Site description
The study site is located in southeastern Wyoming at the Rogers Research Site (RRS) (42.236679 N, −105.344440 E) in the North Laramie Mountains (a mountain range that is considered part of the northern Front Range). RRS is managed by the Wyoming Agricultural Experiment Station and owned by the University of Wyoming (UW). The site is approximately 130 ha in size with moderate to steep slopes (5–50%) and elevations that range from 2000 to 2200 m. Mean annual precipitation is 37.6 cm with mean annual temperature ranging between 14°C and less than 0°C.
The Arapaho Fire was started in 2012 by a lightning strike and burned approximately 39,700 ha, killing 95% of the ponderosa pine trees at the RRS. According to Seymour et al. (2017), ponderosa pines covered approximately 80% of the RRS with trees in different age classes. After the Arapaho Fire, approximately 5% of the trees remained, converting a forested landscape to a shrub- and forb-dominated landscape. Understory vegetation associated with ponderosa pine forests in the Laramie Mountains prior to wildfire disturbance were shrubs—primarily in the Rosaceae family like antelope bitterbrush (Purshia tridentata (Pursh) DC.), chokecherry (Prunus virginiana L.), serviceberry (Amelanchier alnifolia (Nutt.) Nutt. ex M. Roem.), and Woods’ rose (Rosa woodsia Lindl.). Forbs included cinquefoil (Potentilla spp.), prairie sagewort (Artemisia frigida Willd.), geranium (Geranium spp.), milkvetch (Astragalus spp.), and common yarrow (Achillea millefolium L.). The dominant grasses and sedges that occurred were bluebunch wheatgrass (Pseudoroegneria spicata (Pursh) Á. Löve), Idaho fescue (Festuca idahoenis Elmer), prairie Junegrass (Koeleria macrantha (Ledeb.) Schult.), and Geyer’s sedge (Carex geyeri Boott) (Howard 2003).
RRS soils are moderately deep (50–100 cm) and coarse textured on hillsides and ridges, where the water table is high, thick, dark, and fine-textured soil occurs. Soil pH ranges from 7.2 in the top 10 cm and 6.4 below 10 cm (Williams and Waggener 2017; Wilkin et al 2019). Soils at the study site are characterized as moderately developed Alfisols and shallow Entisols with low fertility and low water-holding capacity formed from granitic weathering. Alfisols and Entisols are classified as fine-loamy, mixed, superactive, frigid Typic Haplustalfs moderately deep, and loamy-skeletal, mixed, micaceous, frigid Lithic Ustorthents shallow, respectively (Munn et al 2018). The 2012 Arapaho Fire occurred during one of the driest years on record in the state (Scasta 2015). Temperatures for the Arapaho Fire at RRS reportedly ranged from 200 to 500 °C based on black and white soil surface ash color post-fire, ensuring complete consumption of the organic soil horizon in some areas (Wilkin et al 2019).
Study design
An experimental block design was implemented in the summer of 2015 to determine the best combination of management treatments for ponderosa pine restoration following a high-severity wildfire (Herget et al 2018). Four blocks were established at RRS, each block comprising of 18 plots of 50 × 50 m (0.25 ha) in size. Block location was determined based on the feasibility of implementing the treatments and the topographic variability across the site like boulders, dirt roads, steep slopes, and drainages and is not replicated exactly across the landscape (Herget et al 2018).
A full factorial cross of each treatment was applied to each plot within a block with one pine introduction treatment (natural regeneration, broadcast seeding, and planted seedlings) and one logging treatment (no logging, bole only removal, whole tree removal) nested within an erosion control seeding treatment (no erosion seeding and seeding with a native grass species cultivar mix) for a total of 72 plots across all four blocks (Fig. 1). To account for edge effect, all measurements and surveys were conducted in a subplot of 27 × 27 m (0.07 ha) in the center of each plot. From here on, logging treatments “bole only removal” and “whole tree removal” will be referred to as “bole only” and “whole tree”.
The pine introduction treatments were randomly assigned to each plot within a block. In total, 2400 one-year-old ponderosa pine seedlings grown in 260-cm3 tubes were planted in all “planted seedling” pine introduction treatments in 2015. One hundred seedlings were planted per plot. Seedlings were planted in a grid system 3 m apart within the inner subplot. Nursery stock came from Colorado State Forest Service Nursery in Fort Collins, Colorado. Seedlings were hand planted using sharpshooter shovels in block 4 throughout the month of June, block 3 on July 1st–8th, block 1 on July 14th–21st, and block 2 on July 22nd–23rd. Ponderosa pine seeds used in the “broadcast seeding” plots originated from the Roosevelt National Forest in north-central Colorado. Seeds were kept in cold storage and had 70% germination viability. In October 2015, seeds were dispersed using a hand-held broadcast seeder at 158 g per subplot (~4500 seeds). The “natural regeneration” plots were left to naturally regenerate.
Logging treatments were randomly assigned and implemented in the late spring and summer of 2014 and early summer of 2015. Eight plots did not receive random implementation, but logging treatment plots were selected based on the accessibility of the skidder—avoiding steep slopes, boulders, and wet areas. In both the “whole tree” and “bole only” logging treatments, dead ponderosa pine trees were cut with a chainsaw and removed from the plot by a skidder. In the “bole only” removal plots, woody debris larger than 15 cm was removed. All woody debris remaining was evenly distributed across the entire 50 × 50 m plot (Herget et al 2018).
The erosion treatment was not randomly assigned; one-half of each block received the erosion grass seed mix and the other half was left unseeded. Four native grass cultivar species were included in the grass seed mix: “Bromar” mountain bromegrass (Bromus marginatus Nees ex Steud.), “Lodorm” green needlegrass (Nassella viridula (Trin.) Barkworth), “Pryor” slender wheatgrass (Elymus trachycaulus (Link) Gould ex Shinners), and “Winchester” Idaho fescue (Festuca idahoensis Elmer). Each species had approximately 92% germination viability. The grass seed mix was broadcast seeded using both an ATV and backpack broadcast seeder or evenly by hand (based on ATV access in plots with slash and standing dead trees) in May and June 2015 at approximately 4.7 kg to each 50 × 50 m plot assigned to the erosion treatment (Herget et al 2018). The seed mix was purchased from Western Native Seed in Coaldale, Colorado. The “Lodorm” green needlegrass seed originated from Montana and the other three species of grass seed originated from Washington.
Seedling surveys
Ponderosa pine seedling surveys were conducted for all pine introduction treatments in the summer of 2017. The “planted seedlings” plots were also surveyed in 2015, 1 to 2 months after seedlings were planted, and in the fall of 2016. Surveys were conducted by walking each subplot in 3 × 3 m grids, counting all ponderosa pine seedlings, and marking each as live or dead in planted treatments while seedling presence in broadcast-seeded and natural regeneration treatments were counted and marked live. All seedlings were photographed and marked with a Garmin global positioning system (GPS). Seedling numbers per plot were converted to stems ha−1 for statistical analysis.
Vegetation surveys
Vegetation surveys were done in June and July of 2017 for all plots. Starting at the northeast corner of each plot, five 0.5-m2 quadrats were read at 15, 20, 25, 30, and 35 m along a 50-m transect within the subplot. We recorded the percent of bare ground, ground cover (rock, lichen, litter, and woody litter), and both native and invasive plant species canopy cover in each quadrat.
Statistical analyses
We were interested in the mean abundance of ponderosa pine seedlings, mean percent cover of vegetation functional groups, and ground cover among the three pine introduction treatments, the three logging treatments, the two erosion treatments, and the combination of the pine introduction and logging treatments at the RRS. Comparisons of interest were carefully planned and defined before any data were collected; therefore, no adjustments to the multiple comparisons were used. All statistical analyses were done in R version 4.0.3 (R Core Team 2020).
We fit a Bayesian linear mixed-effect model with a negative binomial distribution using ponderosa pine seedling counts as the response variables, pine introduction treatment and logging treatment and their interaction as fixed effects, and block as a random effect using the blme package (Chung et al. 2013) and lme4 package (Bates et al 2015). We also fit a separate model with ponderosa pine seedling counts as the response variables and erosion treatment as a fixed effect and block as a random effect.
We fit a generalized linear mixed-effect model (GLMM) with a Tweedie distribution to estimate differences in the mean vegetation functional group cover and ground cover between pine introduction, logging, and erosion treatments at RRS using the glmmTMB package (Brooks et al., 2017). The mean functional group cover and ground cover were the response variable; pine introduction, logging, and erosion treatments were fixed effects; and block was the random effect.
The model with the lowest AIC score was chosen to determine which fixed effects were most important to include in our model for vegetation functional group cover and ground cover. Erosion control seeding was a fixed effect in models with the highest AIC score; therefore, we removed it from our models. However, to determine if erosion control seeding did affect the mean percent cover of the four grass species in the erosion control seed mix, invasive species, and bare ground, a separate GLMM was fit with the pine introduction treatment, logging treatment, and erosion control seeding treatment as the fixed effects and block as the random factor.
Residuals from the fitted models were graphically checked with the DHARMa package (Hartig 2021) and model assumptions of constant variance and normality were reasonably met. The blocks in the study were assumed to be independent of one another. The estimated marginal means and contrasts for each model were derived using the emmeans package (Lenth et al. 2021). To test for an overall treatment effect, a Wald chi-square test was performed on the chosen models to determine the degrees of evidence against the null hypothesis for the pine introduction treatment, logging treatment, and erosion control seeding treatment.
A permutational multivariate analysis of variance (PERMANOVA) with a Bray-Curtis dissimilarity index was used to determine the effect of logging and pine introduction treatments and erosion seeding on the vegetation functional groups using the adonis function in the vegan package (Oksanen et al. 2022). Permutations were constrained using block. Pairwise comparisons with a Bonferroni correction were done among logging treatments and pine introduction treatments using the pairwise.perm.manova function in the RVAideMemoire package (Hervé 2021).