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.     

Effects of management practices on annual net N-mineralization in a restored prairie and maize agroecosystems

Nitrogen (N) mineralization is a spatially variable and difficult component of the N cycle to quantify accurately under field conditions. Net N-mineralization was compared by direct measurement, indirect estimate, and laboratory incubation for a restored tallgrass prairie and for deficiently and optimally N-fertilized, no-tillage (NT) and chisel-plowed (CP) maize (Zea mays L.) agroecosystems on Plano silt loam soil (fine-silty, mixed, superactive, mesic Typic Argiudoll) in Wisconsin, USA. Four years of in-situ field measurements using an incubated-soil-core/ion-exchange-resin-bag technique showed that land use significantly affected net N-mineralization. Net N-mineralization was consistently smaller in the restored prairie than in the maize agroecosystems and typically larger in the CP than in the NT maize agroecosystems. Three independent methods for indirectly estimating annual net N-mineralization (i.e., N budget residual, deficiently N-fertilized plant N uptake, and profile-scaled in-situ field measurements) were relatively consistent at capturing land-use and tillage effects on net N-mineralization. Laboratory incubation and periodic leaching of Fall-sampled soils demonstrated that both mineralized N and labile C were co-limiting factors influencing N-mineralization in agricultural soils and generally supported field measurements by showing a significant difference in net N-mineralization with and without added fertilizer-N.     

Dispersal and establishment filters influence the assembly of restored prairie plant communities

Community assembly filters, which in theory determine the suite of species that arrive at and establish in a community, have tremendous conceptual relevance to restoration. However, the concept has remained largely theoretical, with a paucity of empirical tests. As such, the applicability of assembly filters theory to ecological restoration remains incompletely known. We tested the relative strengths of dispersal and establishment filters by comparing the plant species composition, measured by species' presence/absence, in 29 restored prairies with the seed mixes used to restore each prairie. We found that both establishment and dispersal filters limited prairie similarity to the seed mix. Sown species responded differentially to filters, with a few species limited only by dispersal (seed density), many others limited only by establishment conditions (i.e. organic matter and sand content of soils, land use history, and fire frequency), and others limited by both dispersal and establishment filters. A few species, typically those sown most often, were not restricted by dispersal or establishment filters, likely because they were sown in high enough densities and all sites had suitable environmental conditions. Finally, one group of species established poorly, but we could not attribute this to either dispersal or establishment filters. This information can help land managers select species likely to establish in restorations when sown at sufficient densities. These results illustrate that dispersal and establishment filters limit the establishment of species in restored communities and these filters are species‐dependent. Identifying the most limiting filter(s) for species will inform strategies to increase their establishment success.     

Pollination of a bee‐dependent forb in restored prairie: no evidence of pollen limitation in landscapes dominated by row crop agriculture

Restoration is used to conserve biodiversity; however, it is unclear to what extent restoration impacts ecosystem functions. Pollination is an ecosystem function that is critical to plant reproduction and thus restoration success. Few studies have assessed whether pollination is restored within restoration areas themselves. Plant–animal interactions may be affected by factors beyond the scale of the restoration. For example, surrounding landscape context may influence pollinator abundance and consequently the amount of pollen deposited. Decreased pollen receipt might then limit seed set. We hypothesized that in restorations surrounded by more agriculture, pollinator‐dependent forbs would experience greater pollen limitation. This would likely be due to declines in pollinator abundance within the restorations with an increase in surrounding agriculture. We deployed potted Chamaecrista fasciculata (Fabaceae), an obligatorily bee‐pollinated forb, and sampled bee communities in restored prairies in Minnesota, U.S.A. We measured pollen limitation by comparing seed set among open and supplementally pollinated plants. We also sampled native bees in seven of the eight sites. We tested for a relationship between proportion row crop agriculture (corn and soy) surrounding a restoration and pollen limitation, as well as an effect of agriculture on bee abundance. We did not find evidence that increasing proportion of surrounding agriculture negatively affected pollen limitation or bee abundance. Our results indicate that greater surrounding agriculture may not influence pollination of C. fasciculata through declines in pollinator availability, and suggest for some plants that landscape context might not limit pollination in restorations.     

The role of dominant prairie species ecotypes on plant diversity patterns of restored grasslands across a rainfall gradient in the US Great Plains

Questions

A robust ecosystem requires a functionally heterogeneous community of organisms with ecological traits that permit broad resource partitioning. Understanding community diversity patterns can help investigate drivers of community assembly and assess restoration success. Do biodiversity patterns differ among grassland communities sown with different ecotypes of dominant species during restoration along a rainfall gradient in the tallgrass prairie of the central US Great Plains?

Location

Four field sites across a rainfall gradient within the North American Great Plains: Colby, Kansas (39°23′17.8″N, 101°04′57.4″W), Hays, Kansas (38°51′13.2″N, 99°19′08.6″W), Manhattan, Kansas (39°08′22.3″N, 96°38′23.3″W), and Carbondale, Illinois (IL, 37°41′47.0″N, 89°14′19.2″W).

Methods

We applied linear mixed models to assess the effect of dominant species ecotype, year, and location on grassland taxonomic, phylogenetic, and functional diversity.

Results

The non-local grass ecotype (compared to the local ecotype) promoted species richness. In contrast, the effect of the dominant species ecotype on phylogenetic or functional diversity was site-specific over the 10-year restoration. Richness decreased across the rainfall gradient from dry to moist sites, and the wettest site had the highest phylogenetic and functional diversity.

Conclusions

Our results suggest that abiotic filtering by rainfall is a key assembly mechanism that could predict grassland changes in biodiversity in the early restoration phases. Given the community response across the tallgrass prairie, restoration practitioners should consider the impact of regional sources of dominant species used in restoration when biodiversity is a restoration goal. It is recommended for future grassland restoration to detect gaps and limitations in evolutionary and trait structure that will reveal which diversity components to evaluate.     

Incomplete recovery of plant diversity in restored prairie wetlands on agricultural landscapes

Restoration efforts are being implemented globally to mitigate the degradation and loss of wetland habitat; however, the rate and success of wetland vegetation recovery post‐restoration is highly variable across wetland classes and geographies. Here, we measured the recovery of plant diversity along a chronosequence of restored temporary and seasonal prairie wetlands ranging from 0 to 23 years since restoration, including drained and natural wetlands embedded in agricultural and natural reserve landscapes in central Alberta, Canada. We assessed plant diversity using the following structural indicators: percent cover of hydrophytes, native and non‐native species, species richness, and community composition. Our findings indicate that plant diversity recovered to resemble reference wetlands in agricultural landscapes within 3–5 years of restoration; however, restored wetlands maintained significantly lower species richness and a distinct community composition compared to reference wetlands located within natural reserves. Early establishment of non‐native species during recovery, dispersal limitation, and depauperated native seed bank were probable barriers to complete recovery. Determining the success of vegetation recovery provides important knowledge that can be used to improve restoration strategies, especially considering projected future changes in land use and climate.     

Plankton respiration and biomass as functional indicators of recovery in restored prairie wetlands

Reliable ecological indicators of wetland integrity are necessary for assessing recovery of restored wetlands; yet, little consensus currently exists on which indicators are most appropriate. We employed indicators derived from simple, standard measures of ecosystem function selected on the basis of ecological succession theory developed by [Science 164 (1969) 262; Bioscience 35 (1985) 419], which suggests that respiration:biomass ratios should increase in disturbed systems due to the diversion of energy from growth to maintenance. This hypothesis holds potential for the development of a simple ecological indicator and therefore was tested among prairie wetlands restored after drainage disturbance. No difference was observed in respiration:biomass ratios in restored wetlands and reference wetlands designated as controls. Plankton respiration or biomass may be poor indicators of disturbance because plankton responds quickly to re-establishment of a wetland hydrology regime and/or because different plankton species may have redundant function. We suggest employing more revealing assessment techniques that employ simultaneous examination of ecosystem structure and function to better characterize subtle or lingering effects of wetland disturbance after restoration.     

Vegetation and environmental conditions in recently restored wetlands in the prairie pothole region of the USA

How closely the vegetation of restored wetlands resembles that of comparable natural wetlands is a function of the probability of propagules of wetland species reaching reflooded wetlands and how similar environmental conditions in the restored wetland are those in the natural wetlands. Three years after reflooding, we examined the vegetation composition, water level fluctuations, soil organic carbon content, and soil bulk density as well as surface water pH, alkalinity, conductivity, and calcium and magnesium concentrations of 10 restored and 10 natural wetlands. In the restored wetlands, more species of submersed aquatics colonized than were found in natural wetlands, and they rapidly spread to form extensive beds that were larger than those found in natural wetlands. Emergent and wet meadow species in restored wetlands, however, were found in only sparse stands as were a variety of annuals. The vegetation of natural wetlands was predominantly large stands of emergent species. Fluctuations in water storage volume and basin surface area were similar for both restored and natural wetlands. The surface water in restored wetlands had higher pH and lower alkalinity, conductivity, and calcium and magnesium concentrations than that in natural wetlands. Soils of restored wetlands have a lower organic carbon content and higher bulk density than do those of natural wetlands. Our results suggest that for submersed aquatics, dispersal of propagules to restored wetlands is rapid and environmental conditions in restored wetlands are very suitable for their establishment. For other guilds of wetland species, e.g., sedges and other wet meadow species, dispersal to restored wetlands is likely much slower and may pose a serious problem for the re-establishment of these species in restored wetlands. Even if dispersal is not limiting, low surface organic carbon and high bulk density may prevent the establishment of these species in restored wetlands.     

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

Restoration 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.     

Greenhouse gas emissions and carbon sequestration potential in restored wetlands of the Canadian prairie pothole region

North American prairie pothole wetlands are known to be important carbon stores. As a result there is interest in using wetland restoration and conservation programs to mitigate the effects of increasing greenhouse gas concentration in the atmosphere. However, the same conditions which cause these systems to accumulate organic carbon also produce the conditions under which methanogenesis can occur. As a result prairie pothole wetlands are potential hotspots for methane emissions. We examined change in soil organic carbon density as well as emissions of methane and nitrous oxide in newly restored, long-term restored, and reference wetlands across the Canadian prairies to determine the net GHG mitigation potential associated with wetland restoration. Our results indicate that methane emissions from seasonal, semi-permanent, and permanent prairie pothole wetlands are quite high while nitrous oxide emissions from these sites are fairly low. Increases in soil organic carbon between newly restored and long-term restored wetlands supports the conclusion that restored wetlands sequester organic carbon. Assuming a sequestration duration of 33 years and a return to historical SOC densities we estimate a mean annual sequestration rate for restored wetlands of 2.7 Mg C ha⁻¹year⁻¹ or 9.9 Mg CO₂ eq. ha⁻¹ year⁻¹. Even after accounting for increased CH₄ emissions associated with restoration our research indicates that wetland restoration would sequester approximately 3.25 Mg CO₂ eq. ha⁻¹year⁻¹. This research indicates that widescale restoration of seasonal, semi-permanent, and permanent wetlands in the Canadian prairies could help mitigate GHG emissions in the near term until a more viable long-term solution to increasing atmospheric concentrations of GHGs can be found.     

Assessing ecosystem integrity of restored prairie wetlands from species production–diversity relationships

Dense populations of the coccolithophore Pleurochrysis pseudoroscoffensis were found in surface films at several locations around the Salton Sea in February–August, 1999. An unidentified coccolithophorid was also found in low densities in earlier studies of the lake (1955–1956). To our knowledge, this is the first record of this widespread marine species in any lake. Samples taken from surface films typically contained high densities of one or two other phytoplankton species as well as high densities of the coccolithophore. Presence or absence of specific algal pigments was used to validate direct cell counts. In a preliminary screen using a brine shrimp lethality assay, samples showed moderate activity. Extracts were then submitted to a mouse bioassay, and no toxic activity was observed. These results indicate that blooms of P. pseudoroscoffensis are probably not toxic to vertebrates and do not contribute to the various mortality events of birds and fish that occur in the Salton Sea.     

Resource limitation and recruitment patterns in a coral reef fish assemblage

The potential effects of food and shelter availability on the recruitment and early survivorship of coral reef fishes were studied on St. Croix, U.S. Virgin Islands. The faunal assemblage studied included diurnally active fishes found in the “rubble/sand” habitat. The most abundant members were: beaugregory, Stegastes leucostictus (Muller & Troschel), goldspotted goby, Gnatholepis thompsoni Jordan, bridled goby, Coryphopterus glaucofraenum Gill, surgeonfishes, Acanthurus bahianus Castelnau and A. chirurgus (Bloch), and French grunt, Haemulon flavolineatum (Desmarest). Comparisons of recruitment to reefs constructed from substrata that varied in morphological characteristics showed that there were differences in the relative abundances of recruits attracted to and/or surviving on the different reef types. Juveniles of most species appeared to prefer the branching coral Porites porites (Pallas), which provided a large number of small crevices between the branches.Manipulations of the availability of shelter sites for fishes demonstrated that recruitment and/or early survivorship were strongly limited by the number of refuges. This result was found in six separate carried out during different years and in different seasons. Shelter site availability presumably limits fish populations through its effects on prédation rates.Experimental manipulations of food availability indicated that food does not directly influence settlement or early survivorship of coral reef fishes. However, it is probable that correlations between habitat characteristics and food availability have influenced the evolution of settling preferences.     

Resource limitation,predation risk and compensatory growth in a damselfly

Periods of poor nutrition during early development may have negative fitness consequences in subsequent periods of ontogeny. In insects, suppression of growth and developmental rate during the larval stage are likely to affect size and timing of maturity, which in turn may lead to reduced reproductive success or survivorship. In light of these costs, individuals may achieve compensatory growth via behavioural or physiological mechanisms following food limitation. In this study, we examined the effects of a temporary period of food restriction on subsequent growth and age and size at maturity in the larval damselfly Ischnura verticalis (Odonata: Coenagrionidae). We also asked whether this temporary period of reduced nutrition affected subsequent foraging behaviour under predation risk. I. verticalis larvae exposed to a temporary food shortage suffered from a reduced growth rate during this period relative to a control group that was fed ad libitum. However, increased growth rates later in development ensured that adult body size measurements (head and pronotum widths) did not differ between the treatments upon emergence. In contrast, adult dry mass did not catch up to that of the controls, indicating that the increased growth rates for size dimensions occur at the cost of similar gains in mass. Predators reduced foraging effort of larvae, but this reduction did not differ between control larvae and those previously exposed to poor nutrition.     

Seed size and recruitment limitation influence seedling establishment in three tallgrass prairie species

A seed size–seed number tradeoff predicts that large numbers of seeds have an establishment advantage under low levels of competition, while large seed size is advantageous under asymmetric competition. Testing these predictions in the context of tallgrass prairie restoration should increase predictability of outcomes of species composition—restoration technique combinations. I conducted field and greenhouse experiments comparing seedling establishment among three tallgrass prairie species with different seed mass-seed number combinations, but with similar mass of seeds sown. Species were prairie dock [Silphium terebinthinaceum, mass = 33.2 + 2.1 mg ( \(\overline{x}\)  + SE, n = 100)], wild quinine (Parthenium integrifolium, mass = 3.1 + 0.3 mg), and smooth blue aster (Symphyotrichum laeve var. laeve, mass = 0.53 + 0.02 mg). Seeds were sown into mowed or tilled plots with shade and mulch treatments in an old field in northeastern Illinois USA, and seeds were sown into containers with or without vegetative cover in a greenhouse. In mowed plots, species with larger numbers of seeds sown had higher seedling densities. In tilled plots, seedling densities of the large-seeded Silphium were reduced less by shade than the small-seeded Parthenium. Symphyotrichum, however, did not respond to treatments as expected based on its seed size. High relative growth rate possibly increased Symphyotrichum seedling establishment in competitive environments. Results of this study demonstrated recruitment limitation when seedlings established into existing vegetation, and suggest that species composition resulting from specific seeding rates during prairie restoration may be more predictable when species are interseeded into existing vegetation, than when seeds are sown into tilled fields.     

Resource limitation and food quality for cladocerans in a tropical Brazilian lake

Food quality of the natural food (seston) from Lake Monte Alegre was evaluated throughout a series of life-table experiments with cladocerans from the same lake carried out in summer and autumn. Experiments were performed using cohorts of newborns originated from ovigerous females cultured for several generations in the lab or taken directly from the lake. For these tests of food limitation, juveniles of different species were submitted to one of the following treatments: (1) lake seston and (2) lake seston + green algae. The age at first reproduction, mean clutch size, total offspring and the intrinsic rate of natural increase (r) were estimated. Cladocerans responded to both seasonal changes in food resources and to additions of green algae. In summer, food levels were higher and cladocerans grew better than in autumn. The low fecundity and population growth rates of Daphnia gessneri, together with high C:P ratios (>500) in the seston in summer, suggested P limitation. The other cladocerans (Moina micrura, Ceriodaphnia cornuta and Simocephalus mixtus) were apparently less sensitive than D. gessneri to P-limitation in summer. However, energy limitation cannot be disregarded even in summer, although in a lower degree compared to autumn. In this season, the predominance of large diatoms and dinophyceans of low nutritional value and the low food concentration suggested both quantitative and qualitative food limitation. Phytoplankton composition, nutritional value and particle size appeared to be important factors in determining cladocerans reproduction rates with seston diets.     

Interactive effects of mycorrhizae and a root hemiparasite on plant community productivity and diversity

Plant communities can be affected both by arbuscular mycorrhizal fungi (AMF) and hemiparasitic plants. However, little is known about the interactive effects of these two biotic factors on the productivity and diversity of plant communities. To address this question, we set up a greenhouse study in which different AMF inocula and a hemiparasitic plant (Rhinanthus minor) were added to experimental grassland communities in a fully factorial design. In addition, single plants of each species in the grassland community were grown with the same treatments to distinguish direct AMF effects from indirect effects via plant competition. We found that AMF changed plant community structure by influencing the plant species differently. At the community level, AMF decreased the productivity by 15–24%, depending on the particular AMF treatment, mainly because two dominant species, Holcus lanatus and Plantago lanceolata, showed a negative mycorrhizal dependency. Concomitantly, plant diversity increased due to AMF inoculation and was highest in the treatment with a combination of two commercial AM strains. AMF had a positive effect on growth of the hemiparasite, and thereby induced a negative impact of the hemiparasite on host plant biomass which was not found in non-inoculated communities. However, the hemiparasite did not increase plant diversity. Our results highlight the importance of interactions with soil microbes for plant community structure and that these indirect effects can vary among AMF treatments. We conclude that mutualistic interactions with AMF, but not antagonistic interactions with a root hemiparasite, promote plant diversity in this grassland community. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Resource limitation,competition and the influence of life history in a freshwater snail community

Summary Previous work on a snail community occurring throughout lakes in southwestern Michigan showed that predation by molluscivorous sunfish had large impacts on only the rarest snail species. Thus, competition might play a major role in population limitation because dominant members of the snail community are relatively immune to predation. The present experiments were conducted to determine the extent to which the snail community depleted the abundance of food resources (epiphytes) and the extent to which epiphyte abundances limited snail production. An experimental gradient in snail densities showed that removal of snails increased epiphyte biomass by approximately 3-fold relative to that observed at natural snail densities. Enrichment of the environment with phosphorus fertilizer increased epiphyte biomass by approximately 20-fold and provided a test of food limitation in the snail community. All snail taxa exhibited positive numerical or growth responses to enrichment. The observations that snails depleted resources and that resources limited snail production demonstrated that snails competed exploitatively for epiphytes. The response of each snail species to increased food abundance differed depending on the timing of fertilization relative to the snails' life histories. Snails hatched before the experiment began were larger in fertilized treatments, due to increased growth, but their densities were similar among treatments. On the other hand, densities of snails born during the experiment were up to 15-fold greater in fertilized treatments, due in part to increased survival of newborn snails. Comparison of the responses of snails to food addition and to predator removals (based on prior experiments) suggested that food availability limits snail production more than predators do. Additionally, the large responses by algae and snails to fertilization demonstrated that both the producers and herbivores in this simplified food chain were strongly resource limited.