High precipitation and seeded species competition reduce seeded shrub establishment during dryland restoration

Abstract

Drylands comprise 40% of Earth's land mass and are critical to food security, carbon sequestration, and threatened and endangered wildlife. Exotic weed invasions, overgrazing, energy extraction, and other factors have degraded many drylands, and this has placed an increased emphasis on dryland restoration. The increased restoration focus has generated a wealth of experience, innovations and empirical data, yet the goal of restoring diverse, native, dryland plant assemblages composed of grasses, forbs, and shrubs has generally proven beyond reach. Of particular concern are shrubs, which often fail to establish or establish at trivially low densities. We used data from two Great Plains, USA coal mines to explore factors regulating shrub establishment. Our predictor data related to weather and restoration (e.g., seed rates, rock cover) variables, and our response data described shrub abundances on fields of the mines. We found that seeded non-shrubs, especially grasses, formed an important competitive barrier to shrub establishment: With every one standard deviation increase in non-shrub seed rate, the probability shrubs were present decreased ~0.1 and shrub cover decreased ~35%. Since new fields were seeded almost every year for > 20 years, the data also provided a unique opportunity to explore effects of stochastic drivers (i.e., precipitation, year effects). With every one standard deviation increase in precipitation the first growing season following seeding, the probability shrubs were present decreased ~0.07 and shrub cover decreased ~47%. High precipitation appeared to harm shrubs by increasing grass growth/competition. Also, weak evidence suggested shrub establishment was better in rockier fields where grass abundance/competition was lower. Multiple lines of evidence suggest reducing grass seed rates below levels typically used in Great Plains restoration would benefit shrubs without substantially impacting grass stand development over the long term. We used Bayesian statistics to estimate effects of seed rates and other restoration predictors probabilistically to allow knowledge of the predictors' effects to be refined through time in an adaptive management framework. We believe this framework could improve restoration planning in a variety of systems where restoration outcomes remain highly uncertain and ongoing restoration efforts are continually providing new data of value for reducing the uncertainty.

Legacy microsite effect on the survival of bitterbrush outplantings after prescribed fire: capitalizing on spatial variability to improve restoration

“…In support of this postulation, mortality of bitterbrush seedlings increased with greater competition from herbaceous vegetation (Hubbard ). Competition from herbaceous species has also limited other shrub species recovery (Porensky et al ; Rinella et al ).…”

“…All study sites were R023XY310OR North Slope 12-16 PZ Ecological Sites (NRCS 2016). Annual precipitation was 61, 83, and 85% of the 30-year long-term average (1981( -2010( , and 2015( (PRISM 2016. Cattle were excluded the first 2 years after burning.…”

Postwildfire seeding to restore native vegetation and limit exotic annuals: an evaluation in juniper‐dominated sagebrush steppe

“…Seeding mountain big sagebrush is often successful (Davies et al , ; Davies & Bates ). Herbaceous vegetation, particularly non‐native species, may limit shrub establishment (Rinella et al , ; Davies et al ). The effects, however, of seeding sagebrush in combination with native herbaceous vegetation are unknown.…”

Attempting to restore mountain big sagebrush (Artemisia tridentata ssp. vaseyana) four years after fire

“…Compared with successful seeding of mountain big sagebrush (Davies et al ; Davies & Bates ), our current study had approximately 3.5‐fold more cover and 2‐ to 27‐fold greater abundance of perennial grasses in the first growing season after seeding sagebrush. Competition from perennial grasses and other vegetation has limited the establishment of seeded shrubs in other restoration efforts (Allen ; Schuman et al ; Hall et al ; Rinella et al , ). Competition from perennial grasses, in particular, may be a widespread constraint to shrub establishment in arid and semi‐arid lands (Rinella et al ).…”