and others, 1998; Booth, 2002; Schuman and Belden, 2002; Ziegenhagen, 

2003). Results of our study indicate that seeds have higher odds of persisting in the seed bank if they are 

buried. Naturally occurring soil disturbances such as frost-heaving or rodent burrowing would enhance 

seed bank persistence by incorporating a portion of seeds beneath the soil surface. Burial of seeds may 

ameliorate effects of the environment (i.e., fluctuating temperature and moisture levels) and increase 

seed longevity (Facelli and Pickett, 1991; Rotundo and Aguiar, 2005). The same processes that promote 

burial of big sagebrush seeds may operate to bring seeds to the surface when conditions are favorable 

for successful germination (Harper, 1977). It is important to note however that although seeds buried in 

litter remain viable in intact communities they are susceptible to incineration during fires. 

Restoration of sagebrush steppe is often conducted by aerial seeding, which would leave seeds 

on the surface. Aerial seeding of big sagebrush can result in poor establishment of shrubs (Dalzell, 

2004). However, seedling emergence of big sagebrush through four post-seeding growing seasons when 

seeds were broadcast onto the surface of a mulch layer (Booth, 2002) indicates that some portion of the 

seeds may have worked into the soil over time. Providing a soil surface disturbance or mulch that aids in 

seed burial may increase restoration success. Restoration methods that ensure seed contact with soil

such as pressing seeds into the soil surface (Pyke, 1994), creating microdepressions in the soil 

(Haferkamp and others, 1987), and harrowing or raking can greatly increase seedling establishment

(Shaw and others, 2005). An unintended consequence of some of these methods may be burial of a 

small percentage of seed, which had long been thought to be undesirable. Indeed, keeping seeds near the 

surface is critical for germination but there may be a middle ground between surface exposure and deep 

burial that maximizes restoration potential. We argue that techniques to help big sagebrush seeds to 

remain viable in the soil as a hedge against unfavorable germination conditions may be one of the keys 

to successful restoration. A one-time seeding effort with high densities of seed may be sufficient to 

provide a seed source for years and in the long-term may be more economically feasible.
16

Conclusions

The results of our study indicate that dynamics of seed viability over time are similar in 

Wyoming and mountain big sagebrush, and depend on the vertical location of the seed in the seed bank. 

After the initial loss of seeds, most likely due to germination of the physiologically non-dormant 

fraction, both subspecies exhibit patterns of a steadily decreasing abundance of viable seeds on the 

surface and beneath litter. However, 40−60 percent of buried seeds remain viable across all sites for at 

least 24 months. Our study suggests reseeding techniques that promote ‘caching’ of some seeds in the 

soil seed bank may increase restoration potential. 

Acknowledgments

We thank the USGS Forest and Rangeland Ecosystem Science Center, Lava Lake Foundation, 

and the Botany and Plant Pathology Department of Oregon State University for funding this study. We 

thank Tess O’Sullivan at Lava Lake for her guidance in finding study sites and acting as liaison between 

the USGS and the Foundation. We also thank those who have assisted with field or lab work, and 

provided consultation: Stanley Akagi, Doug Bilsland, Jessi Brunson, Nicole DeCrappeo, Elizabeth 

DeLorenze, Meagan Gates, Kevin Knutson, Andrew Lindgren, Adam Nilsson, Emily Orling, Scott 

Shaff, and Troy Wirth. 

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University, Corvallis, OR. 
19

# 

Figure 1. Location of study sites. Within each study site, we located 

one Wyoming and one mountain big sagebrush plot.

UT

OR

ID

NV
20

Figure 2. Median proportion of seeds remaining viable decreased over a period 

of 24 months for Wyoming and mountain big sagebrush, primarily in seeds beneath 

litter and on the surface. Seed were buried 3 cm below the surface (buried), placed

on the soil surface beneath 2 cm of litter (litter), and placed on the surface (surface). 

Bars indicate 95-percent confidence intervals and data points are jittered on the 

x-axis for display.
21

Figure 3. Viability of seeds in bags for Wyoming and mountain big sagebrush was 

higher than viability of free seeds, but only in the surface and litter treatments. 

Buried seeds were not affected by seed bags. Bars indicate 95-percent confidence 

intervals.
22

Figure 4. Median number of viable seeds/m2 in litter and soil samples of 

the Wyoming big sagebrush seed bank. There was no significant effect of 

sampling time on median seed viability in either litter or soil samples across 

all sites. Sites are located west-east (JUWY, MHWY, LLWY) and north-south 

(SAWY, TOWY, HUWY), see table 1 for location abbreviation definitions. 

Bars indicate 95-percent confidence intervals and data points are jittered 

on the x-axis for display.
23

Figure 5. Median number of viable seeds/m2 in litter and soil samples of the 

mountain big sagebrush seed bank. There was no significant effect of sampling time 

on median seed viability in either litter or soil samples across all sites. Sites are located 

west-east (JUVA, MHVA, LLVA) and north-south (SAVA, TOVA, HUVA), see table 1 

for location abbreviation definitions. Bars indicate 95-percent confidence intervals 

and data points are jittered on the x-axis for display. 
24

JUWY

JUVA

MHWY

MHVA

LLWY

LLVA

SAWY

SAVA

TOWY

TOVA

HUWY

HUVA

Apr07sur

Apr07bur

Nov07sur Apr08sur Apr08lit

Apr08bur

Nov08bur

ELEV

MOSS/LC

CANOPY

Axis 1

Axis 3

Subspecies

Wyoming

Mountain

JUWY

JUVA

MHWY

MHVA

LLWY

LLVA

SAWY

SAVA

TOWY

TOVA

HUWY

HUVA

Apr07sur

Apr07bur

Nov07sur Apr08lit

Nov08lit

ELEV

HEAT

CANOPY BARE

Axis 1

Axis 2

Subspecies

Wyoming

Mountain

Figure 6. Nonmetric multidimensional scaling (NMS) ordination of 12 Wyoming and mountain big sagebrush plots

analyzed for plant species composition for a) Axis 1 and Axis 3, and b) Axis 1 and Axis 2. Vectors represent the 

direction and magnitude of correlation between sites and five environmental gradients, as well as eight seed 

viability gradients. Ordination is a 3-dimensional solution.

a) b)
25

JUWY

JUVA

MHWY

MHVA

LLWY

LLVA

SAWY

SAVA TOWY

TOVA

HUWY

HUVA

litterNov06

litterAug07

ELEV

MOSS/LC

CANOPY

Axis 1

Axis 2

Subspecies

Wyoming

Mountain

JUWY

JUVA

MHWY

MHVA

LLWY

LLVA

SAWY

SAVA

TOWY

TOVA

HUWY

HUVA

litterNov06

soilNov06

litterAug07

soilAug07

ELEV

BARE

CANOPY

Axis 2

Axis 3

Figure 7. NMS ordination of twelve Wyoming and mountain big sagebrush plots analyzed for plant species 

composition for a) Axis 1 and Axis 2, and b) Axis 2 and Axis 3. Vectors represent the direction and magnitude of 

correlation between sample units and four environmental gradients, as well as four seed bank responses. 

Ordination is a 3-dimensional solution.

Table 1. Study site information.

Site Name Plot Code Subspecies State

of 

Artemisia tridentata

Slope Aspect in 

percent

Elevation in 

degrees

in 

meters above NAVD

Juntura JUWY wyomingensis Oregon 11.9 83 1072

Juntura JUVA vaseyana Oregon 9.7 150 1277

Mountain Home MHWY wyomingensis Idaho 4.6 175 1096

Mountain Home MHVA vaseyana Idaho 2.9 108 1317

Lava Lake LLWY wyomingensis Idaho 2.3 62 1451

Lava Lake LLVA vaseyana Idaho 2.9 77 1806

Sawtooths SAWY wyomingensis Idaho 2.9 269 1651

Sawtooths SAVA vaseyana Idaho 4.6 51 1919

Tooele TOWY wyomingensis Utah 3.4 102 1903

Tooele TOVA vaseyana Utah 11.9 352 1916

Humboldt HUWY wyomingensis Nevada 5.7 351 2034

Humboldt HUVA vaseyana Nevada 6.3 231 2274

a) b)
26

Table 2. Description of environmental and seed viability vectors in nonmetric multidimensional 

scaling (NMS) graphs. 

Vector Label Description Units

Seed Bag Retrieval

Apr07sur April 2007 surface samples Mean number of seeds viable/seed bag

Apr07bur April 2007 buried samples Mean number of seeds viable/seed bag

Nov07sur November 2007 surface samples Mean number of seeds viable/seed bag

Apr08sur April 2008 surface samples Mean number of seeds viable/seed bag

Apr08lit April 2008 litter samples Mean number of seeds viable/seed bag

Apr08bur April 2008 buried samples Mean number of seeds viable/seed bag

Nov08lit November 2008 litter samples Mean number of seeds viable/seed bag

Nov08bur November 2008 buried samples Mean number of seeds viable/seed bag

Seed Bank

litterNov06 November 2006 litter samples Mean number of seeds viable/m2

soilNov06 November 2006 soil samples Mean number of seeds viable/m2

litterAug07 August 2007 litter samples Mean number of seeds viable/m2

soilAug07 August 2007 soil samples Mean number of seeds viable/m2

Environmental Variables

elev elevation meters

canopy foliar canopy cover percent

moss/lc moss and lichen crust cover percent

bare bare ground cover percent

heat heat load index - 1

1

McCune and Keon (2002).
Wijayratne and others—Investigating Seed Longevity of Big Sagebrush (Artemisia tridentata)—Open-File Report 2009–1146