North American Tallgrass Prairie Research Articles

Contrasting shrub and grass hydraulic responses to experimental drought.

Whole-plant hydraulics provide important information about responses to water limitation and can be used to understand how plant communities may change in a drier climate when measured on multiple species. Here, we measured above- and belowground hydraulic traits in Cornus drummondii, an encroaching shrub within North American tallgrass prairies, and Andropogon gerardii, a dominant C4 grass, to assess the potential hydraulic responses to future drought as this region undergoes woody expansion. Shelters that reduced precipitation by 50% and 0% were built over shrubs and grasses growing in sites that are burned at 1-year and 4-year frequencies. We then measured aboveground (Kshoot), belowground (Kroot), and whole-plant maximum hydraulic conductance (Kplant) in C. drummondii and Kroot in A. gerardii. We also measured vulnerability to embolism (P50) in C. drummondii stems. Overall, we show that: (1) A. gerardii had substantially greater Kroot than C. drummondii; (2) belowground hydraulic functioning was linked with aboveground processes; (3) above- and belowground C. drummondii hydraulics were not negatively impacted by the rainfall reductions imposed here. These results suggest that a multi-year drought will not ameliorate rates of woody expansion and highlight key differences in aboveground and belowground hydraulics for dominant species within the same ecosystem.

Long-term fire and grazing regimes modify soil physical properties and root traits in North American tallgrass prairie

The native Konza tallgrass prairie is maintained by two key natural processes, periodic fire and ungulate grazing. As a potential crucial feedback mechanism, how fire and grazing influence plant root traits and soil properties in the tallgrass prairie ecosystem remains less known. Here, we investigated the effects of long-term fire and grazing regimes on soil physical properties and root traits at the Konza Prairie Biological Station, near Manhattan, KS, USA. Soil cores were collected under control (unburned and ungrazed), fire (annually burned), grazing (annually grazed), and fire + grazing (annually burned and grazed) treatments. Soil water retention, total root volume, specific root length, specific root area, and root biomass were measured. We found that both fire and grazing significantly reduced soil water retention, increased soil bulk density and soil microporosity (<50 μm), and reduced macroporosity (>50 μm) compared to the unburned and ungrazed treatment. Fire and grazing also significantly reduced litter accumulation by 79 % (burned), 57 % (grazed), and 77 % (burned and grazed). Fire increased specific root area by 15 %. Grazing and fire + grazing significantly decreased specific root length by 40 % and 29 %, and specific root area by 33 % and 24 %, respectively. Litter production was positively related to soil volumetric water content and soil pore volume of 300–50 μm, but negatively related to soil bulk density and soil pore volume of less 50 μm. Soil volumetric water content was positively correlated to specific root lengths, and soil bulk density was negatively related to specific root length and specific root area. There was a significant negative relationship between root diameter and soil volumetric water content under burned and grazed treatment. The SEM model showed that fire, grazing, and fire + grazing regimes were associated with dead root biomass (R2 = 0.38) and root length density (R2 = 0.40) indirectly through soil bulk density and litter accumulation. Our findings suggested that the annual fire + grazing regime seriously leads to the degradation of tallgrass prairie by influencing soil physical properties, reducing root trait functions and root biomass, and altering the relationship between root diameter and soil volumetric water content.

Catching up with the trees: empirical advancements to improve herbaceous representation in models.

This article is a Commentary on Curasi et al. (2023), 239: 562–575.

Evolutionary lineage explains trait variation among 75 coexisting grass species.

Evolutionary history plays a key role driving patterns of trait variation across plant species. For scaling and modeling purposes, grass species are typically organized into C3 vs C4 plant functional types (PFTs). Plant functional type groupings may obscure important functional differences among species. Rather, grouping grasses by evolutionary lineage may better represent grass functional diversity. We measured 11 structural and physiological traits insitu from 75 grass species within the North American tallgrass prairie. We tested whether traits differed significantly among photosynthetic pathways or lineages (tribe) in annual and perennial grass species. Critically, we found evidence that grass traits varied among lineages, including independent origins of C4 photosynthesis. Using a rigorous model selection approach, tribe was included in the top models for five of nine traits for perennial species. Tribes were separable in a multivariate and phylogenetically controlled analysis of traits, owing to coordination of important structural and ecophysiological characteristics. Our findings suggest grouping grass species by photosynthetic pathway overlooks variation in several functional traits, particularly for C4 species. These results indicate that further assessment of lineage-based differences at other sites and across other grass species distributions may improve representation of C4 species in trait comparison analyses and modeling investigations.

Invasion of a non‐native forb reduces flammability in a fire‐dependent ecosystem

AbstractFire and grazing are two of the most common global disturbances that maintain and promote many grassland ecosystems worldwide. The presence of non‐native, invasive plant species can alter fire regimes through changes in fuel properties that affect native vegetation in many ecosystems and are a leading threat to biodiversity and ecosystem function. Sericea lespedeza (Lespedeza cuneata), a non‐native, invasive forb frequently present in many North American tallgrass prairies, may reduce flammability by altering the fuel bed moisture and structural characteristics to reduce fire in this fire‐dependent ecosystem. The goal of this research was to examine the influence of sericea lespedeza on fuel bed structure, moisture, and flammability compared to native grassland fuels. Fuel bed burning trials, conducted in a controlled laboratory setting, tested for the effect of fuel load mass of native grass and sericea lespedeza litter, proportion of sericea fuel load, and fuel moisture content on flammability metrics. Regardless of moisture content, sericea lespedeza reduced flammability and reduced maximum fire temperatures by as much as 500°C. Sericea lespedeza poses a major threat to North American tallgrass prairies by decreasing flammability of fuel beds through increased fuel bed density and moisture retention.

Upland Rootzone Soil Water Deficit Regulates Streamflow in a Catchment Dominated by North American Tallgrass Prairie

Intermittent tallgrass prairie streams depend on surface runoff and are highly susceptible to hydrological disturbances such as droughts. The objective of this study was to investigate the timing of intermittent streamflow pulses and upstream rootzone soil water deficit in a watershed dominated by tallgrass prairie. The study was conducted from July to December 2021 in the Kings Creek watershed located within the Konza Prairie Biological station, Kansas, USA. Hourly precipitation and soil moisture observations in the 0–10, 10–30, and 30–50 cm depth were obtained from a hydrological network consisting of 16 monitoring stations across the Kings Creek watershed. Rootzone soil water storage (S) was computed at hourly time steps as the sum of the soil water storage of each soil layer. A drained upper limit (DUL) was estimated as the soil moisture remaining 24 h after the soil had been thoroughly wetted during large (~100 mm) rainfall events. A lower limit (LL) was estimated as the lowest rootzone soil water storage during the study period. Hourly soil water deficit (D) was computed as D = (DUL − S)/(DUL − LL). The study period had 19 precipitation events totaling 436 mm, and only 14 out of the 19 precipitation events exceeded a common canopy and litter interception threshold of 4 mm for tallgrass prairies in this region. Only two precipitation events resulted in measurable streamflow, and the inception of these two streamflow events was associated with a negative weighted soil water deficit (i.e., S &gt; DUL). This pilot study revealed that upland rootzone soil water deficit plays a major role controlling the timing of streamflow in the Kings Creek watershed and possibly in other catchment areas with intermittent prairie streams.

Selection on convergent functional traits drives compositional divergence in early succession of a tallgrass prairie restoration experiment

Abstract Plant biodiversity is often partitioned into taxonomic diversity (species composition and abundance), phylogenetic diversity (breadth of evolutionary lineages) and functional diversity (resource‐use strategies or physical traits). Evaluating the effects and interplay of these dimensions can provide insights into how assembly processes drive compositional changes in plant communities. However, teasing apart the effects of different biodiversity dimensions is challenging in observational studies or retrospective analyses. To evaluate how plant phylogenetic and trait history shape community establishment and turnover in restoration of a species‐rich North American tallgrass prairie, we conducted an experiment with 127 species planted in assemblages representing three levels of phylogenetic diversity (PD) and two of functional trait diversity (FD), holding starting species richness (SR) fixed. We tested whether PD and FD of planted assemblages predicted species diversity, compositional turnover and selection on functional traits. Rank order of initial functional and phylogenetic diversity levels was maintained throughout the experiment, but neither diversity measure correlated positively with species richness by the end of the experiment. Phylogenetic and taxonomic beta diversity increased among all treatments. This increase in compositional beta diversity was associated with directional selection on phylogenetically dispersed functional traits. A set of functional traits associated with competitiveness in tallgrass prairies predicted species' cover for all survey years: stem dry matter content, leaf dry matter content, vegetative height and rhizomatous growth. Although all plots collectively converged on a similar suite of functional traits, functional beta diversity increased among high‐FD plots. Synthesis. Neither higher functional nor phylogenetic diversity maintained higher species richness (SR) over time in our study. Although SR was not maintained, higher levels of PD and FD were. Both types of diversity shaped the rate at which plots changed in composition over time, with high diversity treatment plots increasing in beta diversity. Selection for traits convergent across the tree of life drove phylogenetic and compositional divergence among plots. While optimization of site‐specific functional traits may be most important for maintaining higher SR, our work implies that planting higher initial PD and FD may make grassland restorations more adaptable to site conditions that may be difficult to predict.

Do southern seed or soil microbes mitigate the effects of warming on establishing prairie plant communities?

Restoration in this era of climate change comes with a new challenge: anticipating how best to restore populations to persist under future climate conditions. Specifically, it remains unknown whether locally adapted or warm-adapted seeds best promote native plant community restoration in the warmer conditions predicted in the future and whether local or warm-adapted soil microbial communities could mitigate plant responses to warming. This may be especially relevant for biomes spanning large climatic gradients, such as the North American tallgrass prairie. Here, we used a short-term mesocosm experiment to evaluate how seed provenances (Local Northern region, Non-local Northern region, Non-local Southern region) of 10 native tallgrass prairie plants (four forbs, two legumes, and four grasses) responded to warmer conditions predicted in the future and how soil microbial communities from those three regions influenced these responses. Warming and seed provenance affected plant community composition and warming decreased plant diversity for all three seed provenances. Plant species varied in their individual responses to warming, and across species, we detected no consistent differences among the three provenances in terms of biomass response to warming and few strong effects of soil provenance. Our work provides evidence that warming, in part, may reduce plant diversity and affect restored prairie composition. Because the southern provenance did not consistently outperform others under warming and we found little support for the "local is best" paradigm currently dominating restoration practice, identifying appropriate seed provenances to promote restoration success both now and in future warmer environments may be challenging. Due to the idiosyncratic responses across species, we recommend that land managers compare seeds from different regions for each species to determine which seed provenance performs best under warming and in restoration for tallgrass prairies.

Phylogenetic distance and resource availability mediate direction and strength of plant interactions in a competition experiment.

Phylogenetic ecology uses evolutionary history to improve understanding of plant interactions. Phylogenetic distance can mediate plant interactions such as competition (e.g., via limiting similarity) and facilitation (e.g., via niche complementarity), influencing community assembly patterns. Previous research has found evidence both for and against a relationship between phylogenetic distance and the strength of plant interactions, and has found that other factors, such as trait differences, may be more influential. In addition to phylogenetic distance and species' traits, environmental conditions can also influence competition, with facilitative interactions-particularly among distantly related species-potentially becoming more pronounced under stressful, resource-limited conditions. We tested the prediction that greater phylogenetic distance is associated with decreased competition in a greenhouse experiment using plant species of the North American tallgrass prairie. We calculated the Relative Interaction Index for 81 species pairs using plant height, leaf length, and biomass as indicators of performance. We found that phylogenetic distance alone did not significantly affect competition. However, the interaction between phylogenetic distance and stressful conditions (sandier soils with low nutrient availability and water retention vs. resource-rich potting soil) altered plant traits and competition. Under stressful conditions, more distantly related species competed more strongly, leading to smaller plants. Conversely, under benign conditions more distantly related species pairs competed less and were larger. These results were contrary to our expectations that distant relatives would compete less under stressful conditions. Our experiment provides evidence that, while relatedness alone may not drive competition, phylogenetic distance can nonetheless be influential through interactions with environmental conditions.

Community‐level assisted migration for climate‐appropriate prairie restoration

Climate change threatens the long‐term survival of highly fragmented habitats such as North American tallgrass prairie. De novo plantings into former agricultural sites can contribute to efforts to conserve prairie biodiversity, but past planting practice (including our own) has focused on the use of local species and genotypes. Climate change makes this “local‐only” practice obsolete because of rapidly shifting species' ranges. We propose that new prairie plantings incorporate assisted migration at the community level; all species and genotypes that are viable in the changed climate and potentially could reach the site in an unfragmented landscape should be considered for inclusion. Selecting and sourcing species for climate‐appropriate restorations is possible with minimal additional cost or expertise. Based on best‐available estimates of current and future climate, species richness of future prairie plantings at our Minnesota study site could be increased by over one‐third (26 new species). Potential risks to such extensive use of assisted migration are offset by the drastic conservation challenge of highly fragmented communities under climate change.

Fires slow population declines of a long-lived prairie plant through multiple vital rates.

In grasslands worldwide, modified fire cycles are accelerating herbaceous species extinctions. Fire may avert population declines by increasing survival, reproduction, or both. Survival and growth after fires may be promoted by removal of competitors or biomass and increasing resource availability. Fire-stimulated reproduction may also contribute to population growth through bolstered recruitment. We quantified these influences of fire on population dynamics in Echinacea angustifolia, a perennial forb in North American tallgrass prairie. We first used four datasets, 7-21years long, to estimate fire's influences on survival, flowering, and recruitment. We then used matrix projection models to estimate growth rates across several burn frequencies in five populations, each with one to four burns over 15years. Finally, we estimated the contribution of fire-induced changes in each vital rate to changes in population growth. Population growth rates generally increased with burning. The demographic process underpinning these increases depended on juvenile survival. In populations with high juvenile survival, fire-induced increases in seedling recruitment and juvenile survival enhanced population growth. However, in populations with low juvenile survival, small changes in adult survival drove growth rate changes. Regardless of burn frequencies, our models suggest populations are declining and that recruitment and juvenile survival critically influence population response to fire. However, crucially, increased seedling recruitment only increases population growth rates when enough new recruits reach reproductive maturity. The importance of recruitment and juvenile survival is especially relevant for small populations in fragmented habitats subject to mate-limiting Allee effects and inbreeding depression, which reduce recruitment and survival, respectively.

Panicum virgatum (Poaceae)

Switchgrass (Panicum virgatum L.) is a promising biofuel crop and one of the dominant species in North American tallgrass prairies. It grows rapidly, reaching up to 2.7 m, and planted fields remain productive for 15 years. The large, highly repetitive, and highly heterozygous genome of the generally tetraploid species have always been challenging for sequencing and assembly. Using PacBio long-read sequencing and bacterial artificial chromosome clone technology, researchers recently published a highly continuous genome assembly (v5) of a lowland switchgrass genotype AP13, which consisted of 626 contigs with a contig N50 of 5.5 Mb and a total of 97.2% bases that were assembled in the chromosomes.

Super‐abundant C4 grasses are a mixed blessing in restored prairies

Forbs comprise most of the plant diversity in North American tallgrass prairie and provide vital ecosystem services, but their abundance in prairie restorations is highly variable. Restoration practitioners typically sow C4 grasses in high abundances because they are inexpensive, provide fuel for prescribed fires, can dominate reference sites, and suppress weeds that suppress sown forbs. However, C4 grasses can also suppress sown forbs, calling this practice into question. We evaluated how C4 grasses influence the abundance and diversity of sown forbs in 78 restored prairies across Illinois, Indiana, and Michigan. We found that the direct negative effects of C4 grasses on sown forbs outweighed indirect positive effects that occurred as C4 grasses suppressed nonsown species, which in turn suppressed sown forbs. This pattern was especially strong for the C4 grass big bluestem (Andropogon gerardii). Therefore, strategies to promote big bluestem and other C4 grasses would not promote sown forbs. Although C4 grass cover was not strongly related to two hypothesized drivers (time since fire or site age), seeding density of C4 grasses increased their cover. Sown forb cover also increased with forb seeding density, increased indirectly with fire (through its negative effect on nonsown species), and decreased indirectly with soil water‐holding capacity (through its positive effect on nonsown species). These results highlight the complex interplay of species groups during grassland restoration and show how managers can promote sown forbs in restored prairies: increasing forb seeding density and reducing time since fire and the abundance of C4 grasses and weeds.

Environmentally responsive QTL controlling surface wax load in switchgrass.

Quantitation of leaf surface wax on a population of switchgrass identified three significant QTL present across six environments that contribute to leaf glaucousness and wax composition and that show complex genetic × environmental (G × E) interactions. The C4 perennial grass Panicum virgatum (switchgrass) is a native species of the North American tallgrass prairie. This adaptable plant can be grown on marginal lands and is useful for soil and water conservation, biomass production, and as a forage. Two major switchgrass ecotypes, lowland and upland, differ in a range of desirable traits, and the responsible underlying loci can be localized efficiently in a pseudotestcross design. An outbred four-way cross (4WCR) mapping population of 750 F2 lines was used to examine the genetic basis of differences in leaf surface wax load between two lowland (AP13 and WBC) and two upland (DAC and VS16) tetraploid cultivars. The objective of our experiments was to identify wax compositional variation among the population founders and to map underlying loci responsible for surface wax variation across environments. GCMS analyses of surface wax extracted from 4WCR F0 founders and F1 hybrids reveal higher levels of wax in lowland genotypes and show quantitative differences of β-diketones, primary alcohols, and other wax constituents. The full mapping population was sampled over two seasons from four field sites with latitudes ranging from 30 to 42°N, and leaf surface wax was measured. We identified three high-confidence QTL, of which two displayed significant G × E effects. Over 50 candidate genes underlying the QTL regions showed similarity to genes in either Arabidopsis or barley known to function in wax synthesis, modification, regulation, and transport.

Effects of white‐tailed deer exclusion on the plant community composition of an upland tallgrass prairie ecosystem

AbstractQuestionsGrasslands support ecosystem services, promote diversity, and assist in carbon sequestration. However, grasslands worldwide are diminishing in area, and understanding the drivers shaping the remaining grasslands is critical for their maintenance. The North American tallgrass prairie covers approximately 13% of its historical range and is shaped by fire and herbivory. Fire frequency negatively correlates with plant species richness, while bison (Bos bison) — the historical grazers — offset this effect. However, bison populations have declined, and large browsers are increasing in density. Few studies though have examined the role of large browsers — particularly white‐tailed deer (Odocoileus virginianus) — and their interaction with fire frequency in tallgrass prairies. Here, we addressed two questions: (a) What are the impacts of deer on plant diversity, species identities, and relative abundances; and (b) is there an interactive effect between the pressures exerted by deer and the well‐documented effects of fire in driving plant community responses?LocationThis study took place at the Konza Prairie Biological Station in northeastern Kansas, USA.MethodsUsing a 22‐year deer exclosure experiment, we examined differences in plant species richness, evenness, and plant community composition between plots that were either accessible or inaccessible to deer, in areas burned annually or once every four years.ResultsWe did not find significant effects of deer or interactive effects between deer and burning frequency on any metric of the plant community measured, including plant species richness, evenness, and plant community composition.ConclusionsContrary to the impact that deer have in other ecosystems (e.g. forests), our results indicate that deer do not affect the plant community of herbaceous‐dominated tallgrass prairies. These results indicate that while the loss of bison‐grazers has shifted tallgrass prairie plant communities to C4 grass‐dominated systems, the shift to browsing‐dominated herbivore pressure from deer has a minimal effect on the plant community.

Further evidence of Ceratobasidium D.P. Rogers (Basidiomycota) serving as the ubiquitous fungal associate of Platanthera leucophaea (Orchidaceae) in the North American tallgrass prairie

BackgroundIn the United States and Canada, ca. one-half of native orchid species are now threatened with extinction. A number of these species are restricted to tallgrass prairies of central North America, such as the Eastern Prairie Fringed Orchid, Platanthera leucophaea (Nutt.) Lindl.—a U.S. Federally threatened species.ResultsWe provide new records of fungi recovered from roots of P. leucophaea and five other orchid species inhabiting prairie sites in Illinois and neighboring states during a 10-year period (2008–2017). A total of 39 fungal endophytes were isolated from Cypripedium candidum (1), Platanthera lacera (1), P. leucophaea (32), P. peramoena (3), Spiranthes lacera (1), and S. magnicamporum (1), 31 (79%) of which were assignable to Ceratobasidium and the remainder to Tulasnella. These fungi were acquired from 16 different sites, 13 of which are new records including two new state records (Iowa, Wisconsin). Molecular analysis revealed that some Ceratobasidium strains were virtually identical despite being geographically isolated by > 300 km.ConclusionsThis study, encompassing a decade of work, confirms that Platanthera leucophaea is a mycorrhizal specialist with heavy reliance on Ceratobasidium with the tallgrass prairie ecosystem of North America. Our isolation of Ceratobasidium from P. leucophaea spanning additional sites suggests that the association is widespread. Such information should provide conservationists and land managers with more confidence in developing protocols that facilitate the long-term conservation of this prairie orchid.

White-Tailed Deer in Tallgrass Prairie: Novel Densities of a Native Herbivore in Managed Ecosystems

White-tailed deer (Odocoileus virginianus) are now the dominant large herbivores in tallgrass prairie ecosystems of the U.S. Midwest and have unique ecological impacts compared to the historically dominant bison (Bison bison) and elk (Cervus elaphus) that were extirpated from this region. While most of our knowledge of white-tailed deer impacts on plant communities and ecosystem processes comes from forested habitats, relatively little research has focused on deer effects in prairies. In this review, we discuss the ecological impacts of white-tailed deer in North American tallgrass prairies, potential deer management strategies, and areas of future research.Unlike bison and elk, deer are selective grazers that preferentially eat nutrient-rich forbs. At high deer densities, this consumption of forbs can have negative effects on prairie biodiversity and alter nutrient cycling. Deer can decrease plant diversity by reducing the abundance and reproductive success of their preferred plant species, shifting the plant community to one dominated by species avoided by deer or tolerant of herbivory.At lower densities, deer may maintain biodiversity in accordance with the intermediate disturbance hypothesis. Deer are able to maintain a balance between deer-avoided and deer-preferred species abundance by giving unpreferred or herbivory-tolerant species an advantage, but not completely suppressing herbivory-sensitive species. Nonconsumptive deer impacts also influence prairies through transporting invasive species, trampling on deer trails, and redistributing nutrients in fecal pellets and carcasses. Management strategies such as hunting, use of fences, and patch-burning can alleviate the pressures of overabundant deer populations, while maintaining their ability to promote heterogeneity.

High specificity of a rare terrestrial orchid toward a rare fungus within the North American tallgrass prairie

The Orchidaceae are globally distributed and represent a diverse lineage of obligate mycotrophic plants. Given their dependence on symbiotic fungi for germination and/or plant development, fungal community structure in substrates is expected to influence the distribution and persistence of orchid species. Yet, simultaneous characterization of orchid mycorrhizal fungal (OMF) communities in roots and in soil is rarely reported. To explain the co-distributions of OMF in roots, orchid-occupied, and bulk soil, we characterized mycorrhizal fungi associated with Platanthera praeclara over multiple years across its entire natural distribution within the North American tallgrass prairie. Root derived OMF communities included 24 Ceratobasidiaceae and 7 Tulasnellaceae operational taxonomic units (OTUs) though the orchid exhibited high spatio-temporal specificity toward a single Ceratobasidiaceae OTU, which was strongly stable across population sizes and phenological stages of the sampled individuals. The preferred OMF OTUs were primarily restricted to orchid-occupied locations while infrequent or absent in bulk soil. Variation in soil OMF assemblies was explained most by soil moisture, magnesium, manganese, and clay. In this first study of coupled root and soil OMF communities across a threatened grassland ecosystem, we report a strong relationship, further nuanced by soil chemistry, between a rare fungus and a rare orchid.

Forbs, grasses, and grassland fire behaviour

Abstract In grasslands and savannas, fire regime—frequently a major determinant of woody encroachment, herbaceous species composition and diversity, and nutrient cycling—is influenced by the quantity and characteristics of plant fuel. Laboratory studies reveal variation in flammability among herbaceous species, but field experiments are needed to assess whether herbaceous species composition meaningfully affects ecosystem‐scale fire behaviour. In our North American tallgrass prairie study system, grasses’ thinner leaves and longer leaf retention appeared to create a finer, more aerated, more connected fuel bed than forbs. We tested the hypothesis that grasses promote fire spread area, fire intensity, and associated facets of fire behaviour more strongly than an equivalent mass of forbs. We characterized spring fires over multiple years in 315 annually ignited plots spanning profound gradients of plant biomass, cover, and grass:forb ratio that resulted from species richness and composition treatments, in a 20‐year grassland biodiversity experiment. Grasses increased fire spread and associated facets of fire behaviour, compared with an equivalent biomass or cover of forbs. Grass dominance increased fire spread area—or equivalently increased fire frequency at any given point. For fire to spread through 50% of the 9 m × 9 m plot area required approximately twice as high an abundance of forbs as of grasses. Grass dominance also resulted in fires that advanced faster, were more intense (higher rates of heat release per unit fireline length), caused more damage to plants, and released heat to greater heights. Fire temperature at 50 cm above‐ground was about twice as high in plots with only grasses as in plots with the same biomass of forbs. Synthesis. Even within herbaceous ecosystems that may appear homogenously flammable compared with less flammable woody ecosystems, fuel quality—specifically, the proportional abundance of grasses—combines with fuel quantity and ignitions to determine effective fire regime at a given point. In spring burns, grass‐dominated plots burn more completely and generate higher temperatures, and thus better suppress woody plants and volatilize more nutrients, than forb‐dominated plots (holding all else equal).

Drivers of nocturnal water flux in a tallgrass prairie

Abstract Nocturnal transpiration can impact water balance from the local community to earth‐atmosphere fluxes. However, the dynamics and drivers of nocturnal transpiration among coexisting plant functional groups in herbaceous ecosystems are unknown. Here, we addressed the following questions: (1) How do nocturnal (Enight) and diurnal (Eday) transpiration vary among coexisting grasses, forbs, and shrubs in a tallgrass prairie? (2) What environmental variables drive Enight and do these differ from the drivers of Eday? (3) Is Enight associated with daytime physiological processes? We measured diurnal and nocturnal leaf gas exchange on perennial grass, forb and woody species in a North American tallgrass prairie. Measurements were made periodically across two growing seasons (May–August 2014–2015) on three C4 grasses (Andropogon gerardii, Sorghastrum nutans and Panicum virgatum), two C3 forbs (Vernonia baldwinii and Solidago canadensis), one C3 sub‐shrub (Amorpha canescens) and two C3 shrubs (Cornus drummondii and Rhus glabra). By extending our study to multiple functional groups, we were able to make several key observations: (1) Enight was variable among co‐occurring plant functional groups, with the highest rates occurring in C4 grasses, (2) Enight and Eday exhibited different responses to vapour pressure deficit and other environmental drivers, and (3) rates of Enight were strongly related to predawn leaf water potential for grasses and woody species, and were likely modulated by small‐scale changes in soil moisture availability. Our results provide novel insight into an often‐overlooked portion of ecosystem water balance. Considering the high rates of Enight observed in C4 grasses, as well as the widespread global occurrence of C4 grasses, nocturnal water loss might constitute a greater proportion of global evapotranspiration than previously estimated. Additionally, future predictions of nocturnal water loss may be complicated by stomatal behaviour that differs between the day and at night. Finally, these data suggest a water‐use strategy by C4 grasses wherein the high rates of Enight occurring during wet periods may confer a competitive advantage to maximize resource consumption during periods of greater availability. A plain language summary is available for this article.