Decadal effects of fuel treatments and annual grass invasion on sagebrush ecosystem carbon stocks

Abstract

Fuel treatments are implemented across large spatial scales in sagebrush ( Artemisia spp.) ecosystems of the US Intermountain West to limit wildfire risk and spread, but treatment effects on ecosystem carbon storage remain poorly understood. Fuel treatments can reduce aboveground carbon storage and may also influence belowground carbon storage either directly through disturbance (e.g., disruption of soil surface) or indirectly through altered plant community composition over time (e.g., increases in invasive annual grasses that can impact plant–soil feedbacks and thus soil carbon storage). We analyzed above and belowground carbon stock estimates before and 10 years after treatment to determine whether woody fuel treatments had measurable effects on ecosystem carbon storage in the Sagebrush Steppe Treatment Evaluation Project (SageSTEP) plot network, which includes both pinyon pine and/or juniper‐encroached woodland sites and sagebrush‐dominated shrubland sites. We also assessed whether cheatgrass ( Bromus tectorum L.) invasion, which occurred throughout the study region, impacted soil organic and inorganic carbon storage in the top meter of soil. In woodland sites, prescribed fire and mechanical treatments reduced live tree carbon stocks by 87%–97% and tree litter carbon stocks by 92%–96% compared to starting values, and increased perennial herbaceous carbon stocks by 146%–169%. Prescribed fire also increased annual herbaceous carbon stocks seven‐fold, while mechanical treatments increased shrub carbon stocks by 89% and coarse woody debris carbon stocks five‐fold. In shrubland sites, prescribed fire reduced shrub carbon stocks by 66%. Mowing and woody herbicide treatments reduced shrub carbon stocks between 37% and 46% from pre‐treatment levels, but changes did not differ from those in untreated controls. Belowground carbon stocks were unaffected by treatment, though additional change may occur over time if differences in perennial or annual vegetation are persistent. Regardless of treatment type, cheatgrass invasion correlated with decreases in deep (30–90 cm) soil organic carbon stocks in woodland sites and deep soil inorganic carbon stocks in shrubland sites. While fuel treatments have clear impacts on carbon storage aboveground, their effects belowground (where most dryland ecosystem carbon is stored) appear small compared to the impacts of cheatgrass invasion.