Bitterbrush Restoration Update

Bitterbrush Restoration Update

May 7, 2013

Antelope bitterbrush (Purshia tridentata) dominates many plant communities of the Western Great Basin. The plant is a critical food source for mule deer and elk. This research investigates antelope bitterbrush establishment and survival with the goal of expanding the range and health of these valuable shrub stands.

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Antelope bitterbrush (Purshia tridentata) dominates many plant communities of the Western Great Basin.

Research Questions 1) How does removal of plant competition effect bitterbrush establishment? 2) How do soil nutrients and temperatures effect bitterbrush establishment? 3) Will fertilizer treatments enhance native shrub quality and establishment without benefits to herbaceous annuals?

Competition Removal Study Competition with annual, non-native plants for soil nutrients is a primary cause of bitterbrush seedling mortality and stand degeneration (Hubbard 1957, Ferguson 1972). Holmgren (1956) demonstrated dramatic improvements in bitterbrush survival and vigor when competing cheatgrass was removed for at least the first growing season. Subsequent weed invasions slowed shrub growth but did not result in widespread shrub mortality. This study investigates bitterbrush establishment at the MPG Ranch with the following questions: 1) How does removal of plant competition effect bitterbrush establishment and vigor? 2) How do soil quality and microclimate effect bitterbrush establishment and vigor?

Study Sites Three sites were established in bitterbrush stands of comparable aspect at elevations of 3500, 4000, and 4600 feet. The lower site is dominated by sagebrush, bitterbrush, Sandberg bluegrass and bluebunch wheatgrass. The middle and upper sites consist mainly of bitterbrush and balsamroot invaded by knapweed, cheatgrass, and sulphur cinquefoil. Site elevation and surrounding vegetation could influence soil quality, microclimate, and bitterbrush establishment.

Experimental Design For this study, we removed plant competition in three exclosures (10ft×20ft) at each site according to a randomized complete block design. Treatments were assigned at random within blocks of adjacent plots. This design controls for variation due to spatial effects and allows true treatment differences to be observed. Treatments included: removing all vegetation except existing shrubs (“Shrubs”), removing all vegetation (“cleared”), and removing no vegetation (“Uncleared”). Two 6-month old bitterbrush shrubs and 20 bitterbrush seeds were planted into each plot. We monitored seedling branch number, branch growth and seed germination in each plot.

Gravity-fed drip irrigation systems helped transplanted bitterbrush seedlings survive hot, dry summer months in 2011 and 2012. We pumped water from spring-fed stock tanks into holding barrels and set automatic timers to water each plant every other day through standard drip irrigation tips. Each barrel delivered 0.2 gallons (0.77 liters) of water to 90 shrubs three times a week. We will not irrigate in 2013. Hopefully, the young shrubs will survive the summer on their own.

We monitored soil nutrients, moisture, and temperature within each site and treatment to determine potential effects on shrub germination, survival, and vigor. These photos show soil temperature data loggers and the pipe cleaners used to mark their locations. Average soil temperatures during the hot summer months differed significantly among the treatments. Clearing allowed more solar heat to reach the soil, and these plots remained more than 3 degrees warmer than uncleared plots during these months. This additional heat may have aided seed germination. There was no difference in soil moisture among the treatments.

Shrub Vigor Across all sites, seedling branch numbers and branch lengths were greatest in the “cleared” plots. The “shrub” plots also had greater values than the “uncleared” treatments. Competition from surrounding plants clearly stunts seedling growth. Reduction of competition during the shrub’s first year can improve seedling vigor.

Seed Germination Bitterbrush seeds germinated in greater percentages in the “shrubs” and “cleared” treatments than in the “uncleared”. The dense cover of grasses and weeds limit moisture levels, soil contact, and sun exposure for seeds trying to germinate.

Site Differences Despite different elevations and dominant plant communities (inv = invaded; nat = native), there were no differences in plant vigor among the three study sites. There was a slight increase in seed germination at the middle elevation site.

Site Differences Despite different elevations and dominant plant communities (inv = invaded; nat = native), there were no differences in plant vigor among the three study sites. There was a slight increase in seed germination at the middle elevation site.

Study Conclusions 1) Branch number, branch length, and germination % were greater in the cleared or partially cleared treatments. 2) Nitrate levels were greater in the cleared or partially cleared treatments. 3) Despite higher N and P at the upper and middle sites, there were no siteassociated differences in plant growth. These results reinforce that competition from surrounding plants may stunt or prevent bitterbrush establishment and that reducing year-one competition can improve seed germination and seedling vigor. Future Monitoring We will continue to monitor shrub survival and growth as other plants establish in the cleared plots. Hopefully, the young bitterbrush will compete well in their third summer.

Study Site We established this study on the hill above Tongue Creek. The area contains a wide age range of bitterbrush shrubs as well as several invasive annual plants. We selected 200 bitterbrush plants within the area shown above to receive various treatments.

Experimental Treatments This full factorial experiment consisted of the following treatments applied in 2012: 1) Control – no amendments 2) Carbon – Sucrose amendment can immobilize nitrogen in bacterial biomass. 3) Nitrapyrin – The inhibition of soil nitrification can decrease available nitrate for weeds 4) N, P, & K – These nutrients should feed all plants in a plot. 5) P & K – Only potassium and phosphate amendment may favor N-fixing plants. 6) Legume seeds – These N-fixers may do better in certain treatment groups. Response Variables We will monitor the following response variables in 2013: 1) Bitterbrush foliar nuts 2) Bitterbrush leader length 3) Soil nutrients 4) Forage utilization 5) Plant community response- 6) Seeded species response 7) Native species response

Literature cited

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