Overcoming an “irreversible” threshold: A 15-year fire experiment

Authors

Christine H. Bielski, University of Nebraska-LincolnFollow
Rheinhardt Scholtz, University of Nebraska-LincolnFollow
Victoria Donovan, University of Nebraska-Lincoln
Craig R. Allen, University of Nebraska-LincolnFollow
Dirac Twidwell, University of Nebraska-LincolnFollow

Document Type

Article

Date of this Version

2021

Citation

Published in Journal of Environmental Management 291 (2021) 112550

doi:10.1016/j.jenvman.2021.112550

Comments

Copyright © 2021 Elsevier Ltd. Used by permission.

Abstract

A key pursuit in contemporary ecology is to differentiate regime shifts that are truly irreversible from those that are hysteretic. Many ecological regime shifts have been labeled as irreversible without exploring the full range of variability in stabilizing feedbacks that have the potential to drive an ecological regime shift back towards a desirable ecological regime. Removing fire from grasslands can drive a regime shift to juniper woodlands that cannot be reversed using typical fire frequency and intensity thresholds, and has thus been considered irreversible. This study uses a unique, long-term experimental fire landscape co-dominated by grassland and closed-canopy juniper woodland to determine whether extreme fire can shift a juniper woodland regime back to grassland dominance using aboveground herbaceous biomass as an indicator of regime identity. We use a space-for-time substitute to quantify herbaceous biomass following extreme fire in juniper woodland up to 15 years post-fire and compare these with (i) 15 years of adjacent grassland recovery post-fire, (ii) unburned closed-canopy juniper woodland reference sites and (iii) unburned grassland reference sites. Our results show grassland dominance rapidly emerges following fires that operate above typical fire intensity thresholds, indicating that grassland-juniper woodlands regimes are hysteretic rather than irreversible. One year following fire, total herbaceous biomass in burned juniper stands was comparable to grasslands sites, having increased from 5 ± 3 g m^–2to 142 ± 42 g m^–2(+2785 ± 812 percent). Herbaceous dominance in juniper stands continued to persist 15- years after initial treatment, reaching a maximum of 337 ± 42 g m^–2eight years post-fire. In juniper encroached grasslands, fires that operate above typical fire intensity thresholds can provide an effective method to reverse juniper woodland regime shifts. This has major implications for regions where juniper encroachment threatens rancher-based economies and grassland biodiversity and provides an example of how to operationalize resilience theory to disentangle irreversible thresholds from hysteretic system behavior.