From invasion to rejuvenation: Can we use Western Juniper to restore the rangeland?

From Invasion to Restoration: Can Western Juniper be Used to Improve Rangelands?

by: Kylie Meyer and Kristin Trippe

Summary:

The lack of fire, combined with more frequent droughts, has caused Western juniper (Juniperus occidentalis) woodlands to expanded rapidly in the Intermountain West. Over the last 150 years, millions of acres of grasslands, shrub-steppes, and savannas have been altered. This environmental modification negatively impacts rural economies that rely on agriculture and ecosystem services. One common method of juniper control is to cut, pile, and burn the shrubs. As an alternate management tool, USDA researchers from Corvallis, Oregon explored whether juniper could instead be converted into biochar and applied to the landscape to facilitate rangeland restoration efforts. They conducted greenhouse, lab, and field studies to evaluate whether biochar could aid in establishing native bunchgrass seedlings by improving soil moisture and promoting germination and growth. Results from the greenhouse and lab were used to inform and clarify the 2-year field study, which was conducted in Wheeler County, Oregon from 2018-19. While the promising greenhouse results didn’t translate favorably to the field study outcomes, the projects added valuable information to the growing body of knowledge about the use of biochar in ecological restoration.

The Six Shooter site after cutting and burning of invasive juniper.

The Conundrum: An ‘invasive’ ‘native’ shrub has changed the landscape

Western juniper is native to the arid Intermountain West. Its shrubs are highly adaptable, long-lived, fire-resistant, have deep taproots, and consume large quantities of water. These characteristics enable them to devour soil and surface water, outcompete native plants, encourage the establishment of invasive grasses and forbs, and reduce forage quality and availability. Juniper range has more than doubled in the last century and a half due largely to livestock grazing, reduction in fine fuels, reduced fire frequency due to fire suppression (including burning by Indigenous Peoples), and a warming and drying climate.

Historically, juniper has been used for fence posts and rails, corrals, charcoal, and firewood owing to its durability and rot resistance. Currently, ecological and economic concerns are prompting government and community groups to incentivize juniper removal by creating markets for juniper-based commodities including lumber products, essential oils, and furniture. However, due to juniper’s remote and patchy stand growth, short tree stems, and branching, commercial harvesting and processing are costly and inefficient.

Strategies that remove large stands of juniper have inherent drawbacks. Mechanical control, for example, requires breaking strong support roots and uprooting trees to be effective. Another mitigation method involves cutting, piling, and burning trees; however, this technique releases carbon into the atmosphere and leaves burn scars that are readily colonized by non-native plants. During subsequent site rehabilitation, grass seed broadcasting is generally unsuccessful due to unfavorable germination conditions and competitive grasses. Juniper removal alone has proven insufficient to stimulate the regeneration of other native species.

Rural economies reliant on agriculture and ecosystem services have been negatively impacted by juniper encroachment. Consequently, new strategies for using low-value juniper biomass are needed to further encourage juniper removal.

The Plan:

Dr. Kristin Trippe, Dr. Claire Phillips, and Ms. Kylie Meyer, researchers with USDA Agricultural Research Service in Corvallis, Oregon, explored the scenario of converting juniper biomass to biochar. They outlined a pilot greenhouse trial and two-year field study to evaluate these questions:

Could juniper biochar be produced and applied to the landscape to enhance restoration efforts?
Would biochar provide benefits for establishing native bunchgrass seedlings?
If yes, what biochar application method would encourage maximum germination rate, greatest seedling survival, and improved soil moisture?

Biochar Production

Feedstock

Western juniper feedstock (provided by In The Sticks mill, Fossil, OR) was pyrolyzed in small batches to produce biochar for both the greenhouse and field studies. Feedstock for the greenhouse trial was composed of air-dried mill-ends (Figure 1, top left). Feedstock for the field study was composed of mill-ends and juniper log edge pieces; however much of that material was either rain/snow-wetted or still green (average moisture 17%, n=3).

Figure 1. Clockwise from upper left: juniper feedstock mill ends; greenhouse biochar pyrolysis in “Oregon kiln”; hand-crushed biochar; biochar pyrolysis in Fossil, OR; modified leaf shredder; loading up biochar for field application.

Production Conditions

Biochar was produced in the “Oregon kiln”, a 0.7 m³ open-topped truncated pyramidal flame-cap kiln designed by Wilson Biochar Associates (Figure 1, top center). The kiln is portable and can be lifted by two people making it ideal for on-site biochar pyrolysis. Maximum heating temperatures for the kiln have been measured previously by Ms. Kelpie Wilson at 650 to 700^oC. The properties of the biochar are listed in Table 1.

Table 1. Physicochemical properties of juniper biochar

The burn time for the greenhouse study biochar was approximately 7 hours. The biochar was stored uncovered outside over winter, air-dried in a greenhouse for a week, and hand-crushed using a shovel and mallet.

Burn times for the field study biochar (Figure 1, bottom right) were slower, averaging 8.25 hours. After production, this biochar sat uncovered in central Oregon over winter and was then pulverized with a modified Billy Goat leaf shredder (Figure 1, bottom center). On a dry mass basis, 1 kg of feedstock yielded 0.13 kg of biochar.

Applications

Greenhouse Pilot Project

A six-week-long greenhouse trial was conducted to help decide how best to implement the subsequent field study. Researchers planted bluebunch wheatgrass (Pseudoroegneria spicata, a bunchgrass native to Central Oregon) seeds in soil obtained from Wheeler County.

Figure 2. Left: Schematic diagram of soil treatments and equivalent application rates at a field-scale. Dark grey represents soil, white dashed line represents seed depth, black represents biochar, and light grey represents straw mulch. Right: Images of pre-harvest greenhouse treatments that correspond with the diagram.

Researchers compared a range of juniper biochar amendments with straw mulch and unamended soil (see Figure 2) to evaluate how, and how well, the seeds responded. Study results indicated that incorporating biochar into the soil, rather than applying it as a mulch, would best promote soil moisture retention (important in arid habitat!) while benefitting seed germination and seedling survival.

Wheeler County Field Project

In spring 2018, a field study was established in Wheeler County, Oregon, on the Six Shooter Ranch. Located in the Painted Hills in the John Day/Clarno Uplands of the Blue Mountains, this semiarid region of central Oregon receives 23 to 41 cm of precipitation annually. The site was chosen for its ongoing juniper removal activities: juniper shrubs across a few thousand acres were being mechanically cut and piled and left to dry before being burned. Drs. Trippe and Phillips designed the field study to compare this “business as usual” management strategy (“Burn Scar”) to the results from the greenhouse study (“Biochar”) and a control where shrubs had been removed (“No Residue”).

Dr. Kristin Trippe amending the soil with biochar.

The study area covered 0.3 hectares and each of the three treatments consisted of 5 plots of 5 m x 5 m situated in 5 blocks (rows) along the 5% slope.

Site preparation involved light tillage of the whole area to bury weeds and control for previous site disturbance. Based on the results of the greenhouse study, the biochar plots received a 0.8 cm deep layer (equivalent to 14 Mg dry biochar per hectare) of biochar that was raked into the top 5 cm of soil. A tractor-pulled roller then firmed the seed bed.

Bluebunch wheatgrass seed was spread over the study area using an ATV pulled seeder at the recommended rate of 14.6 kg per hectare. Plot corners were marked with color-coded PVC pipes and a permanent 1 m x 2 m sampling subplot was demarcated in the center of each plot to mitigate plot edge effects on the sampling area. Temperature and moisture sensors were installed at varying depths to continually monitor soil conditions and the study area was fenced to exclude grazers like pronghorn and rabbits.

Results and Conclusions

Figure 3. Left, seedling emergence over the first growing season (error bars = S.D.).Right, Aboveground biomass of bluebunch wheatgrass. Unique letters indicate significantly different treatments (p<0.05). Adapted from Phillips et al. 2021.

During the spring and summer of 2018, the field site was visited every other week to count bluebunch wheatgrass seedlings. Seedling emergence was substantially higher in the burn scars followed by no residue plots and then biochar plots. Peak biomass was determined in July 2018 and in June 2019 by harvesting aboveground biomass. In both years, peak biomass of bluebunch wheatgrass plants was significantly higher in the burn scar plots but did not differ between the no residue and biochar plots (Figure 3).

Although the greenhouse trial indicated that incorporated biochar would increase soil water content, subsequent lab experiments clarified that this would occur only near-saturated conditions and not while the soil dried. Since rainfall at the field site in Central Oregon was below soil saturation levels during seed germination and early plant growth, the greenhouse efforts did not translate favorably to field conditions. Surprisingly, however, seedling establishment in the field trial was highest in the burn scars due to increased nitrogen levels that resulted from the cut-pile-burn method.

Overall, biochar had neither positive nor negative impacts on native seedling regeneration in the field study. However, the lessons that we learned in the field study remain valuable. Rangeland restoration is challenging. Finding affordable ways to prevent juniper encroachment requires creative and thoughtful solutions that are simple to implement and don’t require tillage or soil disturbance. We also learned that, contrary to the greenhouse results, biochar may be more beneficial for improving soil moisture when applied as a surface mulch rather than mixed into the soil. This method of biochar application might inhibit seed germination but could conversely benefit established perennial grasses and shrubs and would eliminate the need for tillage.