PREMISE OF THE STUDYThe evolutionary drivers and proximal regulators of mast-seeding are well understood for species of mesic environments, but how these regulators interact with high spatial and interannual variability in growing-season precipitation for a masting species in a desert environment has never been examined.METHODWe followed flowering and seed production in 16 populations of the North American desert shrub blackbrush (Coleogyne ramosissima) from contrasting environments across its range over an 11-year period to determine patterns of interannual reproductive output variation.KEY RESULTPatterns of reproductive output in blackbrush did not track current growing season precipitation, but instead were regulated by prior-year weather cues. The strength of the response to the masting cue depended on habitat quality, with higher mean reproductive output, shorter intervals between years of high seed production, and lower CVp at more favorable sites. Wind pollination efficiency was demonstrated to be an important evolutionary driver of masting in blackbrush, and satiation of heteromyid seed predator-dispersers was supported as an evolutionary driver based on earlier studies.CONCLUSIONSBoth the evolutionary drivers and proximal regulators of masting in blackbrush are similar to those demonstrated for masting species of mesic environments. Relatively low synchrony across populations in response to regional masting cues occurs at least partly because prior-year environmental cues can trigger masting efforts in years with resource limitation due to suboptimal precipitation, especially in more xeric low-elevation habitats.

The masting phenomenon along with its accompanying suite of seedling adaptive traits has been well studied in forest trees but has rarely been examined in desert shrubs. Blackbrush (Coleogyne ramosissima) is a regionally dominant North American desert shrub whose seeds are produced in mast events and scatter-hoarded by rodents. We followed the fate of seedlings in intact stands vs. small-scale disturbances at four contrasting sites for nine growing seasons following emergence after a mast year. The primary cause of first-year mortality was post-emergence cache excavation and seedling predation, with contrasting impacts at sites with different heteromyid rodent seed predators. Long-term establishment patterns were strongly affected by rodent activity in the weeks following emergence. Survivorship curves generally showed decreased mortality risk with age but differed among sites even after the first year. There were no detectable effects of inter-annual precipitation variability or site climatic differences on survival. Intraspecific competition from conspecific adults had strong impacts on survival and growth, both of which were higher on small-scale disturbances, but similar in openings and under shrub crowns in intact stands. This suggests that adult plants preempted soil resources in the interspaces. Aside from effects on seedling predation, there was little evidence for facilitation or interference beneath adult plant crowns. Plants in intact stands were still small and clearly juvenile after nine years, showing that blackbrush forms cohorts of suppressed plants similar to the seedling banks of closed forests. Seedling banks function in the absence of a persistent seed bank in replacement after adult plant death (gap formation), which is temporally uncoupled from masting and associated recruitment events. This study demonstrates that the seedling establishment syndrome associated with masting has evolved in desert shrublands as well as in forests.

Interacting threats to ecosystem function, including climate change, wildfire, and invasive species necessitate native plant restoration in desert ecosystems. However, native plant restoration efforts often remain unguided by ecological genetic information. Given that many ecosystems are in flux from climate change, restoration plans need to account for both contemporary and future climates when choosing seed sources. In this study we analyze vegetative responses, including mortality, growth, and carbon isotope ratios in two blackbrush (Coleogyne ramosissima) common gardens that included 26 populations from a range-wide collection. This shrub occupies ecotones between the warm and cold deserts of Mojave and Colorado Plateau ecoregions in western North America. The variation observed in the vegetative responses of blackbrush populations was principally explained by grouping populations by ecoregions and by regression with site-specific climate variables. Aridity weighted by winter minimum temperatures best explained vegetative responses; Colorado Plateau sites were usually colder and drier than Mojave sites. The relationship between climate and vegetative response was mapped within the boundaries of the species-climate space projected for the contemporary climate and for the decade surrounding 2060. The mapped ecological genetic pattern showed that genetic variation could be classified into cool-adapted and warm-adapted ecotypes, with populations often separated by steep dines. These transitions are predicted to occur in both the Mojave Desert and Colorado Plateau ecoregions. While under contemporary conditions the warm-adapted ecotype occupies the majority of climate space, climate projections predict that the cool-adapted ecotype could prevail as the dominant ecotype as the climate space of blackbrush expands into higher elevations and latitudes. This study provides the framework for delineating climate change-responsive seed transfer guidelines, which are needed to inform restoration and management planning. We propose four transfer zones in blackbrush that correspond to areas currently dominated by cool-adapted and warm-adapted ecotypes in each of the two ecoregions.

Blackbrush (Colegyne ramosissima) is a desert shrubland species that is currently dominant on over three million acres of the transition zone between the cold desert of the Great Basin and the warm desert of the southwestern United States. Western landscapes are projected to experience unprecedented changes as the climate warms, and researchers at the Rocky Mountain Research Station have been studying the response of this species to assess whether it can move upward in elevation and latitude. Blackbrush was found to have two distinct populations (in the warmer Mojave Desert and the cooler Colorado Plateau), which should give the species greater flexibility in responding to climate change and managers the opportunity to work with locally adapted seeds and plants. There are many barriers to successful dispersal, germination, and establishment of blackbrush under current climate conditions, and any of these may limit the potential of the species to make relatively rapid geographic shifts. Researchers have documented poor seedling survival over the past decade in the warmer Mojave ecoregion, but moving into wetter and cooler areas may not be possible for blackbrush based on current land use and the speed with which the species would have to shift its range. Scientists and managers will need to work together to foster the survival of this important species by identifying priority areas for conservation/restoration, identifying climate-adapted seed sources, and possibly assisting with its long-term migration.

Southern Nevada ecosystems and their associated resources are subject to a number of global and regional/local stressors that are affecting the sustainability of the region. Global stressors include elevated carbon dioxide (CO2) concentrations and associated changes in temperature and precipitation patterns and amounts, solar radiation, and nutrient cycles (Smith and others 2009b). Global stressors are ubiquitous in nature and interact both directly and indirectly with regional or local stressors. Regional/local stressors in southern Nevada include: population growth and urbanization and associated increases in nitrogen deposition, energy development, water development, and recreation; increased effects of insects and disease; ongoing effects of livestock, wild horse and burro grazing; new and expanding invasive species; and altered fire regimes. This chapter provides background information on the stressors affecting southern Nevada’s ecosystems that is needed to address Goal 1.0 in the SNAP Science Research Strategy, which is to restore, sustain, and enhance southern Nevada’s ecosystems (Turner and others 2009).

This synthesis provides information related to the Southern Nevada Agency Partnership (SNAP) Science and Research Strategy Goal 1 - to restore, sustain and enhance southern Nevada’s ecosystems - and Goal 2 - to provide for responsible use of southern Nevada’s lands in a manner that preserves heritage resources and promotes an understanding of human interaction with the landscape. The Science and Research Strategy has nine Sub-goals that address the topics of water and water use, fire, invasive species, biological diversity, restoration, cultural resources, historic content, recreation, and science-based management. This synthesis summarizes the state-of-knowledge related to each of these Sub-goals, addresses knowledge gaps, and provides management implications. It builds on previous efforts to develop the necessary scientific understanding for adaptive management of southern Nevada ecosystems.

Southern Nevada ecosystems are subject to a number of stressors that range in scope from local to regional to global. At the regional scale, human population growth and related activities constitute a major stressor. Nevada has undergone significant change due to unprecedented population growth and ongoing global change processes. Nevada’s growth rate has been the highest in the nation for the last five decades. Clark County has experienced particularly rapid growth with a population increase of more than 40 percent since the 2000 census. Other regional or local stressors, many of which are related to human population growth, include invasive species, changes in land use, and altered fire regimes. Global stressors affecting southern Nevada ecosystems include elevated carbon dioxide (CO2) concentrations, nitrogen (N) deposition, and changes in temperature and precipitation patterns. This chapter provides background information on the stressors affecting southern Nevada’s ecosystems that is needed to address Goal 1.0 in the SNAP Science Research Strategy, which is “Restore, sustain, and enhance southern Nevada’s ecosystems.”

Resource managers in southern Nevada are faced with the challenge of determining appropriate goals and objectives and developing viable approaches for maintaining and restoring sustainable ecosystems in the face of rapid socio-ecological and environmental change. Many of southern Nevada’s ecosystems are being subjected to anthropogenic stressors that span global, regional, and local scales (Chapter 2), and are crossing ecological thresholds to new and often undesirable alternative states (Chapters 4 and 5). Protection, prevention, and restoration are integral parts of managing for sustainable ecosystems that can supply both ecosystem services and habitat for the diversity of plants and animals that occupy the region. This summary addresses the restoration aspects of Sub-goal 1.3 in the SNAP Science Research Strategy, which is to “Restore and sustain proper function of southern Nevada’s watersheds and landscapes” (see table 1.1).

A three-year study was conducted in the Chihuahuan Desert in Southwestern New Mexico to evaluate the effectiveness of revegetating a dryland pasture that was heavily infested with yellow starthistle within the context of the successional weed management model. A prescribed burn treatment of the entire study site (designed disturbance) was followed by single-entry revegetation (controlled colonization) and weed suppression (controlled species performance) treatments. Four native perennial grass species were paired with 4 yellow starthistle suppression treatments. We conclude that an integrated, single-entry approach failed to effectively revegetate yellow starthistle-infested dryland pasture in the Chihuahuan Desert, primarily due to a historic severe drought that occurred soon after grasses were seeded. Different strategies and tactics will be required to manage yellow starthistle in the Southwestern USA than have been previously applied in other areas.

Throughout the southwestern U.S., riparian gallery forests of cottonwood and willow are being invaded by woody exotics, primarily Russian olive and salt cedar. We wondered what effect this might have on native pollinator populations. Pollinators are indispensable contributors to biodiversity, ecosystem health, and human food production. Recent declines in pollinator abundance and health, such as catastrophic declines in honey bee populations due to Colony Collapse Disorder, has renewed interest in native pollinators and the ecosystem services they provide. Insects were collected from willow, Russian olive and salt cedar throughout April and May of 1997 and 1998 using sweep nets. For each collection day, nets were swept over the target shrubs for a specified number for passes to ensure equal collection effort. Insects were counted and identified to family. Total numbers were adjusted by number of sweep-days. Total insect abundance was greatest for willows (33.5 insects per sweep-day), followed by Russian olive (18.0) and salt cedar (6.8). Willows also had the greatest number of insect orders and families represented. Of the four primary insect pollinator orders, willow had the greatest numbers of dipterans, hymenopterans, and lepidopterans collected per sweep-day. Russian olive had the greatest number of coleopterans. When ants and chalcids were excluded from the hymenopterans, willows still had the greatest numbers and proportions of hymenopterans caught. It appears that the willow habitat is important to pollinating insects, especially bees. In contrast, saltcedar consistently had the lowest numbers and proportions of all four of the pollinator orders.

Blackbrush - Coleogyne ramosissima Torr. - grows in the transition zone between warm and cold deserts of southern California, southern Nevada, southern Utah, northern Arizona, and southwestern Colorado. It is found at elevations of 760 to 1,980 m. Ranging from 0.3 to 1.2 m in height, blackbrush forms almost monotypic stands in the lower Mojave-Great Basin ecotone, bounded by creosote bush (Larrea tridentata (Sesse & Moc.) ex DC. Coville.) communities at low elevations and by juniper-big sagebrush (Artemesia tridentata Nutt.) communities at higher elevations. In the eastern part of its range, blackbrush is bordered by Sonoran communities on the south and by big sagebrush, juniper, and mixed shrub communities of the Colorado Plateau in the north. Distribution of blackbrush is limited by soil depth, temperature extremes, and moisture availability.