Upekala Wijayratne, Ph.D. Candidate, Oregon State University
SageSTEP Onaqui study site
5 other sites in Oregon, Idaho, Utah and Nevada with soil and vegetation composition similar to SageSTEP sites (Artemisia tridentata ssp. wyomingensis or ssp. vaseyana on loamy soils)
Study Design and Objectives:
The purpose of this study was to evaluate the longevity and persistence of sagebrush (Artemisia tridentata) seeds in relation to seed burial depth and environmental conditions. Data were collected and analyzed to answer the following questions:
- How long can sagebrush seeds remain viable in soil?
- Does seed longevity depend on a seed’s depth in the seed bank?
- Does seed longevity change with environmental conditions?
To capture variability across the Great Basin, data were collected from six study sites in Oregon, Idaho, Utah and Nevada for two subspecies: Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) and mountain big sagebrush (Artemisia tridentata ssp. vaseyana).
Seed burial experiment:
A seed burial experiment evaluated seed depth (3cm below soil surface, at the soil surface beneath 2 cm of sagebrush leaf litter, and above the soil and litter) and collection time (late spring after normal germination, autumn around time of seed dispersal and one year after placement, after germination during second field season, and autumn at seed dispersal two years after placement). Because sagebrush seeds are so small, researchers used seed bags to insure that seeds could be retrieved.
Seed bank sampling:
Soil cores of 3 cm depth and 5 cm diameter were used to sample the natural seed bank. Forty-eight soil cores were taken under big sagebrush within each of the 12 study plots. Soil cores were placed in a greenhouse for germination during the months of February to April following collection. Greenhouse germination was used to determine whether seeds persisted from one season to the next.
Bag effect experiment:
To account for potential effects of seed bags on seed viability or ability to germinate, researchers reproduced field burial treatments with and without seed bags in an experiment conducted in a greenhouse.
Bag effect experiment:
Seed bags used in this study likely affected the microenvironment of seeds, and researchers found a higher proportion of viable seeds in bags compared to free in soil, on the surface or under litter. Bags reduced ambient light and may have inhibited seed germination. However, bags did not have an effect on the viability of buried seeds. There is also the possibility that bags maintained a higher level of moisture and potentially increased the risk of fungal infection through unnaturally high seed densities, which would result in a conservative estimate of seed persistence. Alternatively, seed bags could inhibit macroinvertebrates in the soil and litter from preying on or damaging seeds, resulting in overestimation of seed persistence. Soil was not added to seed bags before burial, but some buried bags contained soil that was of uniform particle size upon retrieval. This may have provided a similar microenvironment and thus veiled potential heterogeneity in seed persistence within and across regions.
Seed burial experiement and seed bank sampling:
This is the first study to experimentally document that sagebrush can form a transient seed bank and that seed longevity is greatly enhanced by burial of seeds in the soil. Seed bank persistence is affected by many environmental factors, and burial of seeds can ameliorate environmental effects and increase longevity of seeds that might otherwise be lost to death or germination. In this study, researchers found that the proportion of viable buried seeds was fairly constant for both sagebrush subspecies throughout the 2-year study period. Seeds on the surface and under litter decreased in viability over time, but how quickly this happened varied by site. After 24 months, seeds buried at least 3 cm below the soil surface retained 30-40% viability whereas viability of seeds on the surface and under litter declined to 0 and < 11%, respectively.
- The density of naturally dispersed seeds in the seed bank varied within sites and over time, and seed attrition varied significantly by region. Many factors may impact sagebrush seed production including browsing, disease and drought. This may explain why there were lower densities of seeds in the seed bank during the second year as compared to the first year.
- Data from this study showed that more than 75% of all surface and litter samples had signs of fungal infection whereas less than 50% of buried seeds were infected.
- Seeds collected from cold winter sites are nearly 100% light-requiring for germination, perhaps as an adaption to prevent seeds from germinating under snow. Seeds of both subspecies were collected from areas with cold winters; therefore most of these seeds likely needed a light cue for germination.
- Researchers determined density of viable seeds in the seed bank by germinating sagebrush seeds from soil and litter cores instead of extracting seeds physically because the latter method was very labor intensive given our large number of samples. However, the germination method may underestimate seed persistence because dormant seeds or seeds that do not receive a germination cue will remain undetected in samples
Sagebrush steppe restoration often involves aerial seeding, which leaves many seeds on the surface and can lead to poor shrub establishment. Results of this study indicate that providing a soil surface disturbance or mulch that promotes burial of some sagebrush seeds may increase restoration success by providing a seed reservoir as a hedge against establishment failure in the initial year of seeding. This may also prevent some seeds from being incinerated during wildfires.
Wijayratne, U. In review. Seed and seedling ecology of Artemisia tridentata in a restoration context. Ph.D. Dissertation, Oregon State University, Corvallis, Oregon.
Wijayratne, U. and D. Pyke. In review. Burial increases seed longevity of two Artemisia tridentata (Asteraceae) subspecies. American Journal of Botany.