Influence of Aboveground Biomass Removal on Nitrogen Mineralization in a Restored Tallgrass Prairie

Prescribed burning in prairies influences soil nitrogen (N), which is the primary nutrient that limits plant growth and is an important factor in plant competition and diversity. The primary objective of the experiment described here was to better understand the changes in net N mineralization that occur after a fire. We compared soil properties after a fire with those following vegetation removal by mowing and raking in a restored prairie in southeastern Minnesota. The treatments occurred in the spring of two consecutive years. Calcium oxide, burnt lime, was added to some of the raked plots in the first year to mimic the deposition of basic cations during a fire, which cause an increase in soil pH. Aboveground biomass removal by raking or by burning had similar effects on soil moisture, temperature, and inorganic N. The removal treatments caused warmer and drier soil than in the untreated plots. The change in net N mineralization after raking was unaffected by the addition of lime. In the first year, with low rainfall, removal caused net N mineralization rates similar to those in the untreated controls, but during the second year, with heavy rainfall, net N mineralization rates were significantly higher after removal. We predict that if water is sufficient, increased soil temperature after biomass removal will increase soil microbial activity and net N mineralization, but during drought, water will limit microbial activity. Furthermore, depending on soil N concentrations, which are very high at this study site, altered soil microbial activity will have variable effects on net N mineralization.     

Impact of management treatments on waterfowl use of dense nesting cover in the Canadian parklands

Periodic treatment of established stands of dense nesting cover (DNC) is a recommended practice to maintain cover quality, but little information exists on the magnitude and duration of treatment effects on nesting waterfowl. During 1998–2001, we examined the effect of management treatments on vegetative characteristics and waterfowl nest success and density in fields of DNC seeded to introduced and native grass and forb mixes in the parklands of Saskatchewan and Manitoba. We measured vegetation height–density and litter depth within fields and located and monitored 1,927 duck nests within 33–42 fields/yr ranging in size from 6 ha to 62 ha. We considered a series of models examining the influence of grass type and management treatment (GTMT) and years post-management (YPM) on vegetative characteristics, nest success, and nest density while including covariates potentially affecting these response variables. Visual obstruction and litter depth were lowest in native-burned fields and greatest in introduced-hayed fields. Visual obstruction was low the year following management, peaked 2–3 YPM, and remained at intermediate levels through ≥6 YPM. Litter depth remained low for the first 3 YPM and increased thereafter. Nest success and nest density varied little among GTMT. Nest success was high (14.3%) the year following a management treatment, low (6.5%) at 2 YPM, and moderate thereafter. Nest success decreased with percent cropland in the surrounding landscape. Nest density was 0.7 nests/ha the first year following management, increased to approximately 1.3 nests/ha in years 2–3, and declined back to approximately 0.7 nests/ha for ≥6 YPM. Nest density decreased with field size and increased with the area of small wetlands, percent cropland, and percent wetland within surrounding landscapes. Nest density tracked vegetation density as expected and our results indicate a possible trade-off between nest density and nest success. Given ancillary data on small mammal and insect prey in our study fields, and evidence from other studies, we speculate that DNC fields may act as prey reservoirs during years of peak vegetative density with a consequent reduction in nest survival. Therefore, management to increase waterfowl production based on our results needs to consider the interaction of treatment effects, competing habitats, and surrounding landscape composition. © 2011 The Wildlife Society.     

GRASSLAND BIRDS NESTING IN HAYLANDS OF SOUTHERN SASKATCHEWAN: LANDSCAPE INFLUENCES AND CONSERVATION PRIORITIES

Abstract: To determine the benefits to grassland birds of converting cropland to hayland in southern Saskatchewan, Canada, we quantified the relative nest abundance and success of grassland nesting birds in haylands and the influence landscape variables have on these parameters. We found nests of 26 species of grassland nesting birds, primarily waterfowl and vesper sparrow (Pooecetes gramineus). With the exception of the northern pintail (Anas acuta), few nesting attempts were recorded for species of high priority in the Prairie Pothole Bird Conservation Region. Mayfield nest success for all waterfowl (20 and 13% in 1999 and 2000, respectively) was high relative to previously reported nest success estimates in other habitat types—especially spring-seeded cropland—and was near levels thought to be required to sustain populations (15–20%). Vesper sparrow nest success (39 and 33% in 1999 and 2000, respectively) also was high relative to that reported in other studies. Haying destroyed few nests as wet weather delayed operations in 1999 and 2000. More nests may be destroyed by haying in other years as approximately 25% of nests in this study were still active on the long-term average haying date for southern Saskatchewan. Among models we developed to explain waterfowl relative nest abundance, amount of cropland in the surrounding landscape and field area were the most informative. Evidence that a specific set of landscape variables was important to models of waterfowl nest success was equivocal. Landscape variables did not explain variation in vesper sparrow relative nest abundance or nest success. Within our study area, conversion of cropland to hayland appears to provide significant benefits to a variety of grassland species, including some species of high conservation priority (e.g., northern pintail). Grassland species of conservation concern nested less frequently in hayland than in native grassland.     

Effects of prairie and barrens management on butterfly faunal composition

During 1990–1997, we recorded 122 138 adult butterflies in transect surveys at 125 pine-oak barrens in northern Wisconsin and 106 tallgrass prairies in six midwestern states grouped into three prairie subregions. Before analysis, we classified the butterflies into three ecological subgroups: specialist of native herbaceous vegetation, grassland (widely occurring in native and degraded herbaceous vegetation), and generalist. We analyzed this dataset both by ecological subgroups and as total butterflies, and by relative density and species richness, to investigate how these different ways of ordinating the same dataset might affect the results. In multiple linear regressions, density and richness of total butterflies and the subgroups related significantly to many non-management factors. In comparisons of more vs. less recent burning, all significant results for most recent burning were negative. No significant negative relationships were attributed to the longest period since burning. In comparisons of burning vs. idling, all significant results in prairie favored idling, but in barrens favored burning. In comparisons of burning vs. mechanical cutting, all significant results in prairie favored cutting, but no significant differences occurred in barrens. In regressions including all management types, rotational burning (alone or combined with cutting) was significantly positive most often for generalists and never for specialists. Increasing years since last management was always negative in barrens and the southern prairie subregion but always positive in the two northern prairie subregions. Significant management patterns occurred more often in prairie than barrens, which were less fragmented. Specialists were favored by grazing in one northern prairie subregion (but disfavored in the other), haying, single wildfire (testable in barrens only), and increasing years since last treatment in one northern prairie subregion (but disfavored in barrens). Within subregion and subgroup, significant management results for density and richness never conflicted, but density had more significant results than richness. In no instances were the signs opposite when total butterflies and/or any subgroup(s) significantly related to the same management factor in the same type of regression. But what was significant for one sample was often not for another. Thus, management favorable for specialists and total butterflies did not conflict, but the subgroups had varying degrees of sensitivity, rather than opposite responses. Since the specialist (and total) butterflies did not consistently favor one management type over another among subregions, caution should be used in preserve management, to avoid overreliance on one management type over others.     

A literature review of insect responses to fire, compared to other conservation managements of open habitat

This literature review concerns insect responses to fire, compared to other feasible and appropriate conservation managements of open habitats. Many insect groups decline markedly immediately after fire, with the magnitude of reduction related to the degree of exposure to the flames and mobility of the insect. Niche diversity is lower in recently burned habitat, and the rate of insect increase following fire also relates to the species' ability to gain access to the regrowing vegetation. Postburn flora can be quite attractive to some recolonizing insects, possibly to some degree a result of fire-caused insect mortality which provides plants with short-term release from insect herbivory. Insect declines may follow immediately after mowing, but usually of lesser degree and shorter duration than after a fire of comparable timing and size. Season and scale of cutting may affect how much and which species showed positive or negative responses. Cut areas offer the vegetational structure and composition preferred by some insects, but cutting – or cutting at certain scales, seasons, or frequencies – may also be unfavorable for some species. Heavy grazing results in niche and assemblage simplification. Nonetheless, some invertebrates prefer the short turfs and bare ground resulting from heavier grazing. Other species vary in whether they peak in abundance and diversity in intermediate, light, or no grazing. In comparisons of mowing/haying and grazing regimes of similar compatibility with maintenance of the same habitat types, responses of particular species and species groups varied as to whether they had a preference for one or the other. Characteristics associated with insect responses to fire related to the degree of exposure to lethal temperature and stress experienced in the post-fire environment, suitability of post-treatment vegetation as habitat, and ability to rebuild numbers in the site (from survivors and/or colonizers). These factors appear equally useful for explicating insect responses to other managements such as haying, mowing, and grazing. By contrast, the assumption that the most habitat-restricted species will be most adapted to ecological forces believed to be prevalent in that ecosystem appears less efficacious for predicting insect management preferences.