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

Authors: Rinella, Matthew - Matt; HAMMOND, DARCY - Mississippi State University; BRYANT, ISA - Whitman College; KOZAR, BRIAN - Terra Soils, Llc

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2014
Publication Date: 6/1/2015
Publication URL: http://handle.nal.usda.gov/10113/61037
Citation: Rinella, M.J., Hammond, D., Bryant, I., Kozar, B. 2015. High precipitation and seeded species competition reduce seeded shrub establishment during dryland restoration. Ecological Applications. 25(4):1044-1053.

Interpretive Summary: Drylands comprise 40% of Earth’s land mass and are critical to imperiled wildlife species, food security and carbon sequestration. Exotic weed invasions, overgrazing, energy extraction, and other factors have left many of the planet’s drylands in various states of degradation, and this has prompted an increased research and management focus on dryland restoration. The increased emphasis on restoration has generated a wealth of experience, innovations and empirical data, yet the goal of restoring diverse, native dryland plant assemblages comprised of grasses, forbs and shrubs has generally proven beyond reach. Shrubs have been the guild most difficult to restore. When sown with grasses, shrubs usually fail to establish or establish at trivially low densities. We used data from two Great Plains, U.S.A coal mines to explore factors regulating shrub restoration outcomes. Our predictor data related to climate and restoration (e.g. seed rates, rock cover) variables, and our response data described shrub abundances on fields of the mines. We found seeded non-shrubs, particularly grasses, formed important competitive barriers 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%. Because new fields were seeded most years for over 20 years, the data 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%. Several considerations suggest high precipitation harmed shrubs by increasing grass establishment/competition. Also, there was weak evidence shrub establishment was better in rockier fields where grass competition was lower. Multiple lines of evidence suggest reducing non-shrub (i.e. grass) seed rates below levels typically used in Great Plains restoration would benefit shrubs without impacting grass stand development over the long term. In addition to reducing grass seed rates, our study suggested other ways shrub predictors could be adjusted to increase shrub establishment.

Technical Abstract: Drylands comprise 40% of Earth’s land mass and are critical to imperiled wildlife species, food security and carbon sequestration. Exotic weed invasions, overgrazing, energy extraction, and other factors have left many of the planet’s drylands in various states of degradation, and this has prompted an increased research and management focus on dryland restoration. The increased emphasis on restoration has generated a wealth of experience, innovations and empirical data, yet the goal of restoring diverse, native dryland plant assemblages comprised of grasses, forbs and shrubs has generally proven beyond reach. Shrubs have been the guild most difficult to restore. When sown with grasses, shrubs usually fail to establish or establish at trivially low densities. We used data from two Great Plains, U.S.A coal mines to explore factors regulating shrub restoration outcomes. Our predictor data related to climate and restoration (e.g. seed rates, rock cover) variables, and our response data described shrub abundances on fields of the mines. We found seeded non-shrubs, particularly grasses, formed important competitive barriers 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%. Because new fields were seeded most years for over 20 years, the data 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%. Several considerations suggest high precipitation harmed shrubs by increasing grass establishment/competition. Also, there was weak evidence shrub establishment was better in rockier fields where grass competition was lower. Multiple lines of evidence suggest reducing non-shrub (i.e. grass) seed rates below levels typically used in Great Plains restoration would benefit shrubs without impacting grass stand development over the long term. In addition to reducing grass seed rates, our study suggested other ways shrub predictors could be adjusted to increase shrub establishment. We estimated effects of shrub predictors probabilistically to allow shrub restoration strategies to be iteratively improved through time in a Bayesian adaptive management framework.