Restore into the future: post-fire rangeland restoration in the Great Basin

Objective 1. Assess the effects of current post-fire seeding practice on local plant species and genetic diversity in burned areas. We predict that native grass seeding will increase local species and genetic diversity in burned-seeded areas relative to burned-unseeded areas, because burned-unseeded areas are dominated by cheatgrass. We focus on plant diversity at species and genetic level here because we are concerned about the adaptive capacity of plant communities after post-fire seeding. Based on the resilience framework, species diversity buffers against multiple stressors such as drought (Ives and Carpenter 2007, Hautier et al. 2014), and genetic diversity ensures the populations’ potential to evolve when conditions change in the future (Hoffmann and Hercus 2000). Our hypothesis will be supported if species diversity and genetic diversity in burned-seeded areas are higher than those in burned-unseeded areas. Alternatively, seeded areas may have lower diversity than unseeded areas, due to low diversity in seed mixes which contain only 4-5 species from 1-2 source populations (Leger and Baughman 2015). The effect of these low diversity seed mixes on plant diversity of restored communities is not well tested but is likely to reduce the resilience of native perennial grasslands.

Objective 2. Evaluate where on the landscape to prioritize post-fire seeding for native vegetation recovery in burned areas. We predict that dispersal will increase recovery of native species and genetic diversity closer to fire edges, making post-fire seeding more important farther away from the fire edges. Wind dispersal of seeds from unburned areas is the natural process of post-fire recovery in burned areas (Damschen et al. 2014). Seed dispersal exponentially decreases with distance to the nearest seed source or fire edge (Nathan and Muller-Landau 2000). Therefore, in large wildfires, seeds may not reach some burned areas because the distance between unburned areas may be too great for the seeds to disperse. Post-fire seeding attempts to overcome this seed limitation (Foster and Tilman 2003). The effect of seeding on plant diversity may depend on distance from the nearest fire edge. We expect post-fire seeding in the core of these large burns will be most helpful for maintaining a resilient landscape. Our hypothesis will be supported if the effect size of seeding (difference in species and genetic diversity between seeded and unseeded areas) increases with distance from the nearest fire edge.

Objective 3. Identify native grass seed provenance that improves the drought tolerance of restored sites. We predict that the southern populations of native grass species will exhibit higher drought tolerance than the northern populations. This drought tolerance will help provide reliable forage in a hotter/drier future. Our preliminary results indicate that seeds sourced from southern populations in drier sites have higher seedling success rate when grown in a common garden than seeds from local or northern populations in wetter sites (Fig. 1). We expect that this effect is associated with more conservative traits (such as higher water use efficiency) in these populations. A plant trait-based approach using simple-to-measure aboveground traits has been successful in explaining trait-by-environment associations across species (e.g., Baughman et al. 2019, Basey et al. 2015). We will test the association between seed provenance, plant traits, and drought survival. Our hypothesis will be confirmed if southern populations have higher water use efficiency, lower water stress, and higher survival rates than northern populations under drought.

Objective 4. Understand the relationship between the drought tolerance and cheatgrass resistance traits of native grasses. Keeping cheatgrass at bay is important because we expect a tradeoff between drought-tolerant and cheatgrass-resistant traits will decrease survivorship of native grasses under combined drought and invasion. Drought-tolerant individuals have resource-conservative traits (e.g., high water use efficiency (Farquhar et al. 1989) and low growth rates (Craine et al. 2012)) to save energy. In contrast, cheatgrass-resistant individuals are known to have resource-acquisitive traits (e.g., early phenology and high growth rates (Leger et al. 2019)) to compete with annual grasses that emerge in winter and senesce by summer. However, the competitive outcomes for restored natives vs. cheatgrass under contrasting precipitation conditions are still unknown. Understanding the trait association with seedling performance under drought and invasion would inform practitioners which native species and population to use for pre-emptive seeding practices depending on the extent of cheatgrass invasion or the likelihood of future drought in the area. Our hypothesis will be supported if native seedling survival rate increases with drought-tolerance traits under drought but decreases under drought and invasion, and vice versa.

RESEARCH METHODS AND MATERIALS. We will test our hypotheses using two approaches: (1) a Survey where we measure plant diversity at a community and population level in a previously burned area; and 2) a Common Garden Experiment where we measure intraspecific variability in native grass response to drought and cheatgrass treatments.

Objective 1 & 2: Survey of post-fire plant communities.

Sampling design. Our survey took place in Soda Fire, where over 280,000 acres of rangelands burned at the border of Oregon and Idaho in 2015 (MTBS; Fig. 2). Taking advantage of existing USGS monitoring effort, we identified 40 plots stratified by burned-unseeded (n = 15), burned-seeded (n = 15), and unburned-unseeded (n = 10) areas. Using distance from the nearest fire edge as a proxy for landscape dispersal, I further stratified burned-unseeded and burned-seeded plots by five distance classes: <1000 m, 1000-2000 m, 2000-3000 m, 3000-5000 m, >5000 m. Each distance class consisted of four replicates. Burned-seeded plots distributed across three seeded areas were previously drill seeded with bluebunch wheatgrass Pseudoroegneria spicata) in fall 2015-2017. Bluebunch wheatgrass is a native perennial grass species widely used for post-fire restoration for its high fire tolerance (Miller et al. 2013) and relatively high establishment rate after fire (Hudson 2016).

Methods. We collected plant composition data and leaf samples of bluebunch wheatgrass at all 40 plots in June 2021. Each plot was delineated by a 10 x 10 m relevé, where number of species is recorded for species richness. Occurrence of each plant taxon was measured every 0.5 m on four 25-m transects radiating from each corner of the relevé using the point-intercept method, and evenness was calculated. Leaf samples were collected from 25 individuals at each plot (n = 1350), then air-dried and stored at -80F. We isolated DNA from plant tissues using a modified cetyltrimethy ammonium bromide (CTAB)-based method (Aboul-Maaty and Oraby 2019). We amplifed small quantity of DNA with polymerase chain reaction (PCR) assay using ten known primers for bluebunch wheatgrass (Bushman et al. 2008). To measure genetic diversity of bluebunch wheatgrass populations, we used a genotyping service at the Center for Genome Research and Biocomputing at the Oregon State University to obtain microsatellite data in Fall 2021. With the microsatellite data, nucleotide diversity (π) was calculated as a metric of genetic diversity at the population level.

Analysis. We will use ANOVA to test the combined effect seeding on overall species diversity (species richness and evenness) and the genetic diversity (π) of bluebunch wheatgrass, with burned-seeded and burned-unseeded as explanatory variables. To test how seeding effects change with landscape dispersal, we will assess the log response ratio of species and genetic diversity to seeding (burned-seeded/burned-unseeded), and regress these log response ratios against distance from the nearest fire edge in linear models.

Objective 3 & 4: Common Garden Experiment. Experimental design. In October 2019, we established a sagebrush steppe common garden experiment at the Northern Great Basin Experimental Range in Burns, OR. We nested seeding treatments (2 native species x 6 seed provenances) within a factorial design that crosses rainfall (ambient, 50%, 80% reduction) by competitive environment (with and without cheatgrass). We burned the site prior to implementation to mimic post-fire conditions. The experiment is replicated 8 times in a random block design (48 total 1.5 m x 1.5 m plots with nested 25 cm x 25 cm subplots). Precipitation is manipulated using fixed rainout shelters that passively reduce precipitation by a constant percentage (Gherardi and Sala 2013, Knapp et al. 2016). Aluminum flashings are inserted 35 cm deep around the shelters to limit plant roots from accessing water outside the treatment. To account for microclimates from rainout shelters, soil moisture sensors and air + soil temperature sensors are installed year-round. In March 2020, we raked existing vegetation and sowed 20 seeds of bottlebush squirreltail (Elymus elymoides) and Sandberg bluegrass (Poa secunda) from six populations in each subplot. These are desired forage species commonly used for restoration. Bluebunch wheatgrass was not sown because not enough seeds were collected across the climatic gradient to make a meaningful comparison. In July 2021, we added 200 cheatgrass seeds in each competition plot. Seeding of cheatgrass was delayed because cheatgrass usually does not increase until the second year following fire (West and Yorks 2002).

Methods. In July 2021 and 2022, we measured survival rates of seedlings and morphological traits related to growth rate (Perez-Harguindeguy et al. 2013): height (cm), SLA (cm²/g), and LDMC (g/g). In July 2021, we collected leaf samples and process them (dry and grind) for a carbon isotope ratio analysis at the Stable Isotope Laboratory at the University of Oregon. The ratio of carbon isotopes (¹³C/¹²C) indicate carbon gain per unit water loss, which is a measure of water use efficiency (Diefendorf et al. 2010). 

Analysis. To compare the effects of source population and precipitation treatment (ambient, 50 % and 80 % reduction) on drought tolerance traits, we will build mixed models with source population and precipitation as fixed factors and block as a random factor. A tradeoff between drought-tolerant and cheatgrass-resistant traits can be confirmed by using a principal component analysis (PCA) on trait values followed by PERMANOVA; drought-tolerant traits should be orthogonal to cheatgrass-resistant traits. To test the effect of trait tradeoffs on seedling survivorship, we will regress seedling survivorship against traits.