Jeanne C. Chambers

WHAT MAKES GREAT BASIN SAGEBRUSH ECOSYSTEMS INVASIBLE BY BROMUS TECTORUM

Ecosystem susceptibility to invasion by nonnative species is poorly understood, but evidence is increasing that spatial and temporal variability in resources has large-scale effects. We conducted a study in Artemisia tridentata ecosystems at two Great Basin locations examining differences in resource availability and invasibility of Bromus tectorum over elevation gradients and in response to direct and interacting effects of removal of perennial herbaceous vegetation and fire. We monitored environmental conditions, soil variables, and B. tectorum establishment and reproduction over two years. Soil water (measured as the number of days soil matric potential was .� 1.5 MPa) and nitrate availability (measured as micromoles of NO3 � sorbed to resin capsules per day in the ground) decreased with decreasing elevation. Lower-elevation sites had greater annual variability in soil water availability than upper-elevation sites did. Soil nitrate levels were highest at all elevations when soils were wettest; nitrate availability was not more variable at lower elevations. Removal of herbaceous perennials increased soil water and nitrate availability, but burning without removal had only minor effects. Bromus tectorum had low establishment, biomass, and seed production on high-elevation sites and on a mid-elevation site during a cold, short, growing season probably due to ecophysiological limitations resulting from cold temperatures. Establishment, biomass, and seed production were variable at low elevations and best explained by soil characteristics and spatial and temporal variation in soil water. Removal and fire had minor effects on emergence and survival, but biomass and seed production increased two to three times following removal, two to six times after burning, and 10-30 times following removal and burning. Our data indicate that invasibility varies across elevation gradients and appears to be closely related to temperature at higher elevations and soil water availability at lower elevations. High variability in soil water and lower average perennial herbaceous cover may increase invasion potential at lower elevations. Soil water and nitrate availability increase following either fire or removal, but on intact sites native perennials typically increase following fire, limiting B. tectorum growth and reproduction. Following resource fluctuations, invasibility is lowest on sites with relatively high cover of perennial herbaceous species (i.e., sites in high ecological condition).

SEED MOVEMENTS AND SEEDLING FATES IN DISTURBED SAGEBRUSH STEPPE ECOSYSTEMS: IMPLICATIONS FOR RESTORATION

Understanding species establishment patterns and community structure fol- lowing disturbance, and developing effective restoration methods requires knowledge of both the movements and fates of seeds. I used a restoration experiment in a severely disturbed sagebrush steppe ecosystem near Kemmerer, Wyoming to examine the effects of soil surface characteristics and seed morphology on seed entrapment and retention, and the effects of soil surface characteristics on soil water potential and seedling emergence and survival. Seeds of native species with awns, mucilaginous seedcoats, wings, hairy pappi, or no appendages were sown over soil surface treatments consisting of silty loam soil, sand, gravel, surface mulch, shrub mimics, and large and small holes. Seeds that lacked ap- pendages and that had small surface areas did not exhibit significant horizontal movement or redistribution. Seeds with appendages that resulted in exposure of a large surface area to the wind did exhibit significant redistribution, despite apparent adaptations for seed burial or retention. When the entire seed population was considered, the effectiveness of the treatments for trapping and retaining seeds was large holes . small holes

Seed and Seedling Ecology of Pinon and Juniper Species in the Pygmy Woodlands of Western North America

gravel

Alpine Seedling Establishment: The Influence of Disturbance Type

shrub mimics . soil

Resilience to Stress and Disturbance, and Resistance to Bromus tectorum L. Invasion in Cold Desert Shrublands of Western North America

sand. Surface mulch neither gained nor lost seeds. The most effective treatments for seedling emergence had among the least negative soil water potentials and included large holes, surface mulch, and sand. Gravel provided an inadequate growing medium, and both shrub mimics and small holes accumulated fine-textured soils resulting in highly negative water potentials and low seedling emergence. Once a seedling emerged, the probability of survival was reasonably high (56.3% over 2 yr) regardless of treatment. This study indicates that soil surfaces that trap and retain high densities of seeds with large surface areas may have little or no effect on seeds with small surface areas, and may or may not result in high seedling emergence and survival. Restoring diverse native ecosystems requires creating soil surface features that can trap and retain seeds with varying mor- phologies as well as provide favorable conditions for seedling establishment.

SOIL-PLANT RELATIONS ALONG A SOIL-WATER GRADIENT IN GREAT BASIN RIPARIAN MEADOWS

Knowledge of the seed and seedling ecology of the piñon and juniper woodlands of western North America is essential for understanding both the northward migration and expansion of the woodlands during the Holocene (< 11,500 B.P.), and the accelerated expansion of the woodlands since settlement of the West by Anglo-Americans around 200 years ago. We follow the fates of seeds and seedlings of the different piñon and juniper species within the woodlands from seed development to seedling establishment, and discuss the implications of this information for the past and present expansion of the woodlands. While seed development requires about two and one-half years in pinons, it is species-dependent in junipers and can take one, two, or even three years. Substantial seed losses can occur during seed development due to developmental constraints, and before or after seed maturation as a result of insects, pathogens, or predatory animals. In piñon pines, the primary seed dispersers are scatterhoarding birds (corvids) and rodents that harvest seeds from the trees or after seed fall and cache them in the soil. In contrast, most junipers appear to be dispersed primarily by frugivorous birds and mammals that ingest the seeds and defecate them onto the soil surface. We have recently documented that scatter-hoarding rodents also disperse juniper seeds. Disperser effectiveness, or the contribution a disperser makes to the future reproduction of a plant population, may vary among species of piñons and especially junipers. Piñon seeds are short-lived and exhibit little dormancy, and they probably only germinate the spring following dispersal. Juniper seeds are long-lived and seed dispersal can occur over one or more years. Seed germination can be delayed for several years due to impermeable seed coats, embryo dormancy, or the presence of inhibitors. Seedling establishment of piñon pines is facilitated by nurse plants but, while junipers often establish beneath nurse plants, they are capable of establishing in open environments. In the southwestern United States, higher establishment of juniper occurs in open environments due to more favorable precipitation, and competition may be more important than facilitation in determining establishment.When considering the mechanisms involved in the past and present expansion of the woodlands, short-distance dispersal, local population growth, and long-distance dispersal are all important. Different classes of dispersers, some of which appear to have coevolved with the tree species, appear to be responsible for local (short-distance) vs. long-distance dispersal in pinons and junipers. Because ecotones form the interface between the woodlands and adjacent communities, they can provide valuable information on both the seed dispersal and seedling establishment processes responsible for tree expansion.Disturbance regimes and, recently, the effects of humans on those regimes have major effects on the expansion and contraction of the woodlands. Before Anglo-American settlement, fires occurred as frequently as every 50–100 years throughout much of the woodlands. During this century, fire frequencies have been reduced due to the indirect effects of livestock grazing and the direct effects of removing Native Americans from the ecosystem and implementing active fire-prevention programs. The result has been an increase in tree-dominated successional stages at the expense of grass-dominated stages. Various management techniques, including controlled burning and chaining, have been implemented to reduce tree dominance, but their effects depend largely on the life histories of the tree species and the disturbance characteristics. Several areas relating to the seed and seedling ecology of the piñon and juniper require additional research if we are to truly understand the dynamics of the woodlands.

Use of native plants on federal lands: policy and practice.

The effects of disturbance type on seedling environment and establishment of alpine species with different physiological and life history traits were examined during a 2-yr study on the Beartooth Plateau in southwestern Montana, USA. We compared soil temperatures, water potentials, and nutrients on mineral soils of a gravel borrow area with those on highly organic soils of a Geum turf area. Seedling emergence, growth, and survival of six seeded species (Geum rossii, Artemisia scopulorum, Potentilla diversifolia, Sibbaldia procumbens, Deschampsia cespitosa, and Festuca idahoensis) and emergence and survival of five unseeded species (Draba crassifolia, Draba incerta, Cerastium arvense, Arenaria rubella, and Androsace septentrionalis) were evaluated on both areas. The effects of N and P nutrient addition and surface organic mulch on the soil environment and seedling es- tablishment were evaluated on the borrow area, while differences between uncleared turf and turf cleared of vegetation were compared on the Geum turf area. Plots cleared of vegetation on the Geum turf area had higher levels of soil N (NO3-) and P than uncleared turf and both higher levels of N (NO3- + NH4+) and P and higher soil temperatures (surface, 5, and 15 cm depths) than fertilized or not-fertilized borrow area treatments. Fertilization increased N and P on borrow area soils, but after 2 yr N had decreased significantly. Soil water potentials (5 and 15 cm depths) did not differ between cleared plots on the Geum turf area or any of the borrow area treatments and were never low enough to cause plant stress. Vegetated Geum turf had significantly lower water po- tentials than cleared plots, especially late in the growing season. Mulch had no effect on soil water potential or nutrients on the borrow area and increased soil temperatures only on clear days during the first growing season. Wind removed or redistributed the mulch over time, thus decreasing potential effects. Seedling emergence was highly dependent on soil surface stabilization and reflected species life history traits. Growth of seedlings was slow, and varied among species and treatments: 0.005-0.04 and 0.02-0.20 g total mass after the first and second growing seasons, respectively. Significantly higher total seedling mass was observed on cleared Geum turf plots than on any of the borrow area treatments, and on fertilized than on not-fertilized plots on the borrow area. Seedling mortality of most species was much lower than previously found for alpine ecosystems, rarely exceeding 50% even after 2 yr. On the borrow area mulch increased survival, probably through microenvironmental amelioration. The nu- trient pulse from fertilization increased mortality of several species, presumably by creating plant nutrient demands in excess of availability during year 2. Both disturbance characteristics and species life history and physiological traits affected seedling establishment. Pretreatment soil properties of the two disturbance types had the greatest effects on soil temperatures and nutrients and, consequently, on seedling growth and survival. Soil surface characteristics had the largest effects on seedling emergence; surface stabilization was essential for holding both soil and seed in place. Single species responses varied in magnitude but were similar on both disturbance types. In general, there were larger differences among species in emergence and growth than in survival. Thus, successful seedling establishment on different alpine disturbance types may depend more

Relationships between seed fates and seedling establishment in an alpine ecosystem

Alien grass invasions in arid and semi-arid ecosystems are resulting in grass–fire cycles and ecosystem-level transformations that severely diminish ecosystem services. Our capacity to address the rapid and complex changes occurring in these ecosystems can be enhanced by developing an understanding of the environmental factors and ecosystem attributes that determine resilience of native ecosystems to stress and disturbance, and resistance to invasion. Cold desert shrublands occur over strong environmental gradients and exhibit significant differences in resilience and resistance. They provide an excellent opportunity to increase our understanding of these concepts. Herein, we examine a series of linked questions about (a) ecosystem attributes that determine resilience and resistance along environmental gradients, (b) effects of disturbances like livestock grazing and altered fire regimes and of stressors like rapid climate change, rising CO2, and N deposition on resilience and resistance, and (c) interacting effects of resilience and resistance on ecosystems with different environmental conditions. We conclude by providing strategies for the use of resilience and resistance concepts in a management context. At ecological site scales, state and transition models are used to illustrate how differences in resilience and resistance influence potential alternative vegetation states, transitions among states, and thresholds. At landscape scales management strategies based on resilience and resistance—protection, prevention, restoration, and monitoring and adaptive management—are used to determine priority management areas and appropriate actions.

Resilience and Resistance of Sagebrush Ecosystems: Implications for State and Transition Models and Management Treatments

Throughout the western states, riparian ecosystems have been affected by water diversions or spring and seep developments that decrease the quantity of instream flows and result in lowered water-tables. Water extraction is especially damaging in arid and semi-arid regions where the presence of instream and ground-water flows are crucial to riparian vegetation. We examined the temporal and spatial relationships between hydrologic gradients, vegetation, and soils in two central Nevada riparian meadows in order to identify plant species and environmental variables that can serve as indicators of water-table status. Species frequency and aerial cover, ground-cover composition, depth to water-table, and soil morphological and physical properties were measured along hydrologic gradients within two riparian meadow complexes. TWINSPAN, cluster, and multivariate discriminant analyses classified the vegetation into four ecosystem types. These occurred along the hydrologic gradient and included, from wettest to driest, wet meadow, mesic meadow, dry meadow, and basin big sagebrush meadow types. Canonical correspondence analysis (CCA) indicated that the variables most strongly related to plant species frequency within both meadows were those associated with depth to water-table. Integrative variables, including the number of days that depth to water-table was less than 30 cm and less than 70 cm, and degree-days of anaerobiosis were most closely related to the wet and mesic meadow vegetation types. The range in depth to water-talbe, elevation, and aerial cover of gravel and litter were all related to the dry and sagebrush meadow vegetation types. Indicator species associated with particular water-table regimes were identified for each vegetation type based on two-way ordered tables. Carex nebrascensis, an obligate riparian species, occurred at water-tables of 0–30 cm below the surface and was the most reliable indicator of shallow water-tables. Large temporal and spatial variability in water-table depths for the mesic and dry meadow types suggests that species associated with these types could be used only to indicate broad ranges in water-table depth. Integrative environmental variables that incorporated the temporal variation in water-tables (i.e., days that depth to water-table was less than 30 and 70 cm; degree days of anaerobiosis; range in water-table depth during the growing season) demonstrated closer relationships to the vegetation types than water-table alone. They were also more sensitive to the spatial and temporal differences in water-tables than individual plant species or vegetation types. Environmental and integrative environmental variables may respond more quickly to changes in local hydrology than plant species and are possibly more sensitive indicators of both current water-table status and potential vegetation.

Long-term effects of seeding after wildfire on vegetation in Great Basin shrubland ecosystems

Changing social values and advances in ecological knowledge determine native seed policy for revegetating range and forest lands. Natural resource managers are shifting from seeding introduced species for their widespread adaptability to reestablishing native species in order to maintain or restore the genetic and ecological integrity of naive ecosystems. Addressing the problems of reestablishing native plants on a site-specific basis has been increasingly recognized as an integral part of ecosystem management of large landscapes. We review the formation and implementation of native seed policy for fire rehabilitation and mining reclamation by the major federal land management agencies in the United States, the Department of Interiors Bureau of Land Management (BLM) and the Department of Agricultures Forest Service. We then examine native seed policy implementation on specific land revegetation projects over the past 10 years for 4 BLM districts in the state of Nevada. We conclude with an analysis of native seed policy in principle versus practice and suggest implications for future policy review and implementation.