John M. Blair

The Keystone Role of Bison in North American Tallgrass Prairie

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An Ecosystem in Transition: Causes and Consequences of the Conversion of Mesic Grassland to Shrubland

Abstract Woody plant expansion is one of the greatest contemporary threats to mesic grasslands of the central United States. In this article, we synthesize more than 20 years of research to elucidate the causes and consequences of the ongoing transition of C4-dominated grasslands to savanna-like ecosystems codominated by grasses and woody plants. This transition is contingent on fire-free intervals, which provide the opportunity for recruitment both of new individuals and of additional shrub and tree species into this grassland. Once shrubs establish, their cover increases regardless of fire frequency, and infrequent fires accelerate the spread of some shrub species. This process has resulted in a new dynamic state of shrub–grass coexistence in the mesic grasslands of North America. Important consequences of this shift in plant life-form abundance include alterations in plant productivity, species diversity, and carbon storage. Without drastic measures such as mechanical removal of shrubs, it is unlikely that management of fire and grazing regimes alone will be sufficient to restore historic grass dominance in these ecosystems.

Decay Rates, Nitrogen Fluxes, and Decomposer Communiies of Single‐ and Mixed‐Species Foliar Litter

Decomposition rates, N fluxes, and abundances of decomposer organisms were quantified in mixed-species litterbags (containing leaves of two or three of the following tree species: Acer rubrum, Cornus florida, and Quercus prinus) and in litterbags containing leaves of a single species. Data from single-species litterbags were used to generate predicted decay rates, N fluxes, and abundances of decomposer organisms for mixed-species litter- bags, against which observed values could be compared to determine if significant inter- action effects occurred when litter of different species, and different resource quality, was mixed. Decay rates of-mixed-species litterbags during the 1-yr study were not significantly different than predicted from decay rates of individual component species. However, there were significant interaction effects on N fluxes and abundances of decomposer organisms. In the C. florida-A. rubrum and C. florida-A. rubrum-Q. prinus litter combinations there were significantly greater initial releases of N and lower subsequent N immobilization than predicted. In the A. rubrum-Q. prinus and C. florida-A. rubrum-Q. prinus litter combi- nations, lengths of fungal hyphae were significantly less than predicted on at least half the collection dates. Bacterial numbers in the mixed-litter combinations were also generally less than predicted. Nematode abundances, especially fungivores, were generally greater than predicted in mixed-species litterbags until the last sample date. Observed mean abun- dances of nematodes over all dates were 20-30% greater than predicted. Microarthropod abundances were more variable, but tended to be lower than predicted. Our results indicate that measurement of N flux in single-species litterbags may not reflect actual N flux in the field, where leaves of several tree species are mixed together. The differences in N flux between single- and mixed-species litterbags can affect ecosystem-level estimates of N release or accumulation in decomposing litter. For example, estimates of ecosystem-level N fluxes at our field site, based on data from single-species litterbags, resulted in a 64% underestimate of N released by day 75 and a 183% overestimate of N accumulated in the litter by day 375, relative to estimates based on data from mixed-species litterbags. We suggest that the deviation of observed N fluxes in mixed-species litterbags from those predicted using single-species litterbags are the result of differences in the decomposer community, such as lower microbial and microarthropod densities and higher nematode densities, resulting when litter of varied resource quality is mixed together. Longer term studies will be needed to determine if the differences between observed and predicted decomposer communities in mixed-species litter combinations influence the latter stages of decomposition where invertebrate-microbial interactions may have a greater effect on decay rates and nutrient release.

FIRE, N AVAILABILITY, AND PLANT RESPONSE IN GRASSLANDS: A TEST OF THE TRANSIENT MAXIMA HYPOTHESIS

In tallgrass prairie, periodic spring fires often result in enhanced aboveground net primary productivity (ANPP) that exceeds the productivity of either annually burned or unburned sites. This study evaluated two alternate hypotheses for the “pulse” in productivity following an infrequent fire: (1) enhanced ANPP results from increased net N mineralization rates due to the removal of surface litter and elevated soil temperatures following fire (the enhanced mineralization hypothesis) or (2) enhanced ANPP results from a transient release from both light and N limitation during a nonequilibrium period as a switch from energy to N limitation occurs (the transient maxima hypothesis). The former hypothesis predicts greater N availability following an infrequent fire, relative to either annually burned or unburned prairie. The latter predicts that N availability following an infrequent fire will decline to intermediate levels, relative to unburned and annually burned prairie, and continue to decline with successive annual fires. To test these hypotheses, I measured inorganic soil N, net N mineralization rates, and plant productivity and N content at Konza Prairie in sites with several different burn histories (unburned, annually burned, infrequently burned). Inorganic soil N and cumulative net N mineralization rates were greatest on the unburned sites, lowest in annually burned sites, and intermediate in infrequently burned sites. Net N mineralization rates and plant tissue N content both declined with successive spring burning. These results did not support the enhanced mineralization hypothesis but indicated that enhanced ANPP following an infrequent fire resulted from an accumulation of inorganic and mineralizable N in the absence of fire which, under conditions of adequate light availability, was utilized following a spring fire. This is consistent with the transient maxima hypothesis and suggests that nonequilibrium responses to multiple, variable resources (light, energy, N) are an important aspect of tallgrass prairie ecosystem dynamics.

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 ...

Altering Rainfall Timing and Quantity in a Mesic Grassland Ecosystem: Design and Performance of Rainfall Manipulation Shelters

Global climate change is predicted to alter growing season rainfall patterns, potentially reducing total amounts of growing season precipitation and redistributing rainfall into fewer but larger individual events. Such changes may affect numerous soil, plant, and ecosystem properties in grasslands and ultimately impact their productivity and biological diversity. Rainout shelters are useful tools for experimental manipulations of rainfall patterns, and permanent fixed-location shelters were established in 1997 to conduct the Rainfall Manipulation Plot study in a mesic tallgrass prairie ecosystem in northeastern Kansas. Twelve 9 x 14–m fixed-location rainfall manipulation shelters were constructed to impose factorial combinations of 30% reduced rainfall quantity and 50% greater interrainfall dry periods on 6 x 6–m plots, to examine how altered rainfall regimes may affect plant species composition, nutrient cycling, and above- and belowground plant growth dynamics. The shelters provided complete control of growing season rainfall patterns, whereas effects on photosynthetic photon flux density, nighttime net radiation, and soil temperature generally were comparable to other similar shelter designs. Soil and plant responses to the first growing season of rainfall manipulations (1998) suggested that the interval between rainfall events may be a primary driver in grassland ecosystem responses to altered rainfall patterns. Aboveground net primary productivity, soil CO2 flux, and flowering duration were reduced by the increased interrainfall intervals and were mostly unaffected by reduced rainfall quantity. The timing of rainfall events and resulting temporal patterns of soil moisture relative to critical times for microbial activity, biomass accumulation, plant life histories, and other ecological properties may regulate longer-term responses to altered rainfall patterns.

SOIL N AND PLANT RESPONSES TO FIRE, TOPOGRAPHY, AND SUPPLEMENTAL N IN TALLGRASS PRAIRIE

Tallgrass prairie in the Flint Hills region of Kansas is characterized by considerable topographic relief coupled with variation in soil properties. These topoedaphic gradients, together with variation in fire regimes, result in temporal and landscape-level variability in soil resource availability and plant responses. Nitrogen usually is considered to be the nutrient most limiting to primary productivity in tallgrass prairie, but few studies have addressed how N availability varies seasonally, or across the landscape and with fire frequency. We measured soil inorganic N, in situ net N mineralization, aboveground net primary productivity (ANPP), and N mass on plots either fertilized with N in 1993 or in 1994, or unfertilized, in uplands and lowlands of two annually burned and two long-term unburned sites during the 1994 growing season. In addition, our study was conducted in the year following record rainfall, allowing us to assess the potential for high precipitation amounts to affect subsequent N cycling and plant production. Both fire treatment and topography affected soil N availability. In general, N mineralization was greater on unburned than on burned sites and was up to five times greater on uplands than lowlands. Total extractable soil N was highest early in the season and least at midseason, and it also tended to be higher in unburned sites than burned sites on unfertilized plots. Added N increased ANPP, but there were no differences between plots fertilized in 1994 and those fertilized in 1993. In general, patterns of ANPP on control plots were consistent with known production responses to topography and burning (higher in annually burned sites and in lowland sites) but were inversely related or unrelated to patterns of N availability (higher in unburned sites and at upland topographic positions). Potential loss of N by volatilization during spring burning was greater than in years with normal rainfall amounts and represented a significant portion of aboveground plant N mass. Potential N losses did not appear to limit ANPP or N availability in the current growing season. Our results suggest that different factors control soil N mineralization and plant productivity, which explains, in part, why patterns of ANPP are not well correlated with patterns of N availability in tallgrass prairie ecosystems.

Quantifying global soil carbon losses in response to warming

The majority of the Earth’s terrestrial carbon is stored in the soil. If anthropogenic warming stimulates the loss of this carbon to the atmosphere, it could drive further planetary warming. Despite evidence that warming enhances carbon fluxes to and from the soil, the net global balance between these responses remains uncertain. Here we present a comprehensive analysis of warming-induced changes in soil carbon stocks by assembling data from 49 field experiments located across North America, Europe and Asia. We find that the effects of warming are contingent on the size of the initial soil carbon stock, with considerable losses occurring in high-latitude areas. By extrapolating this empirical relationship to the global scale, we provide estimates of soil carbon sensitivity to warming that may help to constrain Earth system model projections. Our empirical relationship suggests that global soil carbon stocks in the upper soil horizons will fall by 30 ± 30 petagrams of carbon to 203 ± 161 petagrams of carbon under one degree of warming, depending on the rate at which the effects of warming are realized. Under the conservative assumption that the response of soil carbon to warming occurs within a year, a business-as-usual climate scenario would drive the loss of 55 ± 50 petagrams of carbon from the upper soil horizons by 2050. This value is around 12–17 per cent of the expected anthropogenic emissions over this period. Despite the considerable uncertainty in our estimates, the direction of the global soil carbon response is consistent across all scenarios. This provides strong empirical support for the idea that rising temperatures will stimulate the net loss of soil carbon to the atmosphere, driving a positive land carbon–climate feedback that could accelerate climate change.

DETERMINANTS OF SOIL CO2 FLUX FROM A SUB-HUMID GRASSLAND: EFFECT OF FIRE AND FIRE HISTORY

Soil CO2 flux (Jc02) was measured at midday over a 2-yr period in undisturbed tallgrass prairie (Konza Prairie, Kansas, USA) to quantify seasonal and annual budgets, to evaluate temperature and moisture as determinants of soil CO2 flux, and to assess the effect of a common land management tool, spring fire, and fire history on soil respiration. We

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.