Robert R. Blank

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).

PRESCRIBED FIRE EFFECTS ON FOREST FLOOR AND SOIL NUTRIENTS IN A SIERRA NEVADA FOREST

The objectives of this study were to quantify the effects of prescribed fire on forest floor C and nutrient content, soil chemical properties, and soil leaching in a Jeffrey pine (Pinus jeffreyi [Grev. and Balf.]) forest in the eastern Sierra Nevada Mountains of California. The study included a prescribed fire and three timber harvest treatments: whole-tree (WT) thinning, cut-to-length (CTL) thinning, and no harvest (CONT). Prescribed fire resulted in significant decreases in forest floor C (−8 to −23 mg ha−1, or 39% to 61% decrease), N (−114 to −252 kg ha−1, or −31% to 51% decrease), S (0 to −15 kg ha−1, or 0% to 48% decrease), and K (−3 to −45 kg ha−1, or 12% to 51% decrease) contents but no significant change in Ca or Mg contents. In each case, the decreases were greatest in the CTL treatment, where slash accumulation before burning was greatest. Burning caused statistically significant effects on soil total nitrogen, C:N ratio, pH, water-extractable ortho-P, and water-extractable SO42− in some cases, but these effects were generally small, inconsistent among harvest treatments and horizons, and in the case of ortho-P much less than the temporal variation in both burned and unburned plots. There were no statistically significant effects of burning on total C, Bray-extractable P, bicarbonate-extractable P, and exchangeable Ca2+, K+, or Mg2+. Burning had no significant effect on soil solution pH, ortho-P, SO42−, NO3−, or NH4+ as measured by ceramic cup lysimeters and no effect on the cumulative leaching of ortho-P, NO3−, or NH4+ as measured by resin lysimeters. Burning had no effect on needle weight or nutrient contents as measured by the vector analysis. We conclude that prescribed fire had minimal effects on soils or water quality at this site, and that the most ecologically significant effect was the loss of N from the forest floor.

Influence of the exotic invasive crucifer, Lepidium latifolium. on soil properties and elemental cycling

Plant species and plant communities can alter element cycling promoting divergent pathways of soil development. We hypothesized that the exotic invasive crucifer, Lepidium latifolium, altered biogeochemical cycling relative to pre-existing vegetation such that sodium-affected subsoils are ameliorated. We tested this hypothesis at the Fleming unit within the Honey Lake Wildlife Refuge in northeastern California by comparing the distribution and cycling of various elements in paired L. latifolium invaded and noninvaded sites occupied by Elytrigia elongata. Soils are fine, smectitic, calcareous, mesic Fluvaquentic Endoaquolls. In 1998, per unit area, live biomass of L. latifolium was more than 3 times greater than that of E. elongata. Content of C, Ca, Mg, K, and S in aboveground tissue of L. latifolium was significantly (P ≤ 0.05) greater than content in E. elongata. Biogeochemical fluxes of C, N, P, Ca, Mg, and S were from 3 to 11 times greater in L. latifolium-occupied areas than in E. elongata-occupied areas. L. latifolium invasion fostered elevated soil solution levels of Mg+2 and Ca+2, thereby reducing sodium adsorption ratios that could lead to sodic soil amelioration. Invasion by L. latifolium has the potential to alter soil properties and processes, thereby altering the trajectory of soil evolution.

Heated substrate and smoke: influence on seed emergence and plant growth.

Combustion products of burning vegetation can increase seed germination of many species of fire-prone plant communities. We tested the influence of heating sagebrush (Artemisia tridentata Nutt.) subcanopy soil, aqueous extracts of artificially burned soil, and sagebrush smoke on the emergence of several range plant species of the sagebrush-steppe. In addition, test seeds were exposed to sagebrush smoke and aqueous slurries of artificially burned sagebrush subcanopy soil to determine their effect on plant growth. As compared to the control, substrates previously heated from 250 to 750 degrees C significantly (P less than or equal to 0.05) increased the emergence of Thurbers needlegrass [Achnatherum thurberianum (Piper) Barkworth] and needle-and-thread [Hesperostipa comata (Trin. &Rupr.) Barkworth]. Sagebrush smoke and aqueous slurries of artificially burned soil significantly increased the emergence of Sierra Nevada needlegrass [Achnatherum occidentalis (Thurber) Barkworth], Indian ricegrass [Achnatherum hymenoides (Roemer &Schultes)Barkworth], and antelope bitterbrush [Purshia tridentata (Pursh) DC.]. Rates of new leaf production and leaf elongation following treatment of seeds with the smoke of burning sagebrush were significantly greater for cheatgrass (Bromus tectorum L.), basin wildrye [Leymus cinereus (Sribner &Merr.) A. Love], Idaho fescue (Festuca idahoensis Elmer ), Sierra Nevada needlegrass, and needle-and-thread as compared to the control. After 83 days of growth, smoke-treated seeds of basin wildrye and needle-and-thread produced significantly greater plant mass than their controls. Smoke treatment of certain seeds before sowing is potentially useful for range plant seedings.

Prescribed Fire, Soil, and Plants: Burn Effects and Interactions in the Central Great Basin

Abstract Pinyon and juniper expansion into sagebrush ecosystems results in decreased cover and biomass of perennial grasses and forbs. We examine the effectiveness of spring prescribed fire on restoration of sagebrush ecosystems by documenting burn effects on soil nutrients, herbaceous aboveground biomass, and tissue nutrient concentrations. This study was conducted in a central Nevada woodland and included control and burn treatment plots sampled before and after a prescribed fire. Six native understory plant species (Crepis acuminata, Eriogonum umbellatum, Eriogonum elatum, Poa secunda secunda, Festuca idahoensis, and Lupinus argenteus) important for native sagebrush obligate foragers were chosen to represent the understory plant community. L. argenteus is also important for system nutrient cycling and nitrogen fixation. Plants were collected from three microsites (under tree canopy, under shrub canopy, and interspace) common in transitional woodlands during peak growth the summer before a spring prescribed burn and each of two summers following the burn. Soils were collected from corresponding locations at two depth intervals (0–8 and 8–52 cm) to determine the relationships between soil and plant nutrients following fire. Microsite affected soil nutrients but did not influence plant tissue concentrations with the exception of F. idahoensis. Burning resulted in increases in soil surface NH+4, NO−3, inorganic N, Ca2+, Mn2+, and Zn2+. Increases in NO−3, inorganic N, and Zn2+ were also observed in deeper horizons. Burning did not affect aboveground plant biomass or nutrient concentrations in the first year with the exception of F. idahoensis, which had increased tissue P. By the second year, all species had statistically significant responses to burning. The most common response was for increased aboveground plant weight and tissue N concentrations. Plant response to burning appeared to be related to the burn treatment and the soil variables surface K+, NO−3, and inorganic N.

Nutrient Availability in Rangeland Soils: Influence of Prescribed Burning, Herbaceous Vegetation Removal, Overseeding with Bromus tectorum, Season, and Elevation

Abstract Soil nutrient availability influences plant invasions. Resin capsules were used to examine soil nutrient bioavailability along 2 sagebrush–grassland elevation transects in the east Tintic Range (Utah) and Shoshone Range (Nevada). In the fall of 2001, treatments were applied to 3 replicate plots at each site, which included prescribed burning, herbaceous vegetation removal, and controls. Cheatgrass (Bromus tectorum L.) was overseeded in small subplots within each treatment. Following treatments in each plot, resin capsules were installed at 15-cm depth in a shrub interspace and a B. tectorum–overseeded area. Nutrient availability was integrated during late fall to spring and spring to late fall for 2 years. Herbaceous vegetation removal increased availability of nitrate (Nevada and Utah) and Ca and Mg (Nevada only) but only during the second sampling period (growing season). Availability of K and ortho-P (Nevada and Utah) and nitrate (Nevada only) was greater on prescribed burned plots. For Utah, availability of ortho-P, K, Ca, Mg, and Fe generally increased with increasing elevation. Availability of Ca, Mg, K, and Fe was greatest during late fall to spring integration periods for Nevada. Overseeding with B. tectorum interacted with the burn treatment to influence availability of Ca, Mg, and Fe (Nevada sites only). Patterns of nutrient availability can be explained by a combination of decreased root uptake in relation to mineralization, differences in soil water content with season and elevation, and nutrient release from vegetation and soil as a consequence of prescribed burning. Herbaceous vegetation removal and burning can raise nitrate availability and increase risk of invasion by nitrophilic species such as B. tectorum. Nutrient availability can be out of phase with plant growth; plants capable of taking up nutrients during cold periods may have a competitive advantage. Resin capsules have utility in quantifying the effects of treatments on the availability of many soil nutrients.

Biogeochemistry of Plant Invasion: A Case Study with Downy Brome (Bromus tectorum)

Abstract Limited data exist on the affect of downy brome invasion on biogeochemical cycling. Biogeochemical cycling was quantified in a winterfat community in northeastern, CA that was separated into three invasion classes: noninvaded (NI), invaded for 3 yr (I3), and 10 yr (I10) by downy brome. On each plot, all aboveground vegetation was harvested and separated by species, dried, weighed, and tissue nutrients quantified. In addition, soil samples were collected from 0- to 30-, 30- to 60-, and 60- to 100-cm depths and various nutrient pools quantified. Aboveground biomass g/m2 was significantly greater, with downy brome averaging over 90% of the plant mass on the I10 plots (280 g) compared to the NI plots (148 g). In comparison to the NI plots, vegetation fluxes (g/m2/yr) of carbon (C) were significantly greater, and fluxes of Ca, Fe, and Cu were significantly less on I10 plots. Soils occupied for 10 yr by downy brome have significantly greater total N and organic C, and greater availability of Fe, Mn, Cu, ortho-P, Ca, and K compared to NI soil. For the I10 plots, available soil N (dominantly NO3−) was greatest in the 60- to 100-cm-depth increment, whereas for the other plots, N availability was greatest in the 0- to 30-cm-depth increment. Net N soil mineralization potential was near 0 on the I10 plots at all depth increments. These data suggest that invasion by downy brome facilitates elevated nutrient availability, possibly increases system leakiness of N, and fosters differential plant nutrient cycling relative to a native noninvaded community. Elevated nutrient availability promulgated by downy brome invasion might increase its competitive stature. Long-term occupation of environments by downy brome might affect the vertical distribution of nutrients, which can alter soil evolution and plant successional patterns. Nomenclature: Downy brome, Bromus tectorum L. BROTE.

Influence of three weed species on soil nutrient dynamics.

The hypotheses that weeds increase soil nutrient availability and differentially alter soil nutrient dynamics were tested in the greenhouse by comparing and contrasting plant-soil relationships among Bromus tectorum L. (cheatgrass, downy brome), Lepidium latifolium L. (perennial pep-perweed, tall whitetop), and Centaurea solstitialis L. (yellow starthistle). These species are highly invasive and produce dense stands that reduce or eliminate native species. A high resource (HR) soil and a low resource (LR) soil were evaluated. To simulate declining soil resources, individual plants were grown in the same pots, harvested, then replanted for a total of three growth cycles. Aboveground mass was greatest in the HR soil in which L. latifolium and C. solstitialis accumulated greatest biomass after the second growth cycle and B. tectorum accumulated greatest biomass after the third growth cycle. In the LR soil, all plants had the greatest aboveground mass after the third growth cycle. Overall, nutrient concentration in aboveground tissue declined significantly with growth cycle. Tissue K and P concentrations were highest in plants grown in the HR soil, whereas Ca and N were highest in plants grown in the LR soil. Nutrient uptake overall decreased with growth cycle for the HR soil and increased with growth cycle for the LR soil. Relative to unplanted controls, all three species facilitated an increase in soil-solution Ca and Mg in the HR soil, greater N-mineralization potential in both soils, and greater enzyme activity in the LR soil by the third growth cycle. The species displayed distinctly different ratios of nutrient uptake and concentration in aboveground tissue, with L. latifolium having much higher tissue Ca/Mg concentration ratios and uptake ratios of Ca/Mg, Ca/N, and Ca/P than C. solstitialis or B. tectorum. Stepwise regression suggests soils with high levels of available Mg may have a greater risk for invasion by C. solstitialis and L. latifolium. These data support the working hypotheses.

Transition From Sagebrush Steppe to Annual Grass (Bromus tectorum): Influence on Belowground Carbon and Nitrogen

Abstract Vegetation changes associated with climate shifts and anthropogenic disturbance have major impacts on biogeochemical cycling. Much of the interior western United States currently is dominated by sagebrush (Artemisia tridentata Nutt.) ecosystems. At low to intermediate elevations, sagebrush ecosystems increasingly are influenced by cheatgrass (Bromus tectorum L.) invasion. Little currently is known about the distribution of belowground organic carbon (OC) on these changing landscapes, how annual grass invasion affects OC pools, or the role that nitrogen (N) plays in carbon (C) retention. As part of a Joint Fire Sciences-funded project called the Sagebrush Treatment Evaluation Project (SageSTEP), we quantified the depth distribution of soil OC and N at seven sites experiencing cheatgrass invasion. We sampled plots that retained sagebrush, but represented a continuum of cheatgrass invasion into the understory. Eighty-four soil cores were taken using a mechanically driven diamond-tipped core drill to a depth of 90 cm, or until bedrock or a restrictive layer was encountered. Samples were taken in 15-cm increments, and soil, rocks, and roots were analyzed for OC and total N. We determined that cheatgrass influences the vertical distribution of OC and N within the soil profile and might result in decreased soil OC content below 60 cm. We also found that OC and total N associated with coarse fragments accounted for at least 10% of belowground pools. This emphasizes the need for researchers to quantify nutrients in deep soil horizons and coarse fragments.

Central Nevada riparian areas: physical and chemical properties of meadow soils.

Despite the importance of soil characteristics for classifying riparian ecosystem types and evaluating ecosystem or range condition, little information exists on western riparian area soils or the factors that influence them. We examined the effects of drainage basin geology and water table depth on soil morphology and soil physical and chemical properties of meadow sites in central Nevada. We described and analyzed the soils of meadows that occurred in 4 drainages with different geology and that exhibited high water tables (0 to ‐20 cm from the surface), intermediate water tables (‐30 to ‐50 cm), and low water tables (‐60 to ‐80 cm). Pedons of high water tables sites had thick O e horizons, dark, fine-textured A horizons, no B horizons, and lower C horizons high in coarse fragments. In contrast, pedons of low water tables sites were characterized by deep, dark and organic-rich A horizons, cambic B horizons, and deep rooting profiles. High water tables sites had higher organic matter, total nitrogen, cation exchange capacity, and extractable potassium, but lower pH than low water table sites. Also, high water table sites had lower percentage sand, lower bulk densities, and higher soil moisture retention. The importance of organic matter was evidenced by strong positive product moment correlations for organic matter and total nitrogen, cation exchange capacity, and extractable potassium. Significant differences in pH, extractable potassium and extractable phosphorus existed among drainages that were explainable largely from the parent materials. Drainages with chert, quartzite, and limestone had higher silt and clay, neutral pH, and high levels of extractable phosphorus. Drainages formed in acidic volcanic tuffs, rhyolites and breccia were characterized by coarser textured soils and low pH and extractable phosphorus. In riparian areas, soil water table depth interacts with soil parent material to significantly affect soil morphology and soil physical and chemical properties. Because these factors vary over both large and small spatial scales, differences among sites must be carefully interpreted when classifying ecosystems or evaluating ecosystem condition.