Trajectories of change in sagebrush steppe vegetation communities in relation to multiple wildfires

Abstract

Repeated perturbations, both biotic and abiotic, can lead to fundamental changes in the nature of ecosystems, including changes in state. Sagebrush steppe communities provide important habitat for wildlife and grazing for livestock. Fire is an integral part of these systems, but there is concern that increased ignition frequencies and invasive species are fundamentally altering them. Despite these issues, the majority of studies of fire effects in systems dominated by Artemisia tridentata wyomingensis have focused on the effects of single burns. The Arid Lands Ecology Reserve (ALE), in south-central Washington (U.S.A.), was one of the largest contiguous areas of sagebrush steppe habitat in the state until large wildfires burned the majority of it in 2000 and 2007. We analyzed data from permanent vegetation transects established in 1996 and resampled in 2002 and 2009. Our objective was to describe how the fires, and subsequent postfire restoration efforts, affected communities' successional pathways. Plant communities differed in response to repeated fire and restoration; these differences could largely be ascribed to the functional traits of the dominant species. Low-elevation communities, previously dominated by obligate seeders, moved furthest from their initial composition and were dominated by weedy, early-successional species in 2009. Higher-elevation sites with resprouting shrubs, native bunchgrasses, and few invasive species were generally more resilient to the effects of repeated disturbances. Shrub cover has been almost entirely removed from ALE, although there was some recovery where communities were dominated by resprouters. Bromus tectorum dominance was reduced by herbicide application in areas where it was previously abundant, but it increased significantly in untreated areas. Several resprouting species, notably Phlox longifolia and Poa secunda, expanded remarkably following competitive release from shrub canopies and/or abundant B. tectorum. Our results suggest that community dynamics can be understood through a state and transition model with two axes (shrub/grass and native/invasive abundance), although such models also need to account for differences in plant functional traits and disturbance regimes. We use our results to develop a conceptual model that will be validated with further research.

Predicting foundation bunchgrass species abundances: model‐assisted decision‐making in protected‐area sagebrush steppe

“…The most robust stands of bunchgrass, characterized by high cover of bluebunch wheatgrass, were most likely to occur on steep north-facing slopes at higher and more remote portions of the Monument. In these cool, mesic sites, productivity is higher, allowing for bluebunch wheatgrass to effectively out-compete invasive annual grasses (Condon et al 2011, Davies et al 2012, Chambers et al 2014. Historic grazing pressure in the landscape prior to Monument establishment was also likely to have been less intense on these steeper slopes.…”

“…We focused on topographic proxies for effective soil moisture because sagebrush steppe communities on wetter, cooler sites have been found to be more resistant to cheatgrass invasion. This is thought to reflect differences in resource availability, productivity, and timing of resource utilization between invasive annuals and native perennial grasses along topographic-soil moisture gradients (i.e., the fluctuating resource hypothesis) (Chambers et al 2007, Condon et al 2011, Davies et al 2012, Reisner et al 2013, Chambers et al 2014. Given the ruggedness of the Monument landscape we expected topography to exert strong influence in models.…”

“…These declines have cascading ecological effects, characterized by positive feedback loops between community invaders and fire, with irreversible changes in ecosystem structure and function (D'Antonio and Vitousek 1992, Brooks et al 2004, Balch et al 2013. The abundances of these steppe foundation species correlate strongly with resilience to fire and resistance to invasion (Chambers et al 2007, Brooks and Chambers 2011, Condon et al 2011, Davies et al 2012, Reisner et al 2013. The uptake of soil nitrogen and water by big sagebrush and by bunchgrasses has been shown through removal experiments to reduce community invasibility (Chambers et al 2007, James et al 2008, Mata-Gonzalez et al 2008, Prevey et al 2010.…”

“…The uptake of soil nitrogen and water by big sagebrush and by bunchgrasses has been shown through removal experiments to reduce community invasibility (Chambers et al 2007, James et al 2008, Mata-Gonzalez et al 2008, Prevey et al 2010. Additionally, the severity of infestations of cheatgrass and another invasive annual grass, medusahead (Taeniatherum caput-medusae), are inversely correlated with the abundances of pre-existing native perennial bunchgrasses (Davies 2008, Condon et al 2011, Davies et al 2012, Reisner et al 2013. Tall tussock-type bunchgrasses like bluebunch wheatgrass seem to effectively reduce dispersal of medusahead seeds, and therefore robust stands of these bunchgrasses may contain incipient infestations (Davies 2008).…”

Sagebrush steppe recovery after fire varies by development phase of Juniperus occidentalis woodland

“…Typically, there is little fire-caused mortality and these species increase 3-5 years after fire (Anderson and Bailey 1979;Wright et al 1979;Sieg and Wright 1996) and are important in recovery of shrub structure on A. t. ssp. vaseyana sites (Davies et al 2012). However, response of S. rotundifolius can be variable and recovery may require up to 15 years or longer if ungulate browsing damages regrowth (Blaisdell 1953;Bartos et al 1994).…”