Paul S. Doescher

Understory vegetation in old and young Douglas-fir forests of western Oregon

Abstract We studied understory composition in thinned and unthinned Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco)/western hemlock ( Tsuga heterophylla (Raf.) Sarg.) stands on 28 sites in western Oregon. These stands had regenerated naturally after timber harvest, 40–70 years before thinning. Commercial thinning had occurred 10–24 years previously, with 8–60% of the volume removed from below with the intent to homogenize spacing among trees. Undisturbed old-growth Douglas-fir stands were present for comparison on 18 of these sites. Total herbaceous cover was greater in thinned (25% cover) stands than in unthinned (13% cover) or old-growth (15% cover) stands. Species richness was also greater in thinned (137) than in unthinned (114) and old-growth (91) stands ( P =0.05). Part of the increased richness was caused by the presence of exotic species in thinned stands, but there were also more native grass and nitrogen-fixing species in thinned stands than in unthinned or old-growth stands. Groups of species differed among stand-types. For example, the frequency of tall cordate-leaved species was greater in old-growth stands ( P =0.009), but their relative cover was different only between old-growth and unthinned stands ( P =0.08). Both the cover and frequency of grasses and sedges in thinned stands were greater than in unthinned or old-growth stands ( P ≤0.002). Ordination of shrub cover showed differences among old-growth and unthinned stands compared to thinned stands, mainly because of the amount of Gaultheria shallon Pursh and Polystichum munitum (Kaulf.) Presl in heavily thinned stands. Ordination of herbaceous community data showed that there were much stronger differences among sites than among stand-types. The lack of difference among stand-types demonstrates the resiliency of herbaceous communities to disturbance associated with past and current forest management.

Conditions favouring Bromus tectorum dominance of endangered sagebrush steppe ecosystems

Summary 1. Ecosystem invasibility is determined by combinations of environmental variables, invader attributes, disturbance regimes, competitive abilities of resident species and evolutionary history between residents and disturbance regimes. Understanding the relative importance of each factor is critical to limiting future invasions and restoring ecosystems. 2. We investigated factors potentially controlling Bromus tectorum invasions into Artemisia tridentata ssp. wyomingensis communities across 75 sites in the Great Basin. We measured soil texture, cattle grazing intensity, gaps among perennial plants and plant cover including B. tectorum, biological soil crusts (BSCs) and bare soil. Using a priori knowledge, we developed a multivariate hypothesis of the susceptibility of Artemisia ecosystems to B. tectorum invasion and used the model to assess the relative importance of the factors driving the magnitude of such invasions. 3. Model results imply that bunchgrass community structure, abundance and composition, along with BSC cover, play important roles in controlling B. tectorum dominance. Evidence suggests abundant bunchgrasses limit invasions by limiting the size and connectivity of gaps between vegetation, and BSCs appear to limit invasions within gaps. Results also suggest that cattle grazing reduces invasion resistance by decreasing bunchgrass abundance, shifting bunchgrass composition, and thereby increasing connectivity of gaps between perennial plants while trampling further reduces resistance by reducing BSC. 4. Synthesis and applications. Grazing exacerbates Bromus tectorum dominance in one of North America’s most endangered ecosystems by adversely impacting key mechanisms mediating resistance to invasion. If the goal is to conserve and restore resistance of these systems, managers should consider maintaining or restoring: (i) high bunchgrass cover and structure characterized by spatially dispersed bunchgrasses and small gaps between them; (ii) a diverse assemblage of bunchgrass species to maximize competitive interactions with B. tectorum in time and space; and (iii) biological soil crusts to limit B. tectorum establishment. Passive restoration by reducing cumulative cattle grazing may be one of the most effective means of achieving these three goals.

The Sagebrush Steppe Treatment Evaluation Project (SageSTEP): a test of state-and-transition theory

The Sagebrush Steppe Treatment Evaluation Project (SageSTEP) is a comprehensive, integrated, long-term study that evaluates the ecological effects of fire and fire surrogate treatments designed to reduce fuel and to restore sagebrush (Artemisia spp.) communities of the Great Basin and surrounding areas. SageSTEP has several features that make it ideal for testing hypotheses from state-and-transition theory: it is long-term, experimental, multisite, and multivariate, and treatments are applied across condition gradients, allowing for potential identification of biotic thresholds. The project will determine the conditions under which sagebrush steppe ecological communities recover on their own following fuel treatment versus the communities crossing ecological thresholds, which requires expensive active restoration.

Seed and seedling traits affecting critical life stage transitions and recruitment outcomes in dryland grasses

Summary Seeding native plants is a key management practice to counter land degradation across the globe, yet the majority of seeding efforts fail, limiting our ability to accelerate ecosystem recovery. Recruitment requires transitions through several seed and seedling stages, some of which may have overriding influences on restoration outcomes. We lack, however, a general framework to understand and predict differences in these critical demographic processes across species. Functional traits influence fitness, and consequently, trait variation could provide the basis for a framework to explain and predict variation in life stage transition probabilities. We used seed and seedling traits, and field probabilities of germination, emergence, seedling establishment, and survival for 47 varieties of drylands grasses under two watering treatments to identify critical life stage transitions and quantify the effect of traits on cumulative survival through the first growing season. Variation in germination and emergence probabilities explained over 90% of the variation in cumulative survival regardless of seedling survival probabilities or watering treatment, with emergence probability being the strongest predictor of cumulative survival. Coleoptile tissue density and seed mass had significant effects on emergence and germination, respectively, explaining 10–23% of the variation in transition probabilities. Synthesis and applications. While the majority of functional trait work has centred on linking leaf and root traits to resource acquisition and utilization, our study demonstrates that traits associated with germination and emergence may have prevailing influences on restoration outcomes. A portion of these traits have been examined, but there is substantial opportunity to identify other key traits driving these demographic processes. These advancements will underpin our ability to develop trait-based frameworks for overcoming recruitment barriers and facilitating recovery of degraded systems across the globe.

Effects of Nitrogen Availability and Cheatgrass Competition on the Establishment of Vavilov Siberian Wheatgrass

Abstract Cheatgrass (Bromus tectorum L.) is the most widespread invasive weed in sagebrush ecosystems of North America. Restoration of perennial vegetation is difficult and land managers have often used introduced bunchgrasses to restore degraded sagebrush communities. Our objective was to evaluate the potential of ‘Vavilov’ Siberian wheatgrass (Agropyron fragile [Roth] P. Candargy) to establish on cheatgrass-dominated sites. We examined Vavilov establishment in response to different levels of soil nitrogen availability by adding sucrose to the soil to promote nitrogen (N) immobilization and examined cheatgrass competition by seeding different levels of cheatgrass. We used a blocked split-split plot design with two sucrose levels (0 and 360 g · m−2), two levels of Vavilov (0 and 300 seeds · m−2), and five levels of cheatgrass (0, 150, 300, 600, and 1 200 seeds · m−2). Seeding was conducted in fall 2003 and 2004, and measurements were taken in June 2004, 2005, and 2006. Sucrose addition decreased availability of soil nitrate but not orthophosphate. In the first year after seeding, sucrose reduced cheatgrass density by 35% and decreased both cheatgrass biomass per square meter and seed production per square meter by 67%. These effects were temporary, and by the second year after seeding, there was a sevenfold increase in cheatgrass density. As a result, the effects of sucrose addition were no longer significant. Sucrose affected Vavilov growth, but not density, during the first year after seeding. Vavilov density decreased as cheatgrass seeding density increased. Short-term reductions in N or cheatgrass seed supply did not have long-term effects on cheatgrass and did not increase Vavilov establishment. Longer-term reductions in soil N, higher seeding densities, or more competitive plant materials are necessary to revegetate areas dominated by cheatgrass.

Persistence of Idaho fescue on degraded rangelands: adaptation to defoliation or tolerance.

Rangelands with histories of overgrazing are frequently depauperate of native grasses. Occasionally, remnant native grasses are found surviving in these areas. We hypothesized that these survivors have responded to livestock grazing, over the past 110 years, through development of genetically based ecotypes that are more tolerant of defoliation than populations protected from heavy use by domestic livestock. Transplanted individuals of a native grass, Idaho fescue (Festuca idahoensis Elmer), from heavily grazed and ungrazed rangelands were compared. Gardens were established in central Oregon at the Central Oregon Agricultural Experiment Station and in eastern Oregon at the Northern Great Basin Experimental Range. Plants were defoliated during the vegetative, boot, and anthesis stages in 1990 and 1991 and subsequent growth evaluated. Parameters measured were end of growing season basal area, relative biomass production, and height and phenology at about biweekly intervals. Grazing history had no consistent effect on Idaho fescue response to defoliation. There were, however, differences between the protected and grazed collections from central Oregon in that the protected population averaged greater height and relative growth than those from the grazed areas even with defoliation. While the limited number of ungrazed sources in this region limits broad speculation, these results suggest idaho fescue survival in heavily grazed areas might be the result of differences in growth form rather than overcompensation or variation in time of phenologic development. Results also suggest that Idaho fescue from this region may elicit some grazing tolerance despite evolving historically with few large herbivores.

Region-Wide Ecological Responses of Arid Wyoming Big Sagebrush Communities to Fuel Treatments

Abstract If arid sagebrush ecosystems lack resilience to disturbances or resistance to annual invasives, then alternative successional states dominated by annual invasives, especially cheatgrass (Bromus tectorum L.), are likely after fuel treatments. We identified six Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis Beetle & Young) locations (152–381 mm precipitation) that we believed had sufficient resilience and resistance for recovery. We examined impacts of woody fuel reduction (fire, mowing, the herbicide tebuthiuron, and untreated controls, all with and without the herbicide imazapic) on short-term dominance of plant groups and on important land health parameters with the use of analysis of variance (ANOVA). Fire and mowing reduced woody biomass at least 85% for 3 yr, but herbaceous fuels were reduced only by fire (72%) and only in the first year. Herbaceous fuels produced at least 36% more biomass with mowing than untreated areas during posttreatment years. Imazapic only reduced herbaceous biomass after fires (34%). Tebuthiuron never affected herbaceous biomass. Perennial tall grass cover was reduced by 59% relative to untreated controls in the first year after fire, but it recovered by the second year. Cover of all remaining herbaceous groups was not changed by woody fuel treatments. Only imazapic reduced significantly herbaceous cover. Cheatgrass cover was reduced at least 63% with imazapic for 3 yr. Imazapic reduced annual forb cover by at least 45%, and unexpectedly, perennial grass cover by 49% (combination of tall grasses and Sandberg bluegrass [Poa secunda J. Presl.]). Fire reduced density of Sandberg bluegrass between 40% and 58%, decreased lichen and moss cover between 69% and 80%, and consequently increased bare ground between 21% and 34% and proportion of gaps among perennial plants > 2 m (at least 28% during the 3 yr). Fire, mowing, and imazapic may be effective in reducing fuels for 3 yr, but each has potentially undesirable consequences on plant communities.

Gas exchange of Idaho fescue in response to defoliation and grazing history.

We tested the hypothesis that prior grazing history would influence the defoliation responses of Idaho fescue (Festuca idahoensis) growing in a common garden environment. Plants were taken from a grazed pasture and adjacent exclosure which had not been grazed since 1937, and established in a common garden at 1 m spacings during spring of 1989. Plants from defoliated and nondefoliated treatments within the 2 populations were sampled during 1992 and 1993. Photosynthesis, conductance to H2O, and xylem potentials were measured during the 2 growing seasons, and carbon isotope ratio (delta 13C) was measured for senescent leaf tissue. Both within exclosure and outside exclosure defoliated plants exhibited compensatory photosynthesis that averaged a 12% increase the first year, and a 52% increase during the second year, compared with nondefoliated plants. No differences in photosynthesis occurred between the 2 collections. However, outside exclosure plants had higher stomatal conductance than did exclosure plants for the dry year 1992. Also, outside exclosure plants exhibited more negative delta 13C (thus lower water use efficiency) than exclosure plants for 1992 and 1993. We suggest that the higher conductance of previously-grazed plants relative to nongrazed plant populations may be an adaptive response to greater soil moisture often found in grazed sites.

Effect of competition by cheatgrass on shoot growth of Idaho fescue.

Ability to compete with alien weeds may be one factor enabling high-seral, native bunchgrasses to persist on degraded rangelands. This study examined the effect of competition from cheatgrass (Bromus tectorum L.) on shoot growth of Idaho fescue (Festuca idahoensis. Elmer). Four Idaho fescue collections were obtained from degraded rangelands, while the fifth was from a site in high ecological condition. Plants were established in pots in a greenhouse with 2 watering regimes, and ratios of Idaho fescue:cheatgrass of 1:0, 1:5, and 1:10. Plants were grown for 56 days. Increasing competition from cheatgrass depleted soil moisture and reduced growth of Idaho fescue. However, Idaho fescue produced greater tiller and leaf numbers than cheatgrass. Idaho fescue plants from the pristine population produced 0.57 g aboveground biomass while plants from the degraded sites produced 0.31 g. Aboveground biomass from the pristine population was reduced 35% and 56% at the 1:5 and 1:10 competition levels respectively, compared to the control (1:0 ratio). Aboveground biomass of plants from the degraded populations was similar to the control at the 1:5 level, and was reduced 32% at the 1:10 level. These results indicated that Idaho fescue from the degraded sites exhibits a different response to competition from cheatgrass than Idaho fescue from the pristine site. This information may prove useful in selecting ecotypes of Idaho fescue for range revegetation.

Effect of temperature on growth of cheatgrass and Idaho fescue.

Development of deep and extensive root systems especially at cold temperatures has been considered an advantage to successful establishment of grass species in arid environments. This study determined the effects of temperature on seedling root and shoot growth of cheatgrass (Bromus tectorum L.) and 5 collections of Idaho fescue (Festuca idahoensis Elmer). Four collections of Idaho fescue were from degraded sites while the fifth Idaho fescue collection was from a site in high ecological condition. Seedlings were grown in environmental chambers (16 hours day/8 hours night) at 5, 10, and 15 degrees C. Root depth was recorded weekly for 9 weeks, and seedlings were harvested after 63 days. Tiller and leaf number, below and above-ground biomass, and total root length were evaluated. At temperatures of 5, 10, and 15 degrees C, cheatgrass grew faster and produced a greater mass of roots and shoots than Idaho fescue. Root and shoot growth were similar for the 5 Idaho fescue collections at all temperatures. Idaho fescue collections produced more tillers than cheatgrass, except at 5 degrees C. These results indicated that cheatgrass produces greater root and shoot growth mass, but tillers less at warmer temperatures than Idaho fescue.