Temporal dynamics of plant community regeneration sources during tallgrass prairie restoration

Ecological restoration aims to augment and steer the composition and contribution of propagules for community regeneration in degraded environments. We quantified patterns in the abundance, richness, and diversity of seed and bud banks across an 11-year chronosequence of restored prairies and in prairie remnants to elucidate the degree to which the germinable seed bank, emerged seedlings, belowground buds, and emerged ramets were related to community regeneration. There were no directional patterns in the abundance, richness, or diversity of the germinable seed bank across the chronosequence. Emerged seedling abundance of sown species decreased during restoration. Richness and diversity of all emerged seedlings and non-sown emerged seedling species decreased across the chronosequence. Conversely, abundance and richness of belowground buds increased with restoration age and belowground bud diversity of sown species increased across the chronosequence. Numbers of emerged ramets also increased across the chronosequence and was driven primarily by the number of graminoid ramets. There were no temporal changes in abundance and richness of sown and non-sown emerged ramets, but diversity of sown emerged ramets increased across the chronosequence. This study demonstrates that after initial seeding, plant community structure in restored prairies increasingly reflects the composition of the bud bank.     

Temporal vegetation dynamics and recolonization mechanisms on different-sized soil disturbances in tallgrass prairie

Assessing the various mechanisms by which plants revegetate disturbances is important for understanding the effects of disturbances on plant population dynamics, plant community structure, community assembly processes, and ecosystem function. We initiated a 2-yr experiment examining temporal vegetation dynamics and mechanisms of recolonization on different-sized soil disturbances created to simulate pocket gopher mounds in North American tallgrass prairie. Treatments were designed to assess potential contributions of the seed rain, soil seed bank, clonal propagation from the edges of a soil mound, and regrowth of buried plants. Small mounds were more rapidly recolonized than large mounds. Vegetative regrowth strategies were the dominant recolonization mechanisms, while the seed rain was considerably less important in maintaining the diversity of forbs and annuals than previously believed. All recolonization mechanisms influenced plant succession, but stem densities and plant mass on soil mounds remained significantly lower than undisturbed controls after two growing seasons. Because natural pocket gopher mounds are indistinguishable from undisturbed areas after two seasons, these results suggest that multiple modes of recruitment concurrently, albeit differentially, contribute to the recolonization of soil disturbances and influence tallgrass prairie plant community structure and successional dynamics.     

Fine-scale spatial organization of tallgrass prairie vegetation along a topographic gradient

Fine-scale spatial patterns of native tallgrass prairie vegetation were studied on Konza Prairie, Kansas, USA. Three sites, upland, slope, and lowland, were sampled in an ungrazed watershed. Presence of vascular plant species was recorded in two 25.6 m long transects of contiguous 5×5 cm micro-quadrats on each topographic position. Spatial patterns of species and functional groups were analyzed by information theory models ofJuhász-Nagy. Within-community variability of coexistence was expressed by the diversity and spatial dependence of local species combinations. Considerable diversity in the local coexistence of species was found on each site. Upland and hillside communities were richer and more diverse in species combinations than lowland. Spatial scale effected coexistence relationships. The maxima of information theory estimates varied between 15 and 30 cm. There was no trend in the variation of characteristic scales along the topographical gradient. Above 10 m, all sites tended to be homogeneous. The analysis of spatial associations revealed that variability in the local coexistence of species was strongly constrained in all topographic positions. Overall spatial association of species was the lowest on lowland. The characteristic scales of maximum association were between 1.2 m and 3 m at all sites. The maxima of information theory estimates for the functional group-based data appeared at smaller plot sizes than for the species based analyses. Only weak spatial associations were detected among the functional groups indicating that individuals of functional groups coexist well at small scales, and form combinations close to random expectations. The length of transects did not effect the relative associations. Strong positive correlations were found between the number of components (species or functional groups) and the maxima, of information theory models suggesting that richness is a good predictor of within-community coexistence relations. However, there was no relationship between richness and the characteristic scales of community patterns.     

Experimental analysis of patch dynamics and community heterogeneity in tallgrass prairie

Effects of fire and small-scale soil disturbances on species richness, community heterogeneity, and microsuccession were investigated in a central Oklahoma tallgrass prairie. In the fall of 1985, 0.2 m² soil disturbances were created on burned and unburned tallgrass prairie. Vegetation on and off disturbances was sampled at monthly intervals over two growing seasons. During the first growing season, the cover of forbs and annuals, and species richness were significantly greater on versus off disturbances, but these differences did not persist through the second year. The variation in species composition among disturbed plots (heterogeneity) was significantly greater compared to undisturbed areas throughout the study. Fire had no consistent effect on richness and heterogeneity of vegetation on soil disturbances but fire reduced heterogeneity on undisturbed vegetation. Rate of succession, based on an increase in cumulative cover of perennial grasses over time, did not differ among treatments during the first growing season. During the second year, rate of succession was significantly greater on burned soil disturbances compared to unburned soil disturbances. These results suggest that while small-scale soil disturbances have primarily short-lived effects on grassland community structure, disturbances do help to maintain spatial and temporal variation in tallgrass prairie communities. Unlike in undisturbed vegetation, however, species richness and heterogeneity on soil disturbances were little effected by fire, but the rate of colonization onto disturbances appeared to be enhanced by fire.     

Precipitation timing and grazer performance in a tallgrass prairie

Changes in precipitation amount and variability have the potential to alter the structure and function of grasslands, but we know little about how changes in the timing of precipitation might affect grasslands. Here, we analyze long‐term records from a tallgrass prairie to show that shifts in the timing of precipitation during the growing season have little effect on primary productivity or grass reproduction, but can greatly affect grazer performance. While greater late‐season precipitation increases the weight gain of adult and young bison, greater mid‐season precipitation decreases their weight gain. In addition, calving rates are lower after years with greater mid‐season precipitation and higher after years with greater late‐season precipitation. As well‐timed drought can actually increase grazer weight gain and reproduction, it will be necessary to generate predictions of within‐season distribution of precipitation to successfully forecast future grazer performance.     

Flowering phenology as a functional trait in a tallgrass prairie

? The timing of flowering is a critical component of the ecology of plants and has the potential to structure plant communities. Yet, we know little about how the timing of flowering relates to other functional traits, species abundance, and average environmental conditions. ? Here, we assessed first flowering dates (FFDs) in a North American tallgrass prairie (Konza Prairie) for 431 herbaceous species and compared them with a series of other functional traits, environmental metrics, and species abundance across ecological contrasts. ? The pattern of FFDs among the species of the Konza grassland was shaped by local climate, can be linked to resource use by species, and patterns of species abundance across the landscape. Peak FFD for the community occurred when soils were typically both warm and wet, while relatively few species began flowering when soils tended to be the driest. Compared with late-flowering species, species that flowered early had lower leaf tissue density and were more abundant on uplands than lowlands. ? Flowering phenology can contribute to the structuring of grassland communities, but was largely independent of most functional traits. Therefore, selection for flowering phenology may be independent of general resource strategies.     

Interspecific nutrient transfer in a tallgrass prairie plant community

Interplant nutrient transfer may be an important ecological process in grasslands, and may significantly influence plant neighborhood interactions. We investigated the potential for phosphorus transfer between the dominant grass Andropogon gerardii and several neighboring plant species in tallgrass prairie via a field ³²PO₄ labelling experiment. The mean amount of ³²P received from donor shoots differed significantly among neighboring species and decreased with increasing distance from the donor. In general, forbs and cool-season C₃ grasses received more labelled ³²P than warm-season C₄ grasses. Phosphorus transfer occurred over distances up to 0.5 m. The effects of species and distance on movement of phosphorus changed with increasing time after labelling. The relative mass of receiver and donor shoots did not affect amounts of ³²P transfer. A benomyl fungicide treatment, applied to suppress mycorrhizal activity, likely did not affect existing vegetative hyphae and did not affect the amount of ³²P transferred. These studies demonstrate that: (1) phosphorus is transferred among neighboring species in tallgrass prairie plant communities, (2) phosphorus may be transferred over significantly greater distances than reported in other grasslands, and (3) there is differential transfer among co-occurring species. Hypothesized mechanisms accounting for these patterns in tallgrass prairie include mycorrhizal hyphal interconnections and/or extensive and differential root and rhizosphere overlap among neighboring species.     

Contrasting bud bank dynamics of two co-occurring grasses in tallgrass prairie: implications for grassland dynamics

Because most shoot recruitment in perennial grasses occurs from belowground axillary buds, bud dynamics determine plant population dynamics and meristem limitation to population growth. Therefore, grassland vegetation responses to environmental change or disturbance may be influenced by interspecific differences in bud banks and the patterns and environmental controls of bud development and demography. We examined bud bank dynamics in Andropogon gerardii and Dichanthelium oligosanthes in tallgrass prairie by enumerating and classifying buds throughout 15?months to determine whether grass buds live for multiple years and accumulate; whether bud natality, dormancy and outgrowth are synchronous or variable; and whether bud bank dynamics differ between these co-occurring species. Andropogon gerardii (a C₄ species) maintained a larger dormant bud bank, showed synchrony in bud development and transition to tiller, and its buds lived for multiple years. Thus, multiple previous years?? bud cohorts contributed to recruitment. By contrast, D. oligosanthes (a C₃ species) maintained a smaller dormant bud bank, had asynchronous bud development with active buds present year-round, and its buds lived for 1?year. Buds played different roles in the dynamics of each species, allowing A. gerardii to over-winter and recruit new spring tillers and D. oligosanthes to survive and recruit new tillers following summer dormancy. These differences in bud bank age structure, phenology, and dynamics between these species suggest greater demographic buffering and time-lag effects in A. gerardii populations. Interspecific differences in bud bank structure and dynamics may explain and help predict grassland responses to environmental change.     

Controls of nitrogen limitation in tallgrass prairie

Summary The relationship between fire frequency and N limitation to foliage production in tallgrass prairie was studied with a series of fire and N addition experiments. Results indicated that fire history affected the magnitude of the vegetation response to fire and to N additions. Sites not burned for over 15 years averaged only a 9% increase in foliage biomass in response to N enrichment. In contrast, foliage production increased an average of 68% in response to N additions on annually burned sites, while infrequently burned sites, burned in the year of the study, averaged a 45% increase. These findings are consistent with reports indicating that reduced plant growth on unburned prairie is due to shading and lower soil temperatures, while foliage production on frequently burned areas is constrained by N availability. Infrequent burning of unfertilized prairie therefore results in a maximum production response in the year of burning relative to either annually burned or long-term unburned sites.Foliage biomass of tallgrass prairie is dominated by C₄ grasses; however, forb species exhibited stronger production responses to nitrogen additions than did the grasses. After four years of annual N additions, forb biomass exceeded that of grass biomass on unburned plots, and grasses exhibited a negative response to fertilizer, probably due to competition from the forbs. The dominant C₄ grasses may out-compete forbs under frequent fire conditions not only because they are better adapted to direct effects of burning, but because they can grow better under low available N regimes created by frequent fire.     

Grasshopper fecundity responses to grazing and fire in a tallgrass prairie

Grasshopper abundance and diversity vary with management practices such as fire and grazing. Understanding how grasshopper life history traits such as fecundity respond to management practices is key to predicting grasshopper population dynamics in heterogeneous environments. Landscape-level experimental fire and bison grazing treatments at the Konza Prairie Biological Station (Manhattan, KS) provide an opportunity to examine how management affects grasshopper fecundity. Here we report on grasshopper fecundity for nine common species at Konza Prairie. From 2007 to 2009, adult female grasshoppers were collected every 3 wk from eight watersheds that varied in fire and grazing treatments. Fecundity was measured by examining female reproductive tracts, which contain a record of past and current reproductive activity. Body size was a poor predictor of fecundity for all species. Despite large differences in vegetation structure and composition with management regime (grazing and fire interval), we observed little effect of management on grasshopper fecundity. Habitat characteristics (grasshopper density, vegetation biomass, and vegetation quality; measured in 2008 and 2009) were better predictors of past fecundity than current fecundity, with species-specific responses. Fecundity increased throughout the summer, indicating that grasshoppers were able to acquire sufficient nutritional resources for egg production in the early fall when vegetation quality is generally low. Because fecundity did not vary across management treatments, population stage structure may be more important for determining population level reproduction than management regime at Konza Prairie.     

Carbon dynamics and microbial activity in tallgrass prairie exposed to elevated CO2 for 8 years

Alterations in microbial mineralization and nutrient cycling may control the long-term response of ecosystems to elevated CO₂. Because micro-organisms constitute a labile fraction of potentially available N and are regulators of decomposition, an understanding of microbial activity and microbial biomass is crucial. Tallgrass prairie was exposed to twice ambient CO₂ for 8 years beginning in 1989. Starting in 1991 and ending in 1996, soil samples from 0 to 5 and 5 to 15 cm depths were taken for measurement of microbial biomass C and N, total C and N, microbial activity, inorganic N and soil water content. Because of increased water-use-efficiency by plants, soil water content was consistently and significantly greater in elevated CO₂ compared to ambient treatments. Soil microbial biomass C and N tended to be greater under elevated CO₂ than ambient CO₂ in the 5–15 cm depth during most years, and in the month of October, when analyzed over the entire study period. Microbial activity was significantly greater at both depths in elevated CO₂ than ambient conditions for most years. During dry periods, the greater water content of the surface 5 cm soil in the elevated CO₂ treatments increased microbial activity relative to the ambient CO₂ conditions. The increase in microbial activity under elevated CO₂ in the 5–15 cm layer was not correlated with differences in soil water contents, but may have been related to increases in soil C inputs from enhanced root growth and possibly greater root exudation. Total soil C and N in the surface 15 cm were, after 8 years, significantly greater under elevated CO₂ than ambient CO₂. Our results suggest that decomposition is enhanced under elevated CO₂ compared with ambient CO₂, but that inputs of C are greater than the decomposition rates. Soil C sequestration in tallgrass prairie and other drought-prone grassland systems is, therefore, considered plausible as atmospheric CO₂ increases. This revised version was published online in June 2006 with corrections to the Cover Date.     

Dominance not richness determines invasibility of tallgrass prairie

Many recent studies suggest that more diverse communities are more resistant to invasion. Community characteristics that most strongly influence invasion are uncertain, however, due to covariation of diversity with competition and crowding. We examined separately the effects of species richness and dominance on invasion by an exotic legume, Melilotus officinalis , in intact, native Kansas grassland. We manipulated dominance of C₄ grasses by reducing their abundance (i.e. ramet densities) by ∼25 and 50%. In addition, richness was reduced by removing species that were mainly rare and uncommon as might be expected with environmental changes such as drought and fragmentation. In both years of the study (2001–2002), invasibility, measured as peak establishment of Melilotus , was not affected by a 3-fold reduction in species richness, nor was there an interaction between loss of species and reduced dominance on invasion. In contrast, reductions in abundance of the dominants significantly reduced invasibility of the grassland plots in both years. Because the abundance of dominants was highly correlated with measures of competition (i.e. ratio of dominant biomass to total biomass) and crowding (total stem densities), this pattern was opposite to that expected if competition were indeed limiting invasion. Rather, invasion appeared to be facilitated by the dominant species, most likely because reduced dominance increased environmental stress. Our results suggest that dominance is the key community characteristic determining invasibility, because highly competitive and space-filling species can either enhance or reduce susceptibility to invasion depending on whether dominants create a more competitive environment or alleviate stressful conditions.     

Functional consequences of climate change-induced plant species loss in a tallgrass prairie

Future climate change is likely to reduce the floristic diversity of grasslands. Yet the potential consequences of climate-induced plant species losses for the functioning of these ecosystems are poorly understood. We investigated how climate change might alter the functional composition of grasslands for Konza Prairie, a diverse tallgrass prairie in central North America. With species-specific climate envelopes, we show that a reduction in mean annual precipitation would preferentially remove species that are more abundant in the more productive lowland positions at Konza. As such, decreases in precipitation could reduce productivity not only by reducing water availability but by also removing species that inhabit the most productive areas and respond the most to climate variability. In support of this prediction, data on species abundance at Konza over 16 years show that species that are more abundant in lowlands than uplands are preferentially reduced in years with low precipitation. Climate change is likely to also preferentially remove species from particular functional groups and clades. For example, warming is forecast to preferentially remove perennials over annuals as well as Cyperaceae species. Despite these predictions, climate change is unlikely to unilaterally alter the functional composition of the tallgrass prairie flora, as many functional traits such as physiological drought tolerance and maximum photosynthetic rates showed little relationship with climate envelope parameters. In all, although climatic drying would indirectly alter grassland productivity through species loss patterns, the insurance afforded by biodiversity to ecosystem function is likely to be sustained in the face of climate change.     

Plant demographic responses to mycorrhizal symbiosis in tallgrass prairie

The effects of mycorrhizal symbiosis on seedling emergence, flowering and densities of several grasses and forbs were assessed in native tallgrass prairie and in sown garden populations at the Konza Prairie in northeastern Kansas. Mycorrhizal activity was experimentally suppressed with the fungicide benomyl. Flowering and stem densities of the cool-season grass, Dichanthelium oligosanthes, sedges (Carex spp.), and the forb Aster ericoides were higher in non-mycorrhizal (benomyl-treated) than in mycorrhizal plots and the magnitude of these differences was significantly affected by burning. Mycorrhizae significantly enhanced flowering of the warmseason grasses Andropogon gerardii and Sorghastrum nutans in burned prairie, but not in unburned sites. These patterns suggest that mycorrhizal effects on the dynamics of cool-season graminoid and forb populations are likely to be mediated indirectly through effects of the symbiosis on the competitive dominance of their neighbors. Seedling emergence rates of the cool-season C₃ grasses Elymus canadensis and Koeleria cristata were significantly reduced in the benomyl-treated plots, whereas benomyl treatment had no significant effect on seedling emergence of the warm-season C₄ grasses A. gerardii and Panicum virgatum. The forbs showed variable responses. Seedling emergence of Liatris aspera was greater under mycorrhizal conditions, but that of Dalea purpurea was unaffected by mycorrhizal treatment. These results show that effects of mycorrhizal symbiosis on the population dynamics of co-occurring prairie plants vary significantly both among species and among different life history stages within species. The results also indicate that mycorrhizas and fire interact to influence competitive interactions and demographic patterns of tallgrass prairie plant populations.     

Vertical distribution of fungal communities in tallgrass prairie soil

We used 454 sequencing of the internal transcribed spacer region to characterize fungal communities in tallgrass prairie soils subdivided into strata 0-10, 10-20, 30-40 and 50-60 cm deep. The dataset included more than 14000 fungal sequences distributed across Basidiomycota, Ascomycota, basal fungal lineages and Glomeromycota in order of decreasing frequency. As expected the community richness and diversity estimators tended to decrease with increasing depth. Although species richness was significantly reduced for samples from the deeper profiles, even the deepest stratum sampled contained richness of more than a third of that in the topmost stratum. More importantly, nonparametric multidimensional scaling (NMS) ordination analyses indicated that the fungal communities differed across vertical profiles, although only the topmost and deepest strata were significantly different when the NMS axis scores were compared by ANOVA. These results emphasize the importance of considering the fungal communities across the vertical strata because the deeper soil horizons might maintain a distinct community composition and thus contribute greatly to overall richness. The majority of operational taxonomic units (OTUs) declined in frequency with increasing depth, although a linear regression analysis indicated that some increased with increasing depth. The OTUs and BLAST-assigned taxa that showed increasing frequencies were mainly unculturable fungi, but some showed likely affinities to families Nectriaceae and Venturiaceae or to genus Pachnocybe. Although the ecological roles of the fungi in the deeper strata remain uncertain, we hypothesize that the fungi with preferences for deeper soil have adequate access to substrates and possess environmental tolerances that enable their persistence in those environments.     

Climate-driven spatial dynamics of plague among prairie dog colonies

We present a Bayesian hierarchical model for the joint spatial dynamics of a host-parasite system. The model was fitted to long-term data on regional plague dynamics and metapopulation dynamics of the black-tailed prairie dog, a declining keystone species of North American prairies. The rate of plague transmission between colonies increases with increasing precipitation, while the rate of infection from unknown sources decreases in response to hot weather. The mean annual dispersal distance of plague is about 10 km, and topographic relief reduces the transmission rate. Larger colonies are more likely to become infected, but colony area does not affect the infectiousness of colonies. The results suggest that prairie dog movements do not drive the spread of plague through the landscape. Instead, prairie dogs are useful sentinels of plague epizootics. Simulations suggest that this model can be used for predicting long-term colony and plague dynamics as well as for identifying which colonies are most likely to become infected in a specific year.     

Impact of management practices on the tallgrass prairie

Summary The ELM ecosystem-level grassland model simulates the flow of water, heat, nitrogen, and phosphorus through the ecosystem and the biomass dynamics of plants, consumers, and the decomposers. This model was adapted to a tallgrass prairie site in northeastern Oklahoma, USA, the Osage Site of the U.S. International Biological Program Grassland Biome. Several range management manipulations were simulated by the model and the results compared to field data and literature information: (1) altering the grazing intensity, grazing system, and grazing time period; (2) adding nitrogen and phosphorus to the grassland; (3) adding water during the growing season; and (4) spring burning of the prairie.The model showed that cattle weight gain per head, above-ground and belowground plant production, transpiration water loss, standing dead biomass, and the net nitrogen balance decrease with increasing grazing intensity, while soil water content and bare soil water loss increase. A moderately stocked year-round cow-calf grazing system is more beneficial to the grassland than a more highly stocked seasonal steer grazing system because the former increases the aboveground and belowground primary production and the plant nutrient uptake rates. Range manipulations, such as fire, which stimulate uniform grazing of a pasture, increase primary production, cattle weight gains, and nutrient uptake of plants and animals. Model results indicated that adding fertilizer was the best strategy for increasing cattle weight gains per head, while adding water would produce the greatest increase in primary production. Simulation of yearly and triennial spring burns suggests that these treatments increase primary production, plant nutrient uptake, and cattle weight gain per head. Burning increases the nitrogen losses from the systems; however, these losses are greater with annual burns. The model results also suggest the spatial grazing pattern of cattle must be considered to correctly represent the impact of grazing on the prairie.The model is used to describe the behavior of the tallgrass prairie ecosystem, evaluate alternative management strategies, and identify future scientific research and management studies.     

Long-term experimental warming decreased labile soil organic carbon in a tallgrass prairie

Plant and Soil - Climate warming has been hypothesized to influence dynamics of soil organic carbon (SOC), especially labile SOC due to its rapid response to changes in temperature and carbon (C)...     

Variation in grazing tolerance among three tallgrass prairie plant species

Three tallgrass prairie plant species, two common perennial forbs (Artemisia ludoviciana and Aster ericoides [Asteraceae]) and a dominant C(4) perennial grass (Sorghastrum nutans) were studied under field and greenhouse conditions to evaluate interspecific variation in grazing tolerance (compensatory growth capacity). Adaptation to ungulate grazing was also assessed by comparing defoliation responses of plants from populations with a 25-yr history of no grazing or moderate ungulate grazing. Under field conditions, all three species showed significant reductions in shoot relative growth rates (RGR), biomass, and reproduction with defoliation. In the two forbs, clipping resulted in negative shoot RGR and reductions in both number and length of shoot branches per ramet. Sorghastrum nutans maintained positive RGR under defoliation due to a compensatory increase in leaf production. Defoliation reduced rhizome production in A. ericoides and S. nutans, but not in A. ludoviciana. Clipping significantly reduced sexual reproductive allocation in all three species, although S. nutans showed a smaller reduction than the forbs. All three species showed similar responses to defoliation in burned and unburned sites. Under greenhouse conditions, a similar clipping regimen resulted in smaller reductions in growth and reproduction than those observed in the field. For all three species, the grazing tolerance indices calculated under natural field conditions were significantly lower than those estimated from greenhouse-grown plants, and the interspecific patterns of grazing tolerance were different. Aster ericoides exhibited the highest overall defoliation tolerance under greenhouse conditions, followed by S. nutans. Artemisia ludoviciana, the only study species that is typically not grazed by ungulates in the field, showed the lowest grazing tolerance. In the field experiment S. nutans showed the highest grazing tolerance and the two forbs had similar low tolerance indices. These patterns indicate that, despite high compensatory growth potential, limited resource availability and competition in the field significantly reduce the degree of compensation and alter interspecific differences in grazing tolerance among prairie plants. In all three species, defoliation suppressed sexual reproduction more than growth or vegetative reproduction. Significant interactions between plant responses to defoliation and site of origin (historically grazed or ungrazed sites) for some response variables (root/shoot ratios, rhizome bud initiation, and reproductive allocation) indicated some degree of population differentiation and genetic adaptation in response to a relatively short history of ungulate grazing pressure. The results of this study indicate that patterns of grazing tolerance in tallgrass prairie are both genetically based and also environmentally dependent.