Vegetation change and anthropogenic development are altering ecosystems and decreasing biodiversity. Successful management of ecosystems threatened by multiple stressors requires development of ecosystem conservation plans rather than single species plans. We selected the big sagebrush (Artemisia tridentata Nutt.) ecosystem to demonstrate this approach. The area occupied by the sagebrush ecosystem is declining and becoming increasingly fragmented at an alarming rate because of conifer encroachment, exotic annual grass invasion, and anthropogenic development. This is causing rangewide declines and localized extirpations of sagebrush associated fauna and flora. To develop an ecosystem conservation plan, a synthesis of existing knowledge is needed to prioritize and direct management and research. Based on the synthesis, we concluded that efforts to restore higher elevation conifer-encroached, sagebrush communities were frequently successful, while restoration of exotic annual grass-invaded, lower elevation, sagebrush communities often failed. Overcoming exotic annual grass invasion is challenging and needs additional research to improve the probability of restoration and identify areas where success would be more probable. Management of fire regimes will be paramount to conserving sagebrush communities, as infrequent fires facilitate conifer encroachment and too frequent fires promote exotic annual grasses. Anthropogenic development needs to be mitigated and reduced to protect sagebrush communities and this probably includes more conservation easements and other incentives to landowners to not develop their properties. Threats to the sustainability of sagebrush ecosystem are daunting, but a coordinated ecosystem conservation plan that focuses on applying successful practices and research to overcome limitations to conservation is most likely to yield success.

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Western juniper (Juniperus occidentalis spp. occidentalis Hook.) has encroached on and now dominates millions of acres of sagebrush/bunchgrass rangeland in the Great Basin and interior Pacific Northwest. On many sites western juniper has significantly increased exposure of the soil surface by reducing density of understory species and surface litter. We used rainfall and rill simulation techniques to evaluate infiltration, runoff, and erosion on cut and uncut field treatments 10 years after juniper removal. Juniper-dominated hillslopes had significantly lower surface soil cover of herbaceous plants and litter and produced rapid runoff from low-intensity rainfall events of the type that would be expected to occur every 2 years. Direct exposure of the soil to rainfall impacts resulted in high levels of sheet erosion (295 kg ? ha 21) in juniper-dominated plots. Large interconnected patches of bare ground concentrated runoff into rills with much higher flow velocity and erosive force resulting in rill erosion rates that were over 15 times higher on juniper-dominated plots. Cutting juniper stimulated herbaceous plant recovery, improved infiltration capacity, and protected the soil surface from even large thunderstorms. Juniper-free plots could only be induced to produce runoff from high-intensity events that would be expected to occur once every 50 years. Runoff events from these higher-intensity simulations produced negligible levels of both sheet and rill erosion. While specific inferences drawn from the current study are limited to juniper-affected sites in the Intermountain sagebrush steppe, the scope of ecosystem impacts are consistent with woody-plant invasion in other ecosystems around the world. Resumen El ''Western juniper'' (Juniperus occidentalis spp. occidentalis Hook.) se ha expandido, y ahora domina millones de hectáreas de pastizal de ''Sagebrush/Bunchgrass'' en la Gran Cuenca y en la región interior del Pacifico Noroeste. En muchos sitio el ''Western juniper'' ha aumentado significativamente la exposición del suelo al reducir la densidad de las especies herbáceas y del mantillo superficial. Utilizamos te´cnicas de simulación de lluvia y canalillos para evaluar la infiltración, el escurrimiento y la erosión en tratamientos de campo con corte y sin corte 10 añ os despue´s de remover el ''Western juniper''. Las laderas de las colinas dominadas por ''Western juniper'' tenían significativamente menos cobertura de plantas herbáceas y mantillo y produjeron un escurrimiento rápido a partir de eventos de lluvia de baja intensidad, del tipo que se esperaría ocurrieran cada dos añ os. En las parcelas dominadas ''Western juniper'', la exposición directa del suelo a los impactos de la lluvia resultó en altos niveles de erosión laminar (295 kg ? ha 21). Grandes parches de suelo desnudo interconectados concentraron el escurrimiento en los canalillos con una mayor velocidad de flujo y fuerza erosiva, resultando en tasas de erosión de surco 15 veces mayor que en las parcelas dominadas por ''Western juniper''. La remoci...

Historical disturbance regimes are often considered a critical element in maintaining native plant communities. However, the response of plant communities to disturbance may be fundamentally altered as a consequence of invasive plants, climate change, or prior disturbances. The appropriateness of historical disturbance patterns under modern conditions and the interactions among disturbances are issues that ecologists must address to protect and restore native plant communities. We evaluated the response of Artemisia tridentata ssp. wyomingensis (Beetle & A. Young) S.L. Welsh plant communities to their historical disturbance regime compared to other disturbance regimes. The historical disturbance regime of these plant communities was periodic fires with minimal grazing by large herbivores. We also investigated the influence of prior disturbance (grazing) on the response of these communities to subsequent disturbance (burning). Treatments were: (1) ungrazed (livestock grazing excluded since 1936) and unburned, (2) grazed and unburned, (3) ungrazed and burned (burned in 1993), and (4) grazed and burned. The ungrazed-burned treatment emulated the historical disturbance regime. Vegetation cover, density, and biomass production were measured the 12th, 13th, and 14th year post-burning. Prior to burning the presence of Bromus tectorum L., an exotic annual grass, was minimal (<0.5% cover), and vegetation characteristics were similar between grazed and ungrazed treatments. However, litter accumulation was almost twofold greater in ungrazed than in grazed treatments. Long-term grazing exclusion followed by burning resulted in a substantial B. tectorum invasion, but burning the grazed areas did not produce an invasion. The ungrazed-burned treatment also had less perennial vegetation than other treatments. The accumulation of litter (fuel) in ungrazed treatments may have resulted in greater fire-induced mortality of perennial vegetation in ungrazed compared to grazed treatments. Our results demonstrate that prior disturbances exert a strong influence on the response of plant communities to subsequent disturbances and suggest that low-severity disturbances may be needed in some plant communities to increase their resilience to more severe disturbances. Modern deviations from historical conditions can alter ecosystem response to disturbances, thus restoring the historical disturbance regime may not be an appropriate strategy for all ecosystems.

Global change drivers (GCDs) are expected to alter community structure and consequently, the services that ecosystems provide. Yet, few experimental investigations have examined effects of GCDs on plant community structure across multiple ecosystem types, and those that do exist present conflicting patterns. In an unprecedented global synthesis of over 100 experiments that manipulated factors linked to GCDs, we show that herbaceous plant community responses depend on experimental manipulation length and number of factors manipulated. We found that plant communities are fairly resistant to experimentally manipulated GCDs in the short term (<10 y). In contrast, long-term (≥10 y) experiments show increasing community divergence of treatments from control conditions. Surprisingly, these community responses occurred with similar frequency across the GCD types manipulated in our database. However, community responses were more common when 3 or more GCDs were simultaneously manipulated, suggesting the emergence of additive or synergistic effects of multiple drivers, particularly over long time periods. In half of the cases, GCD manipulations caused a difference in community composition without a corresponding species richness difference, indicating that species reordering or replacement is an important mechanism of community responses to GCDs and should be given greater consideration when examining consequences of GCDs for the biodiversity–ecosystem function relationship. Human activities are currently driving unparalleled global changes worldwide. Our analyses provide the most comprehensive evidence to date that these human activities may have widespread impacts on plant community composition globally, which will increase in frequency over time and be greater in areas where communities face multiple GCDs simultaneously.

Western juniper (Juniperus occidentalis spp. occidentalis Hook.) expansion into sagebrush steppe plant communities in the northern Great Basin has diminished shrub-steppe productivity and diversity. Chainsaw cutting of western juniper woodlands is a commonly applied practice for removing tree interference and restoring understory composition. Studies reporting understory response following juniper cutting have been limited to early successional stages. This study assessed successional dynamics spanning 13 years following tree cutting. Total herbaceous standing crop and cover increased significantly in the CUT. Total standing crop was 10 times greater in the CUT vs. WOODLAND. Herbaceous standing crop and cover, and densities of perennial grasses in the CUT did not change between 1996 and 2004 indicating that by the 5th year after cutting, remaining open areas had been occupied. In the early successional stages, perennial bunchgrasses and Sandberg's bluegrass were dominant. By the 5th year after treatment, cheatgrass had supplanted Sandberg's bluegrass and was codominant with perennial bunchgrasses. In 2003 and 2004, perennial bunchgrasses dominated herbaceous productivity in the CUT, representing nearly 90% of total herbaceous standing crop. A pretreatment density of 2-3 perennial bunchgrasses m À2 appeared to be sufficient to permit natural recovery after juniper control. Perennial bunchgrass density peaked in the 6th year after treatment and the results suggested that 10-12 plants m À2 were sufficient to fully occupy the site and dominate herbaceous composition in subsequent years. In the CUT, juniper rapidly reestablished from seed and from the presence of seedlings not controlled in the initial treatment. The shifts in herbaceous composition across years suggests that long term monitoring is important for evaluating plant community response to juniper control and to develop appropriate post treatment management to promote continued site improvement. Resumen La expansió n del ''Western juniper'' (Juniperus occidentalis spp. occidentalis Hook.) en las comunidades vegetales de las estepas de ''Sagebrush'' de la Gran Cuenca del norte ha disminuido la productividad de esta estepa arbustiva y su diversidad. La tala de bosques de ''Western juniper'' con sierra de cadena es una prá ctica comú nmente usada para remover la interferencia de los á rboles y restaurar la composision del estrato vegetal inferior. Los estudios que reportan la respuesta del estrato vegetal inferior posterior al corte del ''Western juniper'' han sido limitados a los primeros estados sucesionales. Este estudio aborda las diná micas sucesionales medidas durante un periodo de 13 añ os después del corte de los á rboles. La biomasa total y cobertura de herbá ceas se incrementaron significativamente con el corte de los á rboles. La biomasa total fue 10 veces mayor en las á reas con corte de arboles (CUT) que en las á reas intactas (WOODLAND). En el tratamiento con corte (CUT), la biomasa, cobertura y densidades de zacates perennes no cambiaron durant...

An increase in mega-fires and wildfires is a global issue that is expected to become worse with climate change. Fuel treatments are often recommended to moderate behaviour and decrease severity of wildfires; however, the extensive nature of rangelands limits the use of many treatments. Dormant-season grazing has been suggested as a rangeland fuel treatment, but its effects on fire characteristics are generally unknown. We investigated the influence of dormant-season (winter) grazing by cattle (Bos taurus) on fuel characteristics, fire behaviour and area burned in Wyoming big sagebrush (Artemisia tridentata subsp. wyomingensis) shrub-grassland communities in south-eastern Oregon, USA. Winter grazing was applied for 5 years before burning and compared with ungrazed areas. Winter grazing decreased fine fuels and increased fine fuel moisture, which reduced flame height and depth, rate of spread and area burned. Winter-grazed areas also had lower maximum temperature and heat loading during fires than ungrazed areas, and thereby decreased risk of fire-induced mortality of important herbaceous functional groups. These results suggest that winter grazing may be a fuel management treatment that can be applied across vast shrub-grasslands to decrease wildfire risk and fire intensity to mediate climate change effects on wildfire activity.

Abstract. Woodland ecosystems of the world have been changed by land use demands, altered fire regimes, invasive species and climate change. Reduced fire frequency is recognised as a main causative agent for Pinus-Juniperus L.(piñon-juniper) expansion in North American woodlands. Piñon-juniper control measures, including prescribed fire, are increasingly employed to restore sagebrush steppe communities. We compared vegetation recovery following prescribed fire on Phase 2 (mid-succession) and Phase 3 (late-succession) Juniperus occidentalis Hook. (western juniper) woodlands in Oregon. The herbaceous layer on Phase 2 sites was comprised of native perennial and annual vegetation before and after fire. On Phase 3 sites the herbaceous layer shifted from native species to dominance by invasive Bromus tectorum L. (cheatgrass). After fire, shrubs on Phase 2 sites were comprised of sprouting species and Ceanothus velutinus Dougl. (snowbrush). On Phase 3 woodland sites the shrub layer was dominated by C. velutinus. The results suggest that Phase 2 sites have a greater likelihood of recovery to native vegetation after fire and indicate that sites transitioning from Phase 2 to Phase 3 woodlands cross a recovery threshold where there is a greater potential for invasive weeds, rather than native vegetation, to dominate after fire.