Sara G. Baer

CHANGES IN ECOSYSTEM STRUCTURE AND FUNCTION ALONG A CHRONOSEQUENCE OF RESTORED GRASSLANDS

Changes in aboveground vegetation, roots, and soil characteristics were examined from a 12-yr chronosequence of formerly cultivated fields restored to native C4 grasses through the Conservation Reserve Program (CRP). Following 6–8 yr in the CRP, the native grasses dominated vegetation composition, and the presence of forbs was negligible. Productivity of the restored grasslands did not exhibit any directional changes with the number of years in the CRP, and productivity was generally higher than native prairie in this region. Over time, the restored grasslands accumulated root biomass of decreasing quality as indicated by increasing root biomass and C:N ratio of roots along the 12-yr chronosequence. Root biomass, root C:N ratio, C storage in roots, and N storage in roots of restored grasslands approached that of native tallgrass prairie within the 12 yr of restoration. Establishment of the perennial vegetation also affected soil physical, chemical, and biological characteristics. Soil bulk density in the ...

Soil resources regulate productivity and diversity in newly established tallgrass prairie

In native tallgrass prairie, soil depth and nitrogen (N) availability strongly influence aboveground net primary productivity (ANPP) and plant species composition. We manipulated these factors in a newly restored grassland to determine if these resources similarly constrain productivity and diversity during the initial three years of grassland establishment. Four types of experimental plots with six treatment combinations of deep and shallow soil at reduced-, ambient-, and enriched-N availability formed the basis of this study. The soil responses to the experimental treatments were examined over three years, and patterns in diversity and productivity were examined in year 3. The soil depth treatment did not significantly affect soil carbon (C) and N pools or ANPP and diversity. A pulse amendment of C added to the soil prior to planting increased soil microbial biomass and decreased potential net N mineralization rates to effectively reduce N availability throughout the study. Nitrogen availability declined over time in nonamended soils as a result of plant establishment, but adding fertilizer N alleviated the increasing immobilization potential of the soil. The level of ANPP was lowest and diversity highest in the reduced-N treatment, whereas the enriched-N treatment resulted in high productivity, but low diversity. As a result, diversity was inversely correlated with productivity in these newly established com- munities. The same mechanism invoked to explain decreased diversity under nutrient en- richment in old-field ecosystems and native grasslands (e.g., reduced light availability with increased production) was supported in the restored prairie by the positive relationship between ANPP and intercepted light, and a strong correlation between light availability and diversity. The effects of nutrient availability on plant community composition (diversity and richness) were due primarily to the responses of prairie species, as the productivity of early successional, nonprairie species was less than 1% of total ANPP after three years of establishment. These results show that the effects of resource availability on productivity and diversity are similar in young and mature grasslands, and that manipulation of a limiting nutrient during grassland establishment can influence floristic composition, with conse- quences for long-term patterns of diversity in restored ecosystems.

Contrasting ecosystem recovery on two soil textures: implications for carbon mitigation and grassland conservation

Understanding processes that promote or constrain ecosystem recovery from disturbance is needed to predict the restorative potential of degraded systems. We quantified a suite of ecosystem properties and processes across two chronosequences of restored grasslands on contrasting soil textures to test the hypothesis that restorations on silty clay loam soil would exhibit greater recovery of soil carbon (C) and nitrogen (N) pools and fluxes than on loamy fine sand because soil with higher clay content possesses a greater capacity to physico-chemically protect organic matter. Warm-season grass aboveground net primary productivity was similar between the two soil textures. Root biomass increased and root quality (as indexed by C:N ratio) decreased across both chronosequences. An asymptote in the accumulation of N in roots in the silty clay loam chronosequence resulted in wider C:N ratios of roots than in the loamy fine sand chronosequence. Total soil C (TC) and microbial biomass C (MBC) increased across the si...

GRASSLAND ESTABLISHMENT UNDER VARYING RESOURCE AVAILABILITY: A TEST OF POSITIVE AND NEGATIVE FEEDBACK

The traditional logic of carbon (C) and nitrogen (N) interactions in ecosystems predicts further increases or decreases in productivity (positive feedback) in response to high and low fertility in the soil, respectively; but the potential for development of feedback in ecosystems recovering from disturbance is less well understood. Furthermore, this logic has been challenged in grassland ecosystems where frequent fires or grazing may reduce the contribution of aboveground litter inputs to soil organic matter pools and nutrient supply for plant growth, relative to forest ecosystems. Further, if increases in plant productivity increase soil C content more than soil N content, negative feedback may result from increased microbial demand for N making less available for plant growth. We used a field experiment to test for feedback in an establishing grassland by comparing aboveground net primary productivity (ANPP) and belowground pools and fluxes of C and N in soil with enriched, ambient, and reduced N availability. For eight years annual N enrichment increased ANPP, root N, and root tissue quality, but root C:N ratios remained well above the threshold for net mineralization of N. There was no evidence that N enrichment increased root biomass, soil C or N accrual rates, or storage of C in total, microbial, or mineralizable pools within this time frame. However, the net nitrogen mineralization potential (NMP) rate was greater following eight years of N enrichment, and we attributed this to N saturation of the microbial biomass. Grassland developing under experimentally imposed N limitation through C addition to the soil exhibited ANPP, root biomass and quality, and net NMP rate similar to the ambient soil. Similarity in productivity and roots in the reduced and ambient N treatments was attributed to the potentially high nitrogen-use efficiency (NUE) of the dominant C4 grasses, and increasing cover of legumes over time in the C-amended soil. Thus, in a developing ecosystem, positive feedback between soil N supply and plant productivity may promote enhanced long-term N availability and override progressive N limitation as C accrues in plant and soil pools. However, experimentally imposed reduction in N availability did not feed back to reduce ANPP, possibly due to shifts in NUE and functional group composition.

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.

No effect of seed source on multiple aspects of ecosystem functioning during ecological restoration: cultivars compared to local ecotypes of dominant grasses

Genetic principles underlie recommendations to use local seed, but a paucity of information exists on the genetic distinction and ecological consequences of using different seed sources in restorations. We established a field experiment to test whether cultivars and local ecotypes of dominant prairie grasses were genetically distinct and differentially influenced ecosystem functioning. Whole plots were assigned to cultivar and local ecotype grass sources. Three subplots within each whole plot were seeded to unique pools of subordinate species. The cultivar of the increasingly dominant grass, Sorghastrum nutans, was genetically different than the local ecotype, but genetic diversity was similar between the two sources. There were no differences in aboveground net primary production, soil carbon accrual, and net nitrogen mineralization rate in soil between the grass sources. Comparable productivity of the grass sources among the species pools for four years shows functional equivalence in terms of biomass production. Subordinate species comprised over half the aboveground productivity, which may have diluted the potential for documented trait differences between the grass sources to influence ecosystem processes. Regionally developed cultivars may be a suitable alternative to local ecotypes for restoration in fragmented landscapes with limited gene flow between natural and restored prairie and negligible recruitment by seed.

Vulnerability of Rehabilitated Agricultural Production Systems to Invasion by Nontarget Plant Species

Vast areas of arable land have been retired from crop production and “rehabilitated” to improved system states through landowner incentive programs in the United States (e.g., Conservation and Wetland Reserve Programs), as well as Europe (i.e., Agri-Environment Schemes). Our review of studies conducted on invasion of rehabilitated agricultural production systems by nontarget species elucidates several factors that may increase the vulnerability of these systems to invasion. These systems often exist in highly fragmented and agriculturally dominated landscapes, where propagule sources of target species for colonization may be limited, and are established under conditions where legacies of past disturbance persist and prevent target species from persisting. Furthermore, rehabilitation approaches often do not include or successfully attain all target species or historical ecological processes (e.g., hydrology, grazing, and/or fire cycles) key to resisting invasion. Uncertainty surrounds ways in which nontarget species may compromise long term goals of improving biodiversity and ecosystem services through rehabilitation efforts on former agricultural production lands. This review demonstrates that more studies are needed on the extent and ecological impacts of nontarget species as related to the goals of rehabilitation efforts to secure current and future environmental benefits arising from this widespread conservation practice.

Ecotypes of an ecologically dominant prairie grass (Andropogon gerardii) exhibit genetic divergence across the U.S. Midwest grasslands' environmental gradient.

Big bluestem (Andropogon gerardii) is an ecologically dominant grass with wide distribution across the environmental gradient of U.S. Midwest grasslands. This system offers an ideal natural laboratory to study population divergence and adaptation in spatially varying climates. Objectives were to: (i) characterize neutral genetic diversity and structure within and among three regional ecotypes derived from 11 prairies across the U.S. Midwest environmental gradient, (ii) distinguish between the relative roles of isolation by distance (IBD) vs. isolation by environment (IBE) on ecotype divergence, (iii) identify outlier loci under selection and (iv) assess the association between outlier loci and climate. Using two primer sets, we genotyped 378 plants at 384 polymorphic AFLP loci across regional ecotypes from central and eastern Kansas and Illinois. Neighbour‐joining tree and PCoA revealed strong genetic differentiation between Kansas and Illinois ecotypes, which was better explained by IBE than IBD. We found high genetic variability within prairies (80%) and even fragmented Illinois prairies, surprisingly, contained high within‐prairie genetic diversity (92%). Using Bayenv2, 14 top‐ranked outlier loci among ecotypes were associated with temperature and precipitation variables. Six of seven BayeScanFST outliers were in common with Bayenv2 outliers. High genetic diversity may enable big bluestem populations to better withstand changing climates; however, population divergence supports the use of local ecotypes in grassland restoration. Knowledge of genetic variation in this ecological dominant and other grassland species will be critical to understanding grassland response and restoration challenges in the face of a changing climate.

Environmental and genetic variation in leaf anatomy among populations of Andropogon gerardii (Poaceae) along a precipitation gradient

UNLABELLED PREMISE OF THE STUDY Phenotypes of two Andropogon gerardii subspecies, big bluestem and sand bluestem, vary throughout the prairie ecosystem of North America. This study sought to determine the role of genetics and environment in driving adaptive variation of leaf structure in big bluestem and sand bluestem. • METHODS Four populations of big bluestem and one population of sand bluestem were planted in common gardens at four sites across a precipitation gradient from western Kansas to southern Illinois. Internal leaf structure and trichome density of A. gerardii were examined by light microscopy to separate genetic and environmentally controlled traits. Leaf thickness, midrib thickness, bulliform cells, interveinal distance, vein size, and trichome density were quantified. • KEY RESULTS At all planting sites, sand bluestem and the xeric population of A. gerardii had thicker leaves and fewer bulliform cells compared with mesic populations. Environment and genetic source population were both influential for leaf anatomy. Leaves from plants grown in mesic sites (Carbondale, Illinois and Manhattan, Kansas) had thicker midribs, larger veins, fewer trichomes, and a greater proportion of bulliform cells compared to plants grown in drier sites (Colby and Hays, Kansas). • CONCLUSIONS Water availability has driven adaptive variation in leaf structure in populations of A. gerardii, particularly between sand bluestem and big bluestem. Genetically based differences in leaves of A. gerardii indicate adaptive variation and evolutionary forces differentiating sand bluestem from big bluestem. Environmental responses of A. gerardii leaves suggest an ability to adjust to drought, even in populations adapted to mesic home environments.

Belowground Ecosystem Recovery During Grassland Restoration: South African Highveld Compared to US Tallgrass Prairie

Conversion of cultivated land to grassland is globally practiced to reverse soil degradation, but belowground ecosystem response to restoration has never been compared between old and new world temperate grasslands. We used a chronosequence approach to model change in root biomass and quality (indexed by C:N ratio), microbial biomass and composition [indexed by phospholipid fatty acids (PLFAs)], soil aggregate structure, and soil C and N stocks in the South African Highveld and compared recovery of these variables to a grassland restoration chronosequence in the US tallgrass prairie. We hypothesized soil C recovery, and mechanisms promoting soil C and N accrual would be convergent between these distant temperate grasslands with similar growing season precipitation, history of cultivation, and undergoing restoration with C4-grasses. Total PLFA richness and concentrations of most microbial groups rose to represent uncultivated grassland in the highveld (similar to tallgrass prairie), but in contrast to tallgrass prairie, the fungi:bacteria ratio did not increase with restoration age. In the highveld, root biomass accumulation was lower, but root quality became more representative of the never-cultivated grassland than in restorations in tallgrass prairie. Soil aggregate recovery was slightly faster in tallgrass prairie, and the pattern of macroaggregate C recovery was divergent due to less depletion in cultivated soil and higher stock of C in the uncultivated soil relative to the highveld. More rapid restoration of total soil C and N stocks in the highveld was attributed to greater soil C saturation deficit at the onset of restoration, development of higher quality root systems that promote the microbial biomass and soil aggregation, and climate conditions (distinct periodicity of rainfall and high aridity) that likely impose more limitation to decomposition relative to the tallgrass prairie ecosystem.