No abstract

Plant invasions can affect fuel characteristics, fire behavior, and fire regimes resulting in invasive plant-fire cycles and alternative, self-perpetuating states that can be difficult, if not impossible, to reverse. Concepts related to general resilience to disturbance and resistance to invasive plants provide the basis for managing landscapes to increase their capacity to reorganize and adjust following fire, while concepts related to spatial resilience provide the basis for managing landscapes to conserve resources and habitats and maintain connectivity. New, spatially explicit approaches and decision-tools enable managers to understand and evaluate general and spatial resilience to fire and resistance to invasive grasses across large landscapes in arid and semi-arid shrublands and woodlands. These approaches and tools provide the capacity to locate management actions strategically to prevent development of invasive grass-fire cycles and maintain or improve resources and habitats. In this review, we discuss the factors that influence fire regimes, general and spatial resilience to fire, resistance to invasive annual grasses, and thus invasive grass-fire cycles in global arid and semi-arid shrublands and woodlands. The Cold Deserts, Mediterranean Ecoregion, and Warm Deserts of North America are used as model systems to describe how and why resilience to disturbance and resistance to invasive annuals differ over large landscapes. The Cold Deserts are used to illustrate an approach and decision tools for prioritizing areas on the landscape for management actions to prevent development of invasive grass-fire cycles and protect high value resources and habitats and for determining effective management strategies. The concepts and approach herein represent a paradigm shift in the management of these ecosystems, which allows managers to use geospatial tools to identify resilience to disturbance and resistance to invasive plants in order to target conservation and restoration actions where they will provide the greatest benefits.

Questions: Two of the primary global change factors that threaten shrublands worldwide are loss of native perennial herbaceous species due to inappropriate livestock grazing and loss of native shrubs due to altered fire regimes. We asked:(1) how do the separate and interacting effects of removal of perennial herbaceous species and burning influence relative abundance of plant functional groups over longer time frames; and (2) how do interactions between perennial herbaceous species removal and burning differ along environmental gradients? We discuss implications of our findings for ecosystem resilience to these disturbances.Location: Shoshone Mountain Range, Nevada and East Tintic Range, Utah, USA.Methods: We used a factorial experiment to test effects of perennial herbaceous species removal (0%, 50% and 100%) and burning (burned and not burned) on plant functional group cover along elevation gradients within watersheds characterized by Artemisia tridentata Nutt. vegetation types. The experiment was conducted in two locations (Nevada and Utah) with sites located at low (1960 and 1710 m), mid (2190 and 2085 m) and high (2380 and 2274 m) elevations and was repeated in 2 yr. Percentage cover of native and exotic species and canopy area, density and size of the shrub A. tridentata were evaluated 12 and 13 yr after study implementation.Results: Over a decade later, removal treatments resulted in highly significant decreases (40-62%) in perennial native grass and forb cover across site elevations. Burning decreased overall shrub and A. tridentata cover, but effects on perennial native grass cover differed among elevations. Removal had strong positive effects on A. tridentata seedling recruitment and resulted in progressive increases in density and canopy area following burning. A. tridentata canopy areas on burned plots with 0%, 50% and 100% removal were 0.19, 0.40 and 0.84 m 2 , respectively. Annual invasive grass density also increased with degree of removal, and both density and cover decreased with elevation. Conclusions:Our results show that loss of perennial herbaceous species, which can result from inappropriate livestock grazing, and loss of shrubs, which often results from fire, interact to affect key functional groups. The implications are that ecosystem resilience to disturbance in Cold Desert shrublands decreases when competition from perennial native grasses and forbs for available resources no longer prevents dominance by A. tridentata and other shrubs and/ or annual invasive grasses. Managing livestock grazing to maintain or increase perennial herbaceous species, especially deep-rooted grasses, which contribute to resilience along elevation gradients, can help prevent threshold crossings to undesirable states and retain critical ecosystem services following disturbances such as wildfire.

No abstract