Forb Species Establishment Increases with Decreased Grass Seeding Density and with Increased Forb Seeding Density in a Northeast Kansas, U.S.A., Experimental Prairie Restoration

Chasing the fitness optimum: temporal variation in the genetic and environmental expression of life-history traits for a perennial plant

BACKGROUND AND AIMS

The ability of plants to track shifting fitness optima is crucial within the context of global change, where increasing environmental extremes may have dramatic consequences for life history, fitness, and ultimately population persistence. However, tracking changing conditions relies on the relationship between genetic and environmental variance, where selection may favour plasticity, the evolution of genetic differences, or both depending on the spatial and temporal scale of environmental heterogeneity.

METHODS

Over three years, we compared the genetic and environmental components of phenological and life-history variation in a common environment for the spring perennial Geum triflorum. Populations were sourced from alvar habitats that exhibit extreme but predictable annual flood-desiccation cycles and prairie habitats that exhibit similar but less predictable variation in water availability.

KEY RESULTS

Heritability was generally higher for early life-history (emergence probability) relative to later life-history traits (total seed mass), indicating that traits associated with establishment are under stronger genetic control relative to later life-history fitness expressions, where plasticity may play a larger role. This pattern was particularly notable in seeds sourced from environmentally extreme but predictable alvar habitats relative to less predictable prairie environments. Fitness landscapes based on seed source origin, largely characterized by varying water availability and flower production, described selection as the degree of maladaptation of seed source environment relative to the prairie common garden environment. Plants from alvar populations were consistently closer to the fitness optimum across all years. Annually, the breadth of the fitness optimum expanded primarily along a moisture gradient, with inclusion of more populations onto the expanding optimum.

CONCLUSIONS

These results highlight the importance of temporally and spatially varying selection in life-history evolution, indicating plasticity may become a primary mechanism needed to track fitness for later life-history events within perennial systems.

Testing the hierarchy of predictability in grassland restoration across a gradient of environmental severity

Ecological restoration is critical for recovering degraded ecosystems but is challenged by variable success and low predictability. Understanding which outcomes are more predictable and less variable following restoration can improve restoration effectiveness. Recent theory asserts that the predictability of outcomes would follow an order from most to least predictable from coarse to fine community properties (physical structure > taxonomic diversity > functional composition > taxonomic composition) and that predictability would increase with more severe environmental conditions constraining species establishment. We tested this "hierarchy of predictability" hypothesis by synthesizing outcomes along an aridity gradient with 11 grassland restoration projects across the United States. We used 1829 vegetation monitoring plots from 227 restoration treatments, spread across 52 sites. We fit generalized linear mixed-effects models to predict six indicators of restoration outcomes as a function of restoration characteristics (i.e., seed mixes, disturbance, management actions, time since restoration) and used variance explained by models and model residuals as proxies for restoration predictability. We did not find consistent support for our hypotheses. Physical structure was among the most predictable outcomes when the response variable was relative abundance of grasses, but unpredictable for total canopy cover. Similarly, one dimension of taxonomic composition related to species identities was unpredictable, but another dimension of taxonomic composition indicating whether exotic or native species dominated the community was highly predictable. Taxonomic diversity (i.e., species richness) and functional composition (i.e., mean trait values) were intermittently predictable. Predictability also did not increase consistently with aridity. The dimension of taxonomic composition related to the identity of species in restored communities was more predictable (i.e., smaller residuals) in more arid sites, but functional composition was less predictable (i.e., larger residuals), and other outcomes showed no significant trend. Restoration outcomes were most predictable when they related to variation in dominant species, while those responding to rare species were harder to predict, indicating a potential role of scale in restoration predictability. Overall, our results highlight additional factors that might influence restoration predictability and add support to the importance of continuous monitoring and active management beyond one-time seed addition for successful grassland restoration in the United States.

Seed sourcing for climate-resilient grasslands: The role of seed source diversity during early restoration establishment

Restoration advocates for the use of local seed in restoration, but theory suggests that diverse seed sources may enhance genetic diversity and longer term evolutionary potential within restored communities. However, few empirical studies have evaluated whether species and genetic diversity within species impacts plant community composition following restoration. The goal of this research is to compare the effects of single and multi-sourced seed mix treatments on plant community diversity following restoration. Species establishment, abundance, and diversity were compared following two restoration seed mix treatments created to include 14 species commonly used in grassland restoration. We compared the application of seed mixes designed using a single population per species with those containing five populations per species across sites in Minnesota and South Dakota, United States. Early plant establishment and richness mostly reflected non-seeded species across both sites, although seeded species established at a slightly higher rate in year two following restoration. At the South Dakota site, community composition largely reflected changes associated with establishment across the growing season as opposed to seed mix treatment. This contrasted with the Minnesota site, where community composition appeared to be strongly influenced by seed mix treatment. While there is some evidence seed mix treatment may be influencing the emergent community across sites, spatial heterogeneity across the Minnesota restoration site likely influenced diversity in early emergence over that of seed mix treatment. Indeed, varying land-use history across both sites likely contributed to differences in species composition observed at this early stage of the restoration. This suggests that seed mix treatment may have limited impact on early post-restoration emergence diversity relative to the importance of land-use history. However, future monitoring will be needed to evaluate whether the impact of seed mix treatment on community composition changes over time.

Why are some plant species missing from restorations? A diagnostic tool for temperate grassland ecosystems

The U.N. Decade on Ecosystem Restoration aims to accelerate actions to prevent, halt, and reverse the degradation of ecosystems, and re-establish ecosystem functioning and species diversity. The practice of ecological restoration has made great progress in recent decades, as has recognition of the importance of species diversity to maintaining the long-term stability and functioning of restored ecosystems. Restorations may also focus on specific species to fulfill needed functions, such as supporting dependent wildlife or mitigating extinction risk. Yet even in the most carefully planned and managed restoration, target species may fail to germinate, establish, or persist. To support the successful reintroduction of ecologically and culturally important plant species with an emphasis on temperate grasslands, we developed a tool to diagnose common causes of missing species, focusing on four major categories of filters, or factors: genetic, biotic, abiotic, and planning & land management. Through a review of the scientific literature, we propose a series of diagnostic tests to identify potential causes of failure to restore target species, and treatments that could improve future outcomes. This practical diagnostic tool is meant to strengthen collaboration between restoration practitioners and researchers on diagnosing and treating causes of missing species in order to effectively restore them.

The ecosystem impacts of dominant species exclusion in a prairie restoration

Dominant species often have disproportionately high abundance in restored communities compared to native remnants, which potentially could reduce the conservation value of restorations. Research is needed to determine how the abundance of dominant species in restoration plantings affects community assembly, species diversity, and ecosystem function. Most studies of dominant species in grasslands were modeled after experiments on keystone species, using the short-term experimental removal of dominants to test their functional role in ecosystems. However, the removal of established dominants constitutes a major disturbance that may influence the interpretation of their long-term functional impact. To address this, we experimentally assembled high-diversity tallgrass prairie communities that included or excluded the predicted dominant species (Andropogon gerardii and Sorghastrum nutans) from the seed mix at the time of planting, but without further manipulation of community composition. From 2013 to 2019, we measured several ecosystem functions and community dynamics in the presence or absence of dominants. Communities that included the dominant species had lower species richness, greater aboveground biomass, and reduced light availability at the soil surface. Dominant species presence also increased soil nutrient availability and rates of litter decomposition, although dominant grass litter decomposed more slowly than litter from other common species in both treatments. In the absence of the dominant grasses, communities were instead dominated by a common unplanted forb, Solidago altissima, and there was partial compensation in ecosystem functioning in these forb-dominated communities. The effects of dominant species exclusion may only be apparent in long-term studies of experimentally assembled communities that avoid the legacy effects associated with removal experiments. Furthermore, our results suggest that prairie restorations that limit or exclude the dominant grasses in seed mixes may achieve higher species diversity, increasing the conservation value of these systems.

Using crushed waste bricks for urban greening with contrasting grassland mixtures: no negative effects of brick-augmented substrates varying in soil type, moisture and acid pre-treatment

Ecological restoration aims at supporting biodiversity and ecosystem services, and urban greening is a great opportunity to achieve this goal. This is facilitated by species-rich seed mixtures based on local provenances, which are designed for certain nutrient and moisture regimes based on functional plant traits. Such grassland mixtures might be cultivated on crushed waste bricks, which would be a new component of water-holding urban substrates. Thus, we studied the effects of brick quantity and quality, acid pre-treatment of bricks, soil type and moisture on biomass of designed seed mixtures. Three greenhouse experiments were conducted, with substrates consisting of different brick ratios (5% vs. 30%), brick types (clean production waste vs. demolition material), and brick treatments (acid vs. control) tested on three trait-based mixtures and a non-regional commercial standard mixture. The trait-based mixtures included information on specific leaf area, seed mass and grass-to-legume ratio. There were no negative effects of demolition bricks, soil texture and moisture on grassland biomass. Acid-treated clean porous bricks improved biomass production of the standard and intermediate mixtures, while the effect was minimal with demolition bricks. Designed seed mixtures had a biomass similar to the standard mixture under dry conditions but did not benefit from high moisture like the standard mixture. In conclusion, waste bricks are a useful additive for urban restoration substrates to save raw material, and specifically designed regional mixtures can replace commercial grassland types on these substrates.

Exotic species drive patterns of plant species diversity in 93 restored tallgrass prairies

A primary goal of restoration ecology is to understand the factors that generate variability in species diversity and composition among restorations. Plant communities may assemble deterministically toward a common community type, or they may assemble stochastically, ending differently because of weather conditions during establishment, soil legacy effects, or exotic species propagule pressure. To test these alternative hypotheses, we sampled plant communities and soil at 93 randomly selected restored prairies distributed throughout Iowa, USA. Five remnant sites were sampled as a reference. We tested our hypotheses using multiple regressions and investigated the strength of direct and indirect effects on species diversity and richness using structural equation models. The prairie restorations were highly variable in their age, size, diversity, soil characteristics, and how they were managed post-seeding. The strongest predictor of plant species richness and diversity was the degree of invasion, as measured by the abundance of exotic species. Restorations planted with species-rich seed mixes had reduced exotic species abundance, which led indirectly to higher species richness of restorations. Sites with higher organic matter and a more linear shape had a direct positive effect on exotic abundance, which in turn decreased diversity. We found little support for deterministic assembly, and diversity did not increase with the age of planting. Our results indicate that restored prairie communities tend to assemble into states of high or low diversity, driven by invasion from exotic plant species. Management of exotic species is essential for maximizing species diversity in temperate grassland restorations.

Priority Treatment Leaves Grassland Restoration Vulnerable to Invasion

Priority effects can be used to promote target species during restoration. Early planting can provide an advantage over later-arriving species, increasing abundance of these early-arrivers in restored communities. However, we have limited knowledge of the indirect impacts of priority effects in restoration. In particular, we do not understand how priority effects impact non-target species. Of particular conservation concern is how these priority effects influence establishment by non-native species. We use a field-based mesocosm experiment to explore the impacts of priority effects on both target and non-target species in California grasslands. Specifically, we seeded native grasses and forbs, manipulating order of arrival by planting them at the same time, planting forbs one year before grasses, planting grasses one year before forbs, or planting each functional group alone. While our study plots were tilled and weeded for the first year, the regional species pool was heavily invaded. We found that, while early-arrival of native grasses did not promote establishment of non-native species, giving priority to native forbs ultimately left our restoration mesocosms vulnerable to invasion by non-native species. This suggests that, in some cases, establishment of non-native species may be an unintended consequence of using priority treatments as a restoration tool.

Helianthus maximiliani and species fine-scale spatial pattern affect diversity interactions in reconstructed tallgrass prairies

Biodiversity and Ecosystem Function analyses aim to explain how individual species and their interactions affect ecosystem function. With this study, we asked in what ways do species interact, are these interactions affected by species planting pattern, and are initial (planted) proportions or previous year (realized) proportions a better reference point for characterizing grassland diversity effects?We addressed these questions with experimental communities compiled from a pool of 16 tallgrass prairie species. We planted all species in monocultures and mixtures that varied in their species richness, evenness, and spatial pattern. We recorded species-specific biomass production over three growing seasons and fitted Diversity-Interactions (DI) models to annual plot biomass yields.In the establishment season, all species interacted equally to form the diversity effect. In years 2 and 3, each species contributed a unique additive coefficient to its interaction with every other species to form the diversity effect. These interactions were affected by Helianthus maximiliani and the species planting pattern. Models based on species planted proportions better-fit annual plot yield than models based on species previous contributions to plot biomass.Outcomes suggest that efforts to plant tallgrass prairies to maximize diversity effects should focus on the specific species present and in what arrangement they are planted. Furthermore, for particularly diverse grasslands, the effort of collecting annual species biomass data may not be necessary when quantifying diversity effects with DI models.

A Comparison of Drought-Tolerant Prairie Plants to Support Managed and Wild Bees in Conservation Programs

In response to growing concerns surrounding pollinator health, there have been increased efforts to incorporate wildflower habitat into land management programs, particularly in agricultural systems dependent on bee-mediated pollination. While recommended plant lists abound, there is limited research on which plant species support the greatest bee abundance and diversity. In many farm settings, drought-tolerant plant species adapted to well-drained sandy soils are needed, since wildflower plantings are typically not irrigated. We used a common garden experimental design to evaluate 51 drought-tolerant native perennial plant species, and 2 non-native plant species in three regions of Michigan for their ability to support honey bees (Apis mellifera L. (Hymenoptera: Apidae)) and wild bees. 1,996 honey bees and 2,496 wild bees were recorded visiting study plants. The wild bee community visiting plant species was dominated by Bombus spp. (Hymenoptera: Apidae) (25%), Halictus spp. (Hymenoptera: Halictidae) (23%), and Lasioglossum spp. (Hymenoptera: Halictidae) (16%). The number of honey bees and wild bees visiting study plants varied considerably, suggesting that bee groups have distinct preferences for plant species. Of the plant species assessed, Asclepias syriaca L. (Gentianales: Apocynaceae) (early season), Monarda fistulosa L. (Lamiales: Lamiaceae) (middle season), and Solidago speciosa Nutt. (Asterales: Asteraceae) (late season) were the three most attractive plant species to the entire bee community. Many other plants consistently attracted a high abundance of wild bees, honey bees, or both. Our results inform plant selection to support managed and wild bees as part of pollinator conservation programs in the Great Lakes region of the United States.

Plant community response to switchgrass (Panicum virgatum) population source in establishing prairies

Ecological restoration and revegetation efforts entail the translocation of native plant populations. Risks associated with these efforts include failure of translocated populations to establish or, conversely, such strong establishment that they excessively dominate the recipient community. The role that selective breeding plays in mediating these risks is unclear but of increasing importance as efforts to restore and establish multifunctional grasslands also increase. In a three-year, spatially replicated study, we seeded experimental prairie communities with either domesticated (cultivar) or undomesticated strains of Panicum virgatum (switchgrass), a North American C₄ species under development as a biomass crop. We evaluated the composition, performance, and diversity of the recipient plant communities and compared the performance of cultivar and undomesticated switchgrass in those communities. We found little evidence that switchgrass population source affected community response. Switchgrass cultivars modestly exceeded undomesticated strains with respect to stand establishment, third-year stand density, and aboveground biomass; effect size and significance differed among sites. Our results suggest that including cultivars in ecological restorations and multifunctional grasslands may enhance success of switchgrass establishment with little risk of impairing the composition or diversity of plant communities for up to three years, as reflected in the measures used here. However, the incorporation of undomesticated switchgrass into multifunctional grasslands may enhance landscape-scale genetic variation and mitigate risks associated with gene flow between translocated and local wild switchgrass populations; more research on these dynamics is needed.

Using Publicly Available Data to Quantify Plant-Pollinator Interactions and Evaluate Conservation Seeding Mixes in the Northern Great Plains

Concern over declining pollinators has led to multiple conservation initiatives for improving forage for bees in agroecosystems. Using data available through the Pollinator Library (npwrc.usgs.gov/pollinator/), we summarize plant-pollinator interaction data collected from 2012-2015 on lands managed by the U.S. Fish and Wildlife Service and private lands enrolled in U.S. Department of Agriculture conservation programs in eastern North Dakota (ND). Furthermore, we demonstrate how plant-pollinator interaction data from the Pollinator Library and seed cost information can be used to evaluate hypothetical seeding mixes for pollinator habitat enhancements. We summarize records of 314 wild bee and 849 honey bee (Apis mellifera L.) interactions detected on 63 different plant species. The wild bee observations consisted of 46 species, 15 genera, and 5 families. Over 54% of all wild bee observations were represented by three genera-Bombus, Lassioglossum, and Melissodes. The most commonly visited forbs by wild bees were Monarda fistulosa, Sonchus arvensis, and Zizia aurea. The most commonly visited forbs by A. mellifera were Cirsium arvense, Melilotus officinalis, and Medicago sativa. Among all interactions, 13% of A. mellifera and 77% of wild bee observations were made on plants native to ND. Our seed mix evaluation shows that mixes may often need to be tailored to meet the unique needs of wild bees and managed honey bees in agricultural landscapes. Our evaluation also demonstrates the importance of incorporating both biologic and economic information when attempting to design cost-effective seeding mixes for supporting pollinators in a critically important part of the United States.

An assessment of rehabilitation success in an African grassland using ants as bioindicators

Many studies that evaluate rehabilitation make use of invertebrate bioindicators. Invertebrates, especially ants, make useful indicators as they are sensitive to environmental change. We compared ant assemblages in rehabilitated and control sites in the Rietvlei Nature Reserve, a protected area important for grassland conservation in South Africa. Pitfall traps were used to sample ant assemblages at six control sites and six rehabilitated sites. In addition, environmental and vegetation surveys were conducted at each site. We found that the ant assemblages differed significantly between the control and rehabilitated sites, although there was considerable overlap; the control sites supported a greater species density and higher abundance of ants than the rehabilitated sites. In total, 36 ant species were collected (control sites: 34 species; rehabilitated sites: 26 species). The environmental survey revealed that percentages of bare ground and coarse sand, as well as soil pH, differed significantly between the control and rehabilitated sites. The control and rehabilitated sites also supported significantly different plant assemblages. Three indicator ant species were identified for the control sites: Crematogaster rectinota, Crematogaster amita and Monomorium fastidium. No indicator species were identified for the rehabilitated sites. These results suggest that recovery from the previous agricultural use of the area is still incomplete and highlights the lack of research examining the success of rehabilitation in the grassland biome.Conservation implications: The present study illustrates the need for further research on rehabilitation techniques utilised in the grassland biome. This is of value as the remainder of South African grasslands are considered critically endangered.

Can the Results of Biodiversity-Ecosystem Productivity Studies Be Translated to Bioenergy Production?

Biodiversity experiments show that increases in plant diversity can lead to greater biomass production, and some researchers suggest that high diversity plantings should be used for bioenergy production. However, many methods used in past biodiversity experiments are impractical for bioenergy plantings. For example, biodiversity experiments often use intensive management such as hand weeding to maintain low diversity plantings and exclude unplanted species, but this would not be done for bioenergy plantings. Also, biodiversity experiments generally use high seeding densities that would be too expensive for bioenergy plantings. Here we report the effects of biodiversity on biomass production from two studies of more realistic bioenergy crop plantings in southern Michigan, USA. One study involved comparing production between switchgrass (Panicum virgatum) monocultures and species-rich prairie plantings on private farm fields that were managed similarly to bioenergy plantings. The other study was an experiment where switchgrass was planted in monoculture and in combination with increasingly species-rich native prairie mixtures. Overall, we found that bioenergy plantings with higher species richness did not produce more biomass than switchgrass monocultures. The lack of a positive relationship between planted species richness and production in our studies may be due to several factors. Non-planted species (weeds) were not removed from our studies and these non-planted species may have competed with planted species and also prevented realized species richness from equaling planted species richness. Also, we found that low seeding density of individual species limited the biomass production of these individual species. Production in future bioenergy plantings with high species richness may be increased by using a high density of inexpensive seed from switchgrass and other highly productive species, and future efforts to translate the results of biodiversity experiments to bioenergy plantings should consider the role of seeding density.

Convergent and contingent community responses to grass source and dominance during prairie restoration across a longitudinal gradient

Restoring prairie on formerly cultivated land begins by selecting propagule seed sources and the diversity of species to reintroduce. This study examined the effects of dominant grass propagule source (cultivar vs. non-cultivar) and sown propagule diversity (grass:forb sowing ratio) on plant community structure. Two field experiments were established in Kansas and Illinois consisting of identical split plot designs. Dominant grass source was assigned as the whole-plot factor, and sown dominance of grasses (five levels of seeded grass dominance) as the subplot factor. Species density, cover, and diversity were quantified for 5 years. The effect of dominant grass source on the cover of focal grasses, sown species, and volunteer species was contingent upon location, with variation between dominant grass sources observed exclusively in Kansas. Species density and diversity showed regionally convergent patterns in response to dominant grass source. Contrary to our hypotheses, total species density and diversity were not lower in the presence of grass cultivars, the grass source we had predicted would be more competitive. Sown grass dominance effects on the cover of the focal grass species were contingent upon location resulting from establishment corresponding better to the assigned treatments in Illinois. All other cover groups showed regionally convergent patterns, with lower cover of volunteers and higher cover of sown forbs, diversity, and species density in the lowest sown grass dominance treatment in both sites. Thus, decisions regarding the diversity of propagules to reintroduce had more consequence for plant community structure than cultivar or non-cultivar source of dominant grasses.

High richness and dense seeding enhance grassland restoration establishment but have little effect on drought response

Restorations commonly utilize seed addition to formerly arable lands where the development of native plant communities is severely dispersal limited. However, variation in seed addition practices may profoundly affect restoration outcomes. Theory and observations predict that species-rich seed mixtures and seeding at high densities should enhance native plant community establishment, minimize exotic species cover, and may promote resistance and resilience to, and recovery from, environmental perturbations. We studied the post-seeding establishment of native plant communities in large grassland restoration plots, which were sown at two densities crossed with two levels of species richness on formerly arable land in Nebraska, USA, and their responses to drought. To evaluate drought resistance, recovery, and resilience of restored plant communities, we erected rainfall manipulation structures and tracked the response of seeded species cover and total plant biomass during experimental drought relative to controls and in the post-drought growing season. High seed richness and high-density seeding treatments resulted in greater richness and cover of native, seeded species per 0.5 m2 compared to low-richness and low-density treatments. Cover differences in response to seed mixture richness were driven by native forbs. Richness and cover of exotic species were lowest in high-richness and high-density treatments. We found little evidence of differential drought resistance, recovery, and resilience among seeding treatments. Increases in exotic species across years were restricted to drought subplots, and were not affected by seeding treatments. Grassland restoration was generally enhanced and exotic cover reduced both by the use of high-richness seed mixtures and high-density seeding. Given the lack of restoration treatment effects on the resistance, recovery, or resilience of seeded species exposed to drought, and the increases in exotic species following drought, other forms of active management may be needed to produce restored plant communities that are robust to climate change.