Tallgrass Prairie Restoration Research Articles

Bee communities along a prairie restoration chronosequence: similar abundance and diversity, distinct composition.

Recognition of the importance of bee conservation has grown in response to declines of managed honey bees and some wild bee species. Habitat loss has been implicated as a leading cause of declines, suggesting that ecological restoration is likely to play an increasing role in bee conservation efforts. In the midwestern United States, restoration of tallgrass prairie has traditionally targeted plant community objectives without explicit consideration for bees. However, restoration of prairie vegetation is likely to provide ancillary benefits to bees through increased foraging and nesting resources. We investigated community assembly of bees across a chronosequence of restored eastern tallgrass prairies and compared patterns to those in control and reference habitats (old fields and prairie remnants, respectively). We collected bees for 3yr and measured diversity and abundance of in-bloom flowering plants, vegetation structure, ground cover, and surrounding land use as predictors of bee abundance and bee taxonomic and functional diversity. We found that site-level variables, but not site type or restoration age, were significant predictors of bee abundance (bloom diversity, P=0.004; bare ground cover, P=0.02) and bee diversity (bloom diversity, P=0.01). There were significant correlations between overall composition of bee and blooming plant communities (Mantel test, P=0.002), and both plant and bee assemblages in restorations were intermediate between those of old fields and remnant prairies. Restorations exhibited high bee beta diversity, i.e., restored sites' bee assemblages were taxonomically and functionally differentiated from each other. This pattern was strong in younger restorations (<20yr old), but absent from older restorations (>20yr), suggesting restored prairie bee communities become more similar to one another and more similar to remnant prairie bee communities over time with the arrival of more species and functional groups of bees. Our results indicate that old fields, restorations, and remnants provide habitat for diverse and abundant bee communities, but continued restoration of old fields will help support and conserve bee communities more similar to reference bee communities characteristic of remnant prairies.

Restored tallgrass prairies have reduced phylogenetic diversity compared with remnants

SummaryEcological restoration is critical for mitigating habitat loss and providing ecosystem services. However, restorations often have lower diversity than remnant, reference sites. Phylogenetic diversity is an important component of biodiversity and ecosystem function that has only recently been used to evaluate restoration outcomes. To move towards prediction in the restoration of biodiversity, it is necessary to understand how phylogenetic diversity of restorations compares with that of reference sites, and where deficits are found, to evaluate factors constraining phylogenetic diversity.We quantified plant taxonomic and phylogenetic diversity in eastern tallgrass prairie, one of the most endangered ecosystems on earth. We measured diversity at large (site) and small (plot) scales in 19 restored prairies and compared patterns with those from 41 remnant prairies. To evaluate how environmental conditions and management actions influence outcomes, we tested the effects of soil properties and seed mix composition on diversity of restorations.Restored prairies were less phylogenetically diverse than remnants at both spatial scales. On the other hand, the total species richness of remnant and restored prairies did not significantly differ, but remnants had higher native richness. Restored communities were taxonomically and phylogenetically distinct from remnants.Soil properties (moisture andpH) influenced phylogenetic diversity and composition. There were positive relationships between the taxonomic and phylogenetic diversity of seed mixes and resulting diversity of planted assemblages (excluding volunteer species). Species in seed mixes were more closely related than expected by chance, and several clades found in remnant prairies were missing from seed mixes.Synthesis and applications. Restored tallgrass prairies had lower phylogenetic diversity than remnant prairies, which may contribute to the widely observed phenomenon of restorations not being functionally equivalent to reference sites. It is encouraging for restoration efforts that seed mix phylogenetic diversity predicted phylogenetic diversity of planted assemblages. This indicates that designing phylogenetically diverse seed mixes for restoration is beneficial. In addition, clades found in reference sites that are missing from restoration seed mixes could be added to new or existing restorations to reduce gaps in phylogenetic diversity. Further work on the effects of management on phylogenetic diversity is needed to advance restoration of biodiversity.

Wild bee community change over a 26‐year chronosequence of restored tallgrass prairie

AbstractRestoration efforts often focus on plants, but additionally require the establishment and long‐term persistence of diverse groups of nontarget organisms, such as bees, for important ecosystem functions and meeting restoration goals. We investigated long‐term patterns in the response of bees to habitat restoration by sampling bee communities along a 26‐year chronosequence of restored tallgrass prairie in north‐central Illinois, U.S.A. Specifically, we examined how bee communities changed over time since restoration in terms of (1) abundance and richness, (2) community composition, and (3) the two components of beta diversity, one‐to‐one species replacement, and changes in species richness. Bee abundance and raw richness increased with restoration age from the low level of the pre‐restoration (agricultural) sites to the target level of the remnant prairie within the first 2–3 years after restoration, and these high levels were maintained throughout the entire restoration chronosequence. Bee community composition of the youngest restored sites differed from that of prairie remnants, but 5–7 years post‐restoration the community composition of restored prairie converged with that of remnants. Landscape context, particularly nearby wooded land, was found to affect abundance, rarefied richness, and community composition. Partitioning overall beta diversity between sites into species replacement and richness effects revealed that the main driver of community change over time was the gradual accumulation of species, rather than one‐to‐one species replacement. At the spatial and temporal scales we studied, we conclude that prairie restoration efforts targeting plants also successfully restore bee communities.

Biochar and manure alter few aspects of prairie development: A field test

Biochar and composted manure can impart benefits, such as improved nutrient cycling, increased productivity, and carbon sequestration, when they are applied to agricultural soils. There are limited data, however, on how these additives effect mixed perennial communities. We compare plant and soil community responses by using factorial applications of Royal Oak biochar (0%, 2.6%, and 5.2%) and composted manure (0 and 4.5kgm−2) in a tallgrass prairie restoration in Iowa. We found that biochar contributed to ecosystem function by increasing total carbon in the soil. We also found that biochar increased plant species richness and the 5.2% biochar treatment increased the biomass of plant species in the seed mix. Phosphorus cycling was altered by both the biochar and manure treatments; in soils with manure, biochar addition reduced phosphorus (P) availability compared with the control, but in soils without manure, biochar increased P availability. Belowground, the biochar and manure treatments affected the relative density of gram-negative bacteria, the relative density of actinobacteria, and root colonization by mycorrhizae. Contrary to our hypotheses, however, the biochar and manure treatments did not change total plant cover, total aboveground peak biomass, overall plant community, soil pH, inorganic nitrogen concentrations, extractable soil K, plant tissue N, soil microbial biomass-carbon, nematode density, and overall microbial community composition. Overall, we found several benefits and few externalities associated with biochar and composted manure addition to tallgrass prairie. However, these products can vary considerably and their interactions with soil, plants, and microbes can be situation specific. More study is needed to identify the characteristics of biochar that can affect mixed perennial communities prior to widespread use.

Ecosystem carbon pools, fluxes, and balances within mature tallgrass prairie restorations

AbstractTallgrass prairie restorations can quickly accrue organic C in soil and biomass, but the rate of C accumulation diminishes through time and is highly variable among more mature prairies. Long‐term soil organic carbon (SOC) accumulation in prairies has been linked to edaphic factors such as soil texture, soil moisture, and SOC content, but it is unclear how these factors affect the ecosystem processes that are responsible for observed differences in C accumulation rates in older prairies. We measured belowground plant and SOC pools and fluxes within 27–36‐year‐old restored tallgrass prairies in order to quantify total C storage, determine the net ecosystem production of C (NEP‐C), and explore which edaphic factors influence the ecosystem processes responsible for divergent NEP‐C. We found that 11% of organic C was stored in biomass, and we estimate that one‐third of post‐restoration C sequestration has occurred in biomass, thereby highlighting biomass as a large but often overlooked C pool. Belowground biomass and soil C pools were notably smaller than those reported for remnant prairie, suggesting that future belowground C accumulation could still occur. During this study, the prairies appeared to be a net source of C, although the range of NEP‐C values encompassed zero. Sand content positively affected NEP‐C via increased belowground biomass production‐C inputs, and SOC negatively affected NEP‐C due to increased soil respiration C outputs. However, soil moisture had a smaller negative effect on soil respiration, indicating that both SOC and soil moisture play important roles in determining prairie C balance.

Common Methods for Tallgrass Prairie Restoration and Their Potential Effects on Bee Diversity

Habitat restoration is considered critical for maintaining and restoring biodiversity of many species groups. A better understanding of how species respond to the restoration process is imperative to identifying practices that benefit the target organisms. Using survey responses from land managers about the restoration process in tallgrass prairie habitat, we identify common restoration and management techniques. These responses are then combined with current literature on bee and plant responses to these methods in tallgrass prairie habitat. While some widely used methods, such as burning, are believed to be fairly benign to bees, there are still many knowledge gaps about how bees may respond to many common practices such as tillage, pesticides, and grazing in tallgrass prairie. Other commonly used methods, such as broadcasting of seeds, are known to significantly affect plant diversity, which could hinder bee conservation efforts. The variability in bee and plant response to commonly used methods highlights why it is necessary to better understand how management methods affect plant and bee communities.

Responses of aboveground C and N pools to rainfall variability and nitrogen deposition are mediated by seasonal precipitation and plant community dynamics

Plant productivity and tissue chemistry in temperate ecosystems are largely driven by water and nitrogen (N) availability. Although changes in rainfall patterns may influence nutrient limitation, few studies have considered how these two global change factors could interact to influence terrestrial ecosystem productivity and stoichiometry. Here, we examined the influence of experimentally-increased intra-annual rainfall variability and low-level nitrogen addition on aboveground productivity, C and N pools, and C:N ratios in a restored tallgrass prairie across two growing seasons. In the drier first year of the experiment, increased rainfall variability boosted productivity and C pools. In the wetter second year, aboveground productivity and C pools increased with N addition, suggesting a switch in primary resource limitation from water to N. Increased rainfall variability also reduced aboveground N pools in the second year. Community-level C:N increased under increased rainfall variability in the wetter second year and N addition slightly reduced community C:N in both years. These changes in element pools and stoichiometry were mostly a result of increased forb dominance in response to both treatments. Overall, our findings from a restored prairie indicate that increased rainfall variability and N addition can enhance aboveground productivity and C pools, but that N pools may not have a consistent response to either global change factor. Our study also suggests that these effects are dependent on growing season precipitation patterns and are mediated by shifts in plant community composition.

Site Preparation Drives Long-Term Plant Community Dynamics in Restored Tallgrass Prairie: A Case Study in Southeastern South Dakota

Most tallgrass prairies have been destroyed or altered, making restoration an important component to their conservation. Our goal was to evaluate progress 12-years post-restoration at Spirit Mound Historic Prairie and determine whether the outcomes varied based on different land use and restoration histories across the site. We examined changes in plant diversity, richness, evenness, non-native species relative abundance, and community composition from 2004 to 2013. Areas with different restoration treatments and land-use histories showed divergent results. Seventy percent of the site, previously annual row crop, was reconstructed using herbicide application followed by native seeding (hereafter reconstruction). Areas that were previously grazed, 15% of the site, were restored with only partial seeding and no herbicide treatment (hereafter rehabilitation). Species richness and diversity increased over 40% in the reconstruction since 2004 and remained over 1.9 times higher in the reconstructed areas than rehabilitated areas. Diversity did not change in the rehabilitation, but richness increased 47% since 2004. Evenness decreased 11-26% over time in both areas. Non-native species relative abundance did not change from 2004 to 2013, and remained five times higher in the rehabilitation than the reconstruction. Native C4 grass and forb abundance increased over time in the reconstruction, whereas non-native C3 grasses remained dominant in the rehabilitation. These results showed that restoration outcomes were radically different 12-years post-restoration among areas with different prior land uses that were subjected to different restoration practices. Long-term assessments are important to accurately determine restoration progress and inform management decisions.

Edge Effects and Avian Community Structure in a Restored Tallgrass Prairie

ABSTRACT As a consequence of extensive habitat loss and fragmentation, grassland bird populations have declined substantially across much of North America. To assess management strategies for restoring and maintaining suitable habitat for grassland birds, we conducted a four-year survey of grassland-breeding passerines in the restored tallgrass prairies of the Carleton College Cowling Arboretum in Northfield, Minnesota. Between 2010 and 2013, we sampled bird populations weekly during the breeding season in ten adjacent management units that differed in size and spatial configuration. We used principal components analysis (PCA) to compare avian community composition among the units and to examine the association between habitat properties and makeup of the community. The size of a management unit had no effect on the bird community, but there was a strong impact of forest edge on community composition. Management units with less exposure to forest edge had greater abundance of grassland-obligate but not gr...

Prairies Thrive Where Row Crops Drown: A Comparison of Yields in Upland and Lowland Topographies in the Upper Midwest US

Cellulosic biofuel production is expected to increase in the US, and the targeted establishment of biofuel agriculture in marginal lands would reduce competition between biofuels and food crops. While poorly drained, seasonally saturated lowland landscape positions are marginal for production of row crops and switchgrass (Panicum virgatum L.), it is unclear whether species-diverse tallgrass prairie yield would suffer similarly in saturated lowlands. Prairie yields typically increase as graminoids become more dominant, but it is uncertain whether this trend is due to greater aboveground net primary productivity (ANPP) or higher harvest efficiency in graminoids compared to forbs. Belowground biomass, a factor that is important to ecosystem service provisioning, is reduced when switchgrass is grown in saturated lowlands, but it is not known whether the same is true in species-diverse prairie. Our objectives were to assess the effect of topography on yields and live belowground biomass in row crops and prairie, and to determine the mechanisms by which relative graminoid abundance influences tallgrass prairie yield. We measured yield, harvest efficiency, and live belowground biomass in upland and lowland landscape positions within maize silage (Zea mays L.), winter wheat (Triticum aestivum L.), and restored tallgrass prairie. Maize and winter wheat yields were reduced by more than 60% in poorly drained lowlands relative to well-drained uplands, but diverse prairie yields were equivalent in both topographic settings. Prairie yields increased by approximately 45% as the relative abundance of graminoids increased from 5% to 95%. However, this trend was due to higher harvest efficiency of graminoids rather than greater ANPP compared to forbs. In both row crops and prairie, live belowground biomass was similar between upland and lowland locations, indicating consistent biomass nutrient sequestration potential and soil organic matter inputs between topographic positions. While poorly drained, lowland landscape positions are marginal lands for row crops, they appear prime for the cultivation of species-diverse tallgrass prairie for cellulosic biofuel.

Phylogenetic diversity is maintained despite richness losses over time in restored tallgrass prairie plant communities

Summary Ecosystem restoration is an important tool for mitigating biodiversity loss and recovering critical ecosystem services to humanity, but restoration rarely takes into account the evolutionary attributes of the community being restored. Phylogenetic diversity (PD) represents a potentially valuable measure of restoration success because it can correlate with functional trait diversity that drives ecosystem function. However, PD patterns in restored communities are rarely assessed. We surveyed plant communities in restored tallgrass prairies 2–19 years old and calculated two PD measures, SESMNTD and SESMPD, of the communities and seed mixture applied to sites. We also identified high‐threat exotic species present in each site to determine whether PD of the seed mixture applied was related to resistance against invasion. We show that PD in North American tallgrass prairie restorations, as measured by both SESMNTD and SESMPD, is maintained over time even as richness declines. Neither the resulting community PD nor invasion by high‐threat exotic species was affected by PD of the seed mixture used in site restoration. Thus, simply maximizing PD of seed mixtures without considering the particular component species is unlikely to help achieve restoration goals. Synthesis and applications. These results suggest that species losses over time are not biased towards species with or without close relatives in the community. If phylogenetic diversity (PD) reflects functional trait diversity in communities, then local declines in species richness may not necessarily mean the loss of ecosystem function in restoration projects. However, PD of restored communities may be limited by low establishment rates for most species. Conservation practitioners should consider PD with careful planning to maintain overall community diversity and potentially maximize ecosystem function and services in restorations. This perspective will require a deeper understanding of the relationships between phylogenetic relatedness and traits associated with competition and fitness.

Mammalian Herbivores Alter the Population Growth and Spatial Establishment of an Early-Establishing Grassland Species.

Plant-herbivore interactions influence the establishment context of plant species, as herbivores alter the community context in which individual species establish, and the spatial relationship between individuals and their source population as plants invade. This relationship can be described using an establishment kernel, which takes into account movement through seed dispersal, and subsequent establishment of adults. Mammalian herbivores are hypothesized to influence plant population growth and establishment through a combination of consumption of seeds and seedlings, and movement of seeds. While the movement abilities of plants are well known, we have very few empirical mechanistic tests of how biotic factors like mammalian herbivores influence this spread potential. As herbivores of all sizes are abundant on the landscape, we asked the question, how do mammalian herbivores influence the population growth, spatial establishment, and the community establishment context of an early-recruiting native prairie legume, Chamaecrista fasciculata? We planted C. fasciculata in source populations within a four-acre tallgrass prairie restoration in plots with and without herbivores, and monitored its establishment with respect to distance from the source populations. We found that herbivores decreased population growth, and decreased the mean and range establishment distance. Additionally, C. fasciculata established more often without herbivores, and when surrounded by weedy, annual species. Our results provide insight into how the interactions between plants and herbivores can alter the spatial dynamics of developing plant communities, which is vital for colonization and range spread with fragmentation and climate change. Mammalian herbivores have the potential to both slow rates of establishment, but also determine the types of plant communities that surround invading species. Therefore, it is essential to consider the herbivore community when attempting to restore functioning plant communities.

Rainfall variability and nitrogen addition synergistically reduce plant diversity in a restored tallgrass prairie

Summary Climate change is expected to bring fewer, larger rainfall events and prolonged droughts (i.e. increased rainfall variability). Concurrently, the burning of fossil fuels and reliance on nitrogen (N) fertilizers are expected to continue to increase N availability in many ecosystems. These changes in water and N availability have the potential to alter plant community composition and structure. We manipulated rainfall variability and N inputs in a restored tallgrass prairie over the course of two growing seasons. Greater rainfall variability led to wetter soils throughout the majority of both growing seasons and provided punctuated relief from a severe drought that occurred during the first three months of the experiment. Both rainfall variability‐ and fertilization‐induced increases in resource availability favoured fast‐growing, deeply rooted C3 forbs, particularly the dominant Solidago canadensis, at the expense of species adapted to low resource conditions, particularly C4 grasses and N‐fixing forbs. This change in community composition decreased plant community diversity and evenness in plots receiving both supplemental N and more variable rainfall. Synthesis and applications. These results suggest that future increases in rainfall variability and nitrogen (N) deposition could synergistically alter the structure of prairie restorations and jeopardize restoration targets related to increasing floral diversity. Mitigating N availability in restoration sites may help to maintain prairie diversity as rainfall patterns become more variable.

Seed source impacts germination and early establishment of dominant grasses in prairie restorations

Summary Land managers choose seed from a variety of provenances for restoration projects. By selecting seed of the local ecotype, managers can increase establishment in the short term and prevent the disruption of local adaptations and genetic swamping in the long term. However, local seed may be disadvantageous if populations are inbred or maladapted to managed restoration environments. Seed selection may be further confounded by propagation methods. Three dominant C4 grasses, Andropogon gerardii, Bouteloua curtipendula and Sorghastrum nutans, from three types of seed provenances (remnant, restoration and nursery) were planted as seeds and plugs into experimental plots at three established tallgrass prairie restorations in western Minnesota, USA. Using a common garden design, we tested whether (i) provenance and (ii) site of planting influence germination and first‐season survival and growth both (iii) for seeds directly planted in the field and for transplants (plugs). Seed provenance impacted germination and seedling survival in all cases, except S. nutans seeded directly in the field. Andropogon gerardii and B. curtipendula nursery seedlings were consistently taller than those of the other provenance types. When directly seeded, germination, survival and vigour differed among restoration sites; however, the results were species specific. Sorghastrum nutans germination varied among sites depending on provenance, indicating that this species may be particularly sensitive to environmental conditions. Germination was 3–12 times greater for plugs than for seeds directly planted in the field in summer, but mortality after planting in the field was low for both groups. Synthesis and applications. Provenance and restoration site had varying effects among species, indicating that the dominant C4 grasses used in this study ought not to be considered ecological equivalents. While we found little evidence of local adaptation, use of local remnant seeds diminishes the risk of spreading maladapted genotypes. Germination limited establishment when sowing seeds directly in summer. Supplemental use of plugs may increase species diversity in restorations.

Urban Rights-of-Way as Reservoirs for Tall-Grass Prairie Plants and Butterflies.

Urban rights-of-way may be potential reservoirs of tall-grass prairie plants and butterflies. To determine if this is true, in 2007-2008, we conducted vegetation surveys of species richness and cover, and butterfly surveys of species richness and abundance, along 52 transmission lines and four remnant prairies in Winnipeg, Manitoba. We detected many prairie plants and butterflies within transmission lines. Some unmowed and infrequently managed transmission lines had native plant species richness and total percent cover of native plants comparable to that of similar-sized remnant tall-grass prairies in the region. Although we did not find significant differences in overall native butterfly numbers or species richness between rights-of-way and remnant prairies, we found lower numbers of some prairie butterflies along frequently mowed rights-of-way than within remnant tall-grass prairies. We also observed higher butterfly species richness along sites with more native plant species. By reducing mowing and spraying and reintroducing tall-grass prairie plants, urban rights-of-way could serve as extensive reservoirs for tall-grass prairie plants and butterflies in urban landscapes. Eventually, managing urban rights-of-way as reservoirs for tall-grass prairie plants and animals could contribute to the restoration of tall-grass prairie in the North American Midwest.

Does tallgrass prairie restoration enhance the invasion resistance of post-agricultural lands?

There is building interest in the use of ecological restoration to enhance the biotic invasion resistance of disturbed lands. However, few studies have rigorously examined the effect of community restoration on biotic invasion resistance under conditions of controlled invader propagule pressure. Results are presented from a field experiment conducted in a post-agricultural grassland in eastern Kansas to explore the interplay of tallgrass prairie restoration and invader propagule pressure in modulating plant invasion. Seed additions of multiple native and non-native species were used to provide a general test of biotic invasion resistance under varied propagule availability. Restoration increased plant diversity, increased above ground productivity, reduced the availability of light, soil moisture and bare soil microsites and strongly suppressed the invasion of all species sown into the experiment, including the highly invasive exotic legume, Lespedeza cuneata. In the absence of restoration, L. cuneata rapidly dominated plots where it had been sown, particularly at the highest propagule pressure. Results of multiple regression modelling suggested that restoration most likely increased community resistance to L. cuneata invasion through changes in functional guild composition rather than through changes in species diversity. Overall our study indicates that restoration of abandoned agricultural land to native tallgrass prairie can enhance invasion resistance in the face of substantial invader propagule pressures by altering community composition to dominance by native species that are efficient in utilizing resources.

Plant‐soil feedbacks as drivers of succession: evidence from remnant and restored tallgrass prairies

Plant‐soil feedbacks can contribute to the coexistence of plant species and may predict the abundance of plant species within communities. Here, we test if plant‐soil feedbacks act as drivers of secondary succession. We found that the strength of feedback experienced by a plant species was positively correlated with that species' successional stage, indicating that plant‐soil feedbacks can contribute to shifts in plant species abundance during succession. We did not observe a significant relationship between strength of feedback and plant species abundance at our study sites, but the positive relationship of feedback and successional stage would generate positive relationships between feedback and the abundance of plant species in communities at equilibrium. This result is supported by spatially explicit simulation models that demonstrate the potential for plant‐soil feedbacks to determine changes in species abundance over time and the increasing strength of the relationship between feedback and plant species abundance during succession.

Edaphic controls on soil organic carbon stocks in restored grasslands

Cultivation of undisturbed soils dramatically depletes organic carbon stocks at shallow depths, releasing a substantial quantity of stored carbon to the atmosphere. Restoration of native ecosystems can help degraded soils rebuild a portion of the depleted soil organic matter. However, the rate and magnitude of soil carbon accrual can be highly variable from site to site. Thus, a better understanding of the mechanisms controlling soil organic carbon stocks is necessary to improve predictions of soil carbon recovery. We measured soil organic carbon stocks and a suite of edaphic factors in the upper 10cm of a series of restored tallgrass prairies representing a range of drainage conditions. Our findings suggest that factors related to soil organic matter stabilization mechanisms (texture, polyvalent cations) were key predictors of soil organic carbon, along with variables that influence plant and microbial biomass (available phosphorus, pH) and soil moisture. Exchangeable soil calcium was the strongest single predictor, explaining 74% of the variation in soil organic carbon, followed by clay content, which explained 52% of the variation. Our results demonstrate that the cumulative effects of even relatively small differences in these edaphic properties can have a large impact on soil carbon stocks when integrated over several decades.

Nitrogen conservation decreases with fertilizer addition in two perennial grass cropping systems for bioenergy

Warm-season prairie grasses are promising bioenergy crops that exhibit conservative nitrogen use and cycling, which promise to reduce expensive N amendments and contributions to environmental N pollution associated with annual cropping systems. However, efforts to maximize crop yields with fertilizer may reduce N conservation in these systems. We used two perennial grass systems of differing diversity levels – a restored tallgrass prairie and a Panicum virgatum (switchgrass) monoculture—to determine the effects of N fertilizer level and harvest timing on biomass yields, N concentrations and N removal at harvest. To address plant N conservation, we measured N resorption efficiency (proportion of N resorbed), resorption proficiency (minimum N level attained after senescence), and timing of resorption under different N fertilizer rates. Yield responses to N fertilizer were not consistent between cropping system, year, or harvest timing, and were generally weak, resulting in an average of only 1.27 times more biomass compared to unfertilized plots. In contrast, fertilized plots removed 1.67 times more N relative to unfertilized plots, as N removal was largely driven by increases in biomass N concentrations rather than increases in yield. N resorption was affected by fertilizer in switchgrass, but not in selected prairie species. Fertilized switchgrass plants took longer to reach their maximum resorption levels and had reduced resorption proficiency, despite higher resorption efficiencies. Our results suggest that striving to increase biomass yields with N fertilizer may be an imprudent approach to sustainable bioenergy production, because yield responses are highly variable and N conservation mechanisms are compromised.

Characterisation of the fuel and fire environment in southern Ontario’s tallgrass prairie

Prescribed burning can be an integral part of tallgrass prairie restoration and management. Understanding fire behaviour in this fuel is critical to conducting safe and effective prescribed burns. Our goal was to quantify important physical characteristics of southern Ontario’s tallgrass fuel complex prior to and during prescribed burns and synthesise our findings into useful applications for the prescribed fire community. We found that the average fuel load in tallgrass communities was 0.70 kg m–2. Fuel loads varied from 0.38 to 0.96 kg m–2. Average heat of combustion did not vary by species and was 17 334 kJ kg–1. A moisture content model was developed for fully cured, matted field grass, which was found to successfully predict moisture content of the surface layers of cured tallgrass in spring. We observed 25 head fires in spring-season prescribed burns with spread rates ranging from 4 to 55 m min–1. Flame front residence time averaged 27 s, varying significantly with fuel load but not fire spread rate. A grassland spread rate model from Australia showed the closest agreement with observed spread rates. These results provide prescribed-burn practitioners in Ontario better information to plan and deliver successful burns.