Research and Publications
Adaptive variation among plant populations must be known for effective conservation and restoration of imperiled species and predicting their responses to a changing climate. Common‐garden experiments, in which plants sourced from geographically distant populations are grown together such that genetic differences may be expressed, have provided much insight on adaptive variation. Common‐garden experiments also form the foundation for climate‐based seed‐transfer guidelines. However, the spatial scale at which population differentiation occurs is rarely addressed, leaving a critical information gap for parameterizing seed‐transfer guidelines and assessing species’ climate vulnerability. We asked whether adaptation was evident among populations of a foundational perennial within a single “empirical” seed‐transfer zone (based on previous common‐garden findings evaluating very distant populations) but different “provisional” seed zones (groupings of areas of similar climate and are not parameterized from common‐garden data). Seedlings from three populations originating from similar conditions within an intermediate elevation were planted into gardens nearby at the same elevation, or 250–450 m higher or lower in elevation and 0.4–25 km away. Substantial variation was observed between gardens in survival (ranging 2%–99%), foliar crown volume (7.8–22.6 dm3), and reproductive effort (0%–65%), but not among the three transplanted populations. The between garden variation was inversely related to climatic differences between the gardens and seed‐source populations, specifically the site differences in maximum–minimum annual temperatures. Results suggest that substantial site‐specificity in adaptation can occur at finer scales than is accounted for in empirical seed‐transfer guidance when the guidance is derived from broadscale common‐garden studies. Being within the same empirical seed zone, geographic unit, and even within 10 km distance may not qualify as “local” in the context of seed transfer. Moving forward, designing common‐garden experiments so that they allow for testing the scale of adaptation will help in translating the resulting seed‐transfer guidance to restoration projects.
Western US sagebrush ecosystems are threatened due to multiple interacting factors: encroachment by conifer woodlands, exotic annual grass invasion, severe wildfire, climate change, and anthropogenic development. Restoration of these communities is primarily focused on reducing conifer species such as western juniper, with the goal of increasing native herbaceous perennials and sagebrush and decreasing exotic annual grass invasion. Assessing the long-term success of restoration treatments is critical for informing future management and treatment strategies since short-term patterns do not generally predict long-term trends. Using a designed experiment from a Wyoming big sagebrush community that was established in 2008, we examined the long-term vegetation response to juniper removal and seeding (cultivar and local) in disturbed and undisturbed areas (slash pile, skid trails, no disturbance). We also examined the landscape scale plant response to juniper removal using repeatedly measured randomly located transects across two restoration units. We found that seeded species persisted in the long term and also mitigated exotic grass increases.
Big sagebrush and scabland sagebrush communities responded uniquely to multiple fires, due to different fuel loadings, fire severities, succession and invasion dynamics. Big sagebrush experienced nearly complete shrub loss and conversion from exotic invaded‐shrubland to exotic annual grassland after only one fire. In contrast, scabland sagebrush retained a minor shrub component and higher relative cover of native herbaceous species, even after three fires. Both communities retained cover of native perennial grasses, including shallow‐ and deep‐rooted species, likely reflecting decreasing fire intensity with number of times burned.
Woodland encroachment is a global issue linked to diminished ecosystem services, prompting the need for restoration efforts. However, restoration outcomes can be highly variable, making it difficult to interpret the ecological benefits and risks associated with woodland-reduction treatments within semiarid ecosystems. We addressed this uncertainty by assessing the magnitude and direction of vegetation change over a 15-year period at 129 sagebrush (Artemisia spp.) sites following pinyon (Pinus spp.) and juniper (Juniperus spp.) (P–J) reduction. Pretreatment vegetation indicated strong negative relationships between P–J cover and the abundance of understory plants (i.e., perennial grass and sagebrush cover) in most situations and all three components differed significantly among planned treatment types. Thus, to avoid confounding pretreatment vegetation and treatment type, we quantified overall treatment effects and tested whether distinct response patterns would be present among three dominant plant community types that vary in edaphic properties and occur within distinct temperature/precipitation regimes using meta-analysis (effect size = lnRR = ln[posttreatment cover/pretreatment cover]). We also quantified how restoration seedings contributed to overall changes in key understory vegetation components. Meta-analyses indicated that while P–J reduction caused significant positive overall effects on all shrub and herbaceous components (including invasive cheatgrass [Bromus tectorum] and exotic annual forbs), responses were contingent on treatment- and plant community-type combinations. Restoration seedings also had strong positive effects on understory vegetation by augmenting changes in perennial grass and perennial forb components, which similarly varied by plant community type. Collectively, our results identified specific situations where broad-scale efforts to reverse woodland encroachment substantially met short-term management goals of restoring valuable ecosystem services and where P–J reduction disposed certain plant community types to ecological risks, such as increasing the probability of native species displacement and stimulating an annual grass-fire cycle. Resource managers should carefully weigh these benefits and risks and incorporate additional, appropriate treatments and/or conservation measures for the unique preconditions of a given plant community in order to minimize exotic species responses and/or enhance desirable outcomes.
This study evaluated the long-term (13 years post-treatment) effectiveness of prescribed fire and mechanical tree removal to re-establish sagebrush steppe vegetation and associated spatial patterns in ground surface conditions and soil hydrologic properties of two woodland-encroached sites. Specifically, we assessed the effects of tree removal on: (1) vegetation and ground cover at the hillslope scale (990 m2 plots) and (2) associated spatial patterns in point-scale ground surface conditions and soil hydrologic properties along transects extending from tree bases and into the intercanopy areas between trees. Both sites were in mid to late stages of woodland encroachment with extensive bare conditions (~60–80% bare ground) throughout a degraded intercanopy area (~75% of the domain) surrounding tree islands (~25% of domain, subcanopy areas). All treatments effectively removed mature tree cover and increased hillslope vegetation. Enhanced herbaceous cover (4–15-fold increases) in burned areas reduced bare interspace (bare area between plants) by at least 4-fold and improved intercanopy hydraulic conductivity (> than 2-fold) and overall ecohydrologic function. Mechanical treatments retained or increased sagebrush and generally increased the intercanopy herbaceous vegetation. Intercanopy ground surface conditions and soil hydrologic properties in mechanical treatments were generally similar to those in burned areas but were also statistically similar to the same measures in untreated areas in most cases. This suggests that vegetation and ground surface conditions in mechanical treatments are trending toward a significantly improved hydrologic function over time. Treatments had limited impact on soil hydrologic properties within subcanopy areas; however, burning did reduce the soil water repellency strength and the occurrence of strong soil water repellency underneath trees by three- to four-fold. Overall, the treatments over a 13-year period enhanced the vegetation, ground surface conditions, and soil hydrologic properties that promote infiltration and limit runoff generation for intercanopy areas representing ~75% of the area at the sites. However, ecological tradeoffs in treatment alternatives were evident. The variations in woodland responses across sites, treatments, and measurement scales in this long-term study illustrate the complexity in predicting vegetation and hydrologic responses to tree removal on woodland-encroached sagebrush sites and underpin the need and value of multi-scale long-term studies.
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Shifts from longer seasonal snowpacks to shorter, ephemeral snowpacks (snowpacks that persist for <60 days) due to climate change will alter the timing and rates of water availability. Ephemeral snowmelt has less predictable timing and lowers soil water availability during the growing season. The Great Basin, United States is an ideal system to study snow ephemerality across a broad climate gradient. To identify the climatic controls on snow ephemerality, we analysed moderate resolution imaging spectroradiometer (MODIS) snow‐covered products from water years 2001–2015 using an object‐based mapping approach and a random forest model. Winter temperature and precipitation were the most influential variables on the maximum snow duration. We predict that warming the average winter air temperature by 2 and 4°C would reduce the areal extent of seasonal snow by 14.7 and 47.8%, respectively (8.8% of the Great Basin’s areal extent is seasonal in the historical record), with shifts to ephemeral snowpack concentrated in lower elevations and warmer regions. The combination of warming and interannual precipitation variability (i.e., reductions of 25%) had different effects on vegetation types. Vegetation types that have had consistent seasonal snow cover in their historical record are likely to have lower resilience to a new hydrologic regime, with earlier and more intermittent snowmelt causing a longer but drier growing season. Implications of increased snow ephemerality on vegetation productivity and susceptibility to disturbance will depend on local topography, subsurface water storage, and physiological adaptations. Nevertheless, patterns found in this study can help target management intervention to species that are most at risk.
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What may appear at first glance as a sea of sagebrush is in reality a complex and diverse ecosystem with a wide variety of plants and animals. The sagebrush steppe teems with life, but threats such as wildfire, grazing and invasive species are affecting the resilience of rangeland across the Northwest. Learn more about the groups of plants that make up a healthy rangeland ecosystem.
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Grazing by domestic livestock is a ubiquitous land use in the sagebrush (Artemisia spp.) biome of western North America. Widespread, long‐term population declines in greater sage‐grouse (Centrocercus urophasianus) have elicited concern about potential negative effects of livestock management practices on sage‐grouse populations. We evaluated how recently implemented rotational grazing systems affected sage‐grouse nesting habitat quality as part of a large‐scale, replicated, natural experiment in central Montana, USA. We used Bayesian methods to assess support for effects of rotational grazing management and rest from grazing on daily survival rates of nearly 500 sage‐grouse nests monitored over 6 years, and mixed effects models to test for effects of rotational grazing and rest on vegetation structure. Though nests on rotationally grazed ranches displayed a trend toward greater daily survival rates, the evidence for an effect was weak. There was no evidence that rest from grazing (≥12 months) increased daily survival rates. Furthermore, rotational grazing systems and rest had negligible effects on herbaceous vegetation height and cover relative to other grazing strategies used in the study area. Results do not support the hypothesis that rotational grazing systems or rest from grazing increase nest success in the northern Great Plains. Estimated nest success, however, was comparable to range‐wide averages, suggesting concealing cover for nests is unlikely to be limiting growth of this population regardless of grazing strategy. In light of these results and recent research questioning reported relationships between grass height and nest survival, maximization of hiding cover may be overemphasized in grazing management guidelines and policies. Rather, our findings suggest a variety of locally appropriate grazing strategies focused on fundamental range health principles may provide adequate habitat quality for nesting sage‐grouse.
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In coniferous western forests, recent widespread tree mortality provided opportunities to test the long-held theory that forest cover loss increases water yield. We reviewed 78 studies of hydrologic response to standing-replacing (severe wildfire, harvest) or nonstand-replacing (drought, insects, low-severity wildfire) disturbances, and reassessed the question: Does water yield or snowpack increase after forest disturbance? Collective results indicate that postdisturbance streamflow and snowpack may increase, not change, or even decrease, and illuminate factors that may help improve predictability of hydrologic response to disturbance. Contrary to the expectation that tree mortality reduces evapotranspiration, making more water available as runoff, postdisturbance evapotranspiration sometimes increased—particularly following nonstand-replacing disturbance—
because of (a) increased evaporation resulting from higher subcanopy radiation, and (b) increased transpiration resulting from rapid postdisturbance growth. Postdisturbance hydrologic response depends on vegetation structure, climate, and topography, and new hypotheses continue to be formulated and tested in this rapidly evolving discipline.
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In recent years restoration project efforts in arid grasslands of the Pacic Northwest have increased; however, little isknown about the bee communities in these areas or how restoration affects them. Native bees provide an essential ecosystemservice through pollination of crops and native plants and understanding their response to restoration is a high priority. Toaddress this issue, we conducted a three-year study in an arid bunchgrass prairie with three objectives: (1) describe the beecommunity of this unique grassland type and its temporal variability; (2) investigate environmental variables inuencing thecommunity; and (3) examine effects of restoration on the community. We identified 62 bee species and found strong seasonaland inter-annual variation in bee abundance, richness, diversity, and species composition. Unexpectedly, these temporaltrends did not correspond with patterns in floral resources; however, several variables were associated with variation inbee abundance, richness, and diversity among sites. Sites with high levels of litter cover had more bees, while sites withtaller vegetation or more blooming flowers had greater species richness but lower diversity. We found no detectable effectof restoration on bee abundance, richness, diversity, or composition. Species composition at native sites differed from those inactively and passively restored sites, which did not differ from each other. Restored sites also had fewer flowers and differingfloral composition relative to native sites. These results suggest that if grassland restoration is to benefit bees, efforts shouldfocus on both expanding floral resources and enhancing variables that influence nesting habitat.