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Fire and Grazing Influence Site Resistance to Bromus Tectorum

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Fire and Grazing Influence Site Resistance to Bromus Tectorum Ecosystems https://doi.org/10.1007/s10021-018-0230-8 Ó 2018 Springer Science+Business Media, LLC, part of Springer Nature (outside the USA) Fire and Grazing Influence Site Resistance to Bromus tectorum Through Their Effects on Shrub, Bunchgrass and Biocrust Communities in the Great Basin (USA) Lea A. Condon1,2* and David A. Pyke2 1Department of Botany and Plant Pathology, Oregon State University, 2082 Cordley Hall, Corvallis, Oregon 97333, USA; 2Present address: Forest and Rangeland Ecosystem Science Center, U.S. Geological Survey, Corvallis, Oregon 97331, USA ABSTRACT Shrubs, bunchgrasses and biological soil crusts cover. Characteristics of fire and grazing did not (biocrusts) are believed to contribute to site resis- directly relate to cover of B. tectorum. Relationships tance to plant invasions in the presence of cattle were mediated through shrub, bunchgrass and grazing. Although fire is a concomitant disturbance biocrust communities. Increased site resistance with grazing, little is known regarding their com- following fire was associated with higher bunch- bined impacts on invasion resistance. We are the grass cover and recovery of bunchgrasses and first to date to test the idea that biotic communities mosses with time since fire. Evidence of grazing mediate the effects of disturbance on site resistance. was more pronounced on burned sites and was We assessed cover of Bromus tectorum, shrubs, na- positively correlated with the cover of B. tectorum, tive bunchgrasses, lichens and mosses in 99 burned indicating an interaction between fire and grazing and unburned plots located on similar soils where that decreases site resistance. Lichen cover showed fires occurred between 12 and 23 years before a weak, negative relationship with cover of B. tec- sampling. Structural equation modeling was used torum. Fire reduced near-term site resistance to B. to test hypothesized relationships between envi- tectorum on actively grazed rangelands. Indepen- ronmental and disturbance characteristics, the dent of fire, grazing impacts resulted in reduced site biotic community and resistance to B. tectorum resistance to B. tectorum, suggesting that grazing management that enhances plant and biocrust communities will also enhance site resistance. Key words: biological soil crusts; gap size; Great Received 19 September 2017; accepted 21 January 2018 Basin; lichens; mosses; sagebrush ecosystem; Electronic supplementary material: The online version of this article shrub–steppe; structural equation model. (https://doi.org/10.1007/s10021-018-0230-8) contains supplementary material, which is available to authorized users. Author Contributions LAC and DAP conceived the study, analyzed the data, and wrote the paper. Data for this article can be found at https://doi.org/10.5066/F72Z14S7. *Corresponding author; e-mail: [email protected] L. A. Condon and D. A. Pyke INTRODUCTION native perennials to reach reproductive maturity (Brooks and others 2004). Bradley and others Ecosystem responses to compound disturbances are (2017) estimate B. tectorum is at 15% or greater of concern across biomes and continents as climate, cover on almost one-third of the Great Basin land use, and fire interact (Westerling and others (210,000 km2) and that 10.7% of this area, over 2006). Shrub–steppe ecosystems (also known as 22,000 km2, burned between 2000 and 2014, temperate deserts and semi-deserts), where the demonstrating the extent of the grass–fire cycle in primary land use is livestock grazing, make up 15% the region. of Eurasia and 3% of North America (West 1983a). Biotic communities can limit the magnitude of Grazing has likely reduced fire size in western invasions by exotic species (Levine and others Eurasia following millennia of livestock use with 2004). We use the Great Basin sagebrush steppe as shifts in livestock types from grass-dependent to a model to understand potential mechanisms that more browse-dependent animals as herbaceous lead to ecosystem resistance to B. tectorum in tem- vegetation has become limited (Breckle 1983; West perate shrub–steppe ecosystems. The Great Basin 1983b). Unlike western Eurasia, the Great Basin of sagebrush steppe is a comparable ecosystem to North America has only been grazed by livestock those of Eurasia, containing many similar life- for less than 130 years. Over the last 80 years, in- forms (for example, perennial bunchgrasses, fire- creased livestock management in the Great Basin intolerant shrubs, and biological soil crusts [lichens, has led to improved herbaceous production and mosses, and cyanobacteria on the soil surface rangeland condition (Box 1990). Although these hereafter called biocrusts]; Walter and Box 1983). ecosystems have not and may never return to their Understanding the strength of interactions between pre-livestock-grazing condition, they do allow for biotic communities and site resistance to B. tectorum the examination of impacts related to both fire and in the presence of disturbances such as fire and grazing on a system that is less altered by anthro- grazing is crucial to the management of sagebrush pogenic influences compared to similar ecosystems steppe ecosystems. Here we use ‘‘resistance’’ in the globally. When fire and grazing occur in the pres- ecological sense as it is used by D’Antonio and ence of invasive species, fire regimes can change others (2009) to mean the ‘‘ability of a community and further promote invasive species (Rossiter and to withstand encroachment by nonnative species.’’ others 2003). Our goal in this study is to provide a compre- In Great Basin shrub–steppe ecosystems, fire and hensive understanding of the role of the biotic grazing are thought to regulate dominance of the community: shrubs, native bunchgrasses, and bio- exotic annual grass, Bromus tectorum L. (Pyke and crusts in maintaining site resistance to B. tectorum in others 2016), although they have rarely been the presence of both fire and grazing across the examined simultaneously (Davies and others region. We build upon two models of site resistance 2016). Bromus tectorum was introduced into the to B. tectorum, one in the presence of fire (Condon region from Eurasia in the late 1800 s and attained and others 2011) and one in the presence of graz- its current distribution by the 1930 s (Mack 1981). ing (Reisner and others 2013), linking these models The invasion history of B. tectorum is long, as it is with the findings of many studies on interactions classified as an archaeophyte in Central Europe, between biotic, environmental and disturbance having established there from its native steppe indicators (Table 1). Resistance in these models communities of southern Europe and the Middle depended on cover of perennial vegetation, cover East where it can dominate shrub–steppe ecosys- of biocrusts, or the distance among perennial tems following excessive grazing (Terpo´ and others plants. Expanding upon these models, we charac- 1999; Kaczmarski 2000). Populations from Central terize fire and grazing with multiple factors that Europe are known to exhibit reduced biomass and could be employed by land managers to predict and fewer inflorescences when situated with more na- possibly facilitate site resistance (grazing animal tive plant communities, particularly high cover of stocking levels, fire severity, extent of fire, and time native bunchgrasses (Fenesi and others 2011). In since fire). Additionally, we anticipated an inter- the Great Basin, B. tectorum often creates a contin- action between fire and grazing, since cattle uous fuel among perennial vegetation, where it preferably use areas with light to moderate burn dominates interspaces among vascular plants that severity over unburned areas (Clark and others were previously occupied by biological soil crusts 2014). We expected that biocrusts would be more (for example, lichens, mosses, and cyanobacteria sensitive to disturbance, particularly grazing, com- on the soil surface), thus allowing fires to become pared to native bunchgrasses. In areas surrounding more frequent than the time needed for many the Great Basin, results from surveys inside and Bunchgrasses and Biocrusts Affect Site Resistance Table 1. Components of Hypotheses Represented by Initial Conceptual Model (Figure S1) Site resistance Abiotic factors HLE fi + Bromus fl Increases in heat load result in reduced site resistance to Bromus tectorum L. (Chambers and others 2007; Condon and others 2011) HLE fi - grass cover fl Increases in heat load result in decreased productivity of bunchgrasses (Davies and others 2007)1 SPP fi + bare ground fl Increased sand content is associated with increases in bare ground (BG) and B. tectorum establishment1 BG fi + Bromus fl Safe sites in the form of bare ground (BG) increase establishment rates of B. tectorum (Fowler 1988)1 SPP fi + Bromus fl Soils with deeper, coarser textures are associated with increased cover of B. tectorum (Stewart and Hull 1949)1 SPP fi ± grass comp fl›Increases in sand content are associated with different compositions of native bunch- grasses1 Grazing impacts Grazing fi - biocrusts fl Grazing results in trampling of biocrusts and increases safe sites for establishment of B. tectorum (Ponzetti and others 2007)1 Grazing fi ± Bromus fl›Grazing results in a decrease in cover of B. tectorum by herbivory (Hempy-Mayer and Pyke 2009), an increase through propagule pressure via animal mediated seed dis- persal (Schiffman 1997) or no direct effect in some areas1 Grazing fi + gaps fl Increases in grazing intensity relate to positive increases in community gap structure (Condon and Pyke 2016)1 Grazing
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