David M. Engle

Invertebrate Community Response to a Shifting Mosaic of Habitat

Abstract Grazing management has focused largely on promoting vegetation homogeneity through uniform distribution of grazing to minimize area in a pasture that is either heavily disturbed or undisturbed. An alternative management model that couples grazing and fire (i.e., patch burning) to promote heterogeneity argues that grazing and fire interact through a series of positive and negative feedbacks to cause a shifting mosaic of vegetation composition and structure across the landscape. We compared patch burning with traditional homogeneity-based management in tallgrass prairie to determine the influence of the two treatments on the aboveground invertebrate community. Patch burning resulted in a temporal flush of invertebrate biomass in patches transitional between unburned and patches burned in the current year. Total invertebrate mass was about 50% greater in these transitional patches within patch-burned pastures as compared to pastures under traditional, homogeneity-based management. Moreover, the mosaic of patches in patch-burned pastures contained a wider range of invertebrate biomass and greater abundance of some invertebrate orders than did the traditionally managed pastures. Patch burning provides habitat that meets requirements for a broad range of invertebrate species, suggesting the potential for patch burning to benefit other native animal assemblages in the food chain.

Is Altering Grazing Selectivity of Invasive Forage Species With Patch Burning More Effective Than Herbicide Treatments

Abstract Invasion of rangeland by exotic forage species threatens ecosystem structure and function and can cause catastrophic economic losses. Herbicide treatments often are the focus of management efforts to control invasions. Management with the fire-grazing interaction (or patch burning) might suppress an invasive forage species that has grazing persistence mechanisms developed apart from the fire-grazing interaction. We studied tallgrass prairies invaded by sericea lespedeza (Lespedeza cuneata [Dum.-Cours.] G. Don) to compare rate of invasion between traditional management and management with patch burning, to evaluate the effect of burn season on sericea lespedeza invasion within pastures managed with patch burning, and to correlate canopy cover of sericea lespedeza to canopy cover of other functional groups with and without herbicides. Sericea lespedeza canopy cover increased from 1999 to 2005 in both traditional- and patch-burn pastures, but sericea lespedeza increased from 5% to 16% canopy cover in traditionally managed pastures compared to 3% to 5% in the patch-burn pastures. Rate of increase in canopy cover of sericea lespedeza was less in patches burned in summer (0.41% · year−1) than in patches burned in spring (0.58% · year−1) within patch-burn pastures. Most plant functional groups, including forbs, were weak-negatively correlated with canopy cover of sericea lespedeza. Although herbicide application reduced mass of sericea lespedeza, other components of the vegetation changed little. Herbicide treatments temporarily reduced sericea lespedeza but did not predictably increase other plant functional groups. Patch burning reduced the rate of invasion by sericea lespedeza by maintaining young, palatable sericea plants in the burn patch, and could play a vital role in an integrated weed management strategy on rangelands.

Landscape heterogeneity and fire behavior: scale-dependent feedback between fire and grazing processes

Fire and grazing are ecological processes that frequently interact to modify landscape patterns of vegetation. There is empirical and theoretical evidence that response of herbivores to heterogeneity is scale-dependent however the relationship between fire and scale of heterogeneity is not well defined. We examined the relationship between fire behavior and spatial scale (i.e., patch grain) of fuel heterogeneity. We created four heterogeneous landscapes modeled after those created by a fire–grazing interaction that differed in grain size of fuel patches. Fire spread was simulated through each model landscape from 80 independent, randomly located ignition points. Burn area, burn shape complexity and the proportion of area burnt by different fire types (headfire, backfire and flankfire) were all affected by the grain of fuel patch. The area fires burned in heterogeneous landscapes interacted with the fuel load present in the patch where ignition occurred. Burn complexity was greater in landscapes with small patch grain than in landscapes with large patch grain. The proportion of each fire type (backfire, flankfire and headfire) was similar among all landscapes regardless of patch grain but the variance of burned area within each of the three fire types differed among treatments of patch grain. Our landscape fire simulation supports the supposition that feedbacks between landscape patterns and ecological processes are scale-dependent, in this case spatial scale of fuel loading altering fire spread through the landscape.

Effects of Patch-Burn Management on Dickcissel Nest Success in a Tallgrass Prairie

Abstract Grassland birds have declined more than any other North American habitat-associated bird community. Because most species of grassland birds evolved within heterogeneous landscapes created by the interaction of fire and grazing, traditional rangeland management that promotes homogeneity, including annual dormant-season burning combined with early-intensive grazing, might be partly responsible for these declines, especially in some regions of the Great Plains, USA. Recently, an alternative grassland management practice known as patch-burning has been promoted as a means of restoring heterogeneity to grasslands by mimicking the grazing–fire interaction that once occurred on the prairie before European settlement. From 2003 to 2004, we examined effects of patch-burning and traditional management (annual burning followed by early-intensive grazing) on the reproductive success of dickcissels (Spiza americana) in tallgrass prairie in Oklahoma. We monitored 296 dickcissel nests and found that dickcissel nesting phenology differed between traditional and patch-burned pastures. Specifically, dickcissels tended to initiate their nests later in the traditional pasture. Mean number of eggs laid and fledglings produced were similar between the treatments, but nest densities were higher in traditional pastures. Predation was the predominant cause of nest failure and was higher in traditional pastures than in patch-burned pastures. Brown-headed cowbird (Molothrus ater) parasitism was higher in traditional pastures than in patch-burned pastures. Overall, dickcissel nest success was higher in patch-burned pastures than in traditional pastures. The positive response of dickcissel nest success to patch-burn management provides further evidence that this practice can be a useful tool for grassland bird conservation. By creating a mosaic of different stature vegetation, patch-burn management enhances productivity of grassland bird species by providing a refuge area in the unburned patches that affords dickcissels and other nesting grassland birds some protection from the direct (e.g., trampling) and indirect (e.g., cowbird parasitism and predation) effects of grazing, which are not available under traditional management. Patch-burn management should be encouraged as a conservation strategy for grassland birds throughout the Great Plains.

The Combined Influence of Grazing, Fire, and Herbaceous Productivity on Tree–Grass Interactions

Although Juniperus communities are native to most regions of North America, they have proliferated in many areas of the Great Basin and Great Plains that historically supported grasslands, shrublands, and savannas. Explanations for the observed increases in Juniperus dominance, as well as other woody plant communities, are the subject of ongoing debate. The balance between herbaceous and woody vegetation is regulated by complex interactions between climate (e.g., amount and seasonality of rainfall), soils (e.g., soil texture and depth), and disturbance regimes (e.g., fire, gazing, browsing) (Walker 1987; Scholes and Archer 1997; Higgins et al. 2000). Changes in one or more of these factors can potentially elicit a change in the ratio of woody to herbaceous plants. Accordingly, climate change, intensification of grazing, elimination of fire and browsing (Hastings and Turner 1965; Grover and Musick 1990; Archer 1994; Fuhlendorf et al. 1996), atmospheric CO 2 enrichment (Idso 1992; Johnson et al. 1993), and nitrogen deposition (Kochy and Wilson 2001) have all been invoked as potential reasons for woody plant proliferation over the past century (see reviews by Archer 1994; Van Auken 2000). However, because these factors are correlative and interact across multiple spatiotemporal scales, it is neither feasible nor realistic to assess their relative importance using traditional, short-term factorial experiments. Field studies based on space-for-time substitutions and comparisons of landscapes with differing management histories have been used to assess long-term changes, but results from such studies are difficult to replicate, interpolate, or extrapolate and do not explicitly test causality. As a result, there is still considerable debate as to the relative importance of grazing, climate, and fire influences on community dynamics in drylands (O’Connor 1995; Fernandez-Gimenez and Allen-Diaz 1999; Illius and O’Connor 1999; Fuhlendorf et al. 2001). Dynamic simulation modeling is an underutilized tool that can be used to evaluate how climate or climate–disturbance interactions potentially affect tree– grass ratios and to test competing hypotheses attempting to account for woody plant increases over the past century. Grazing, fire, and climate strongly interact to influence woody plant abundance via effects on herbaceous production and composition. High rainfall can promote

Seed dispersal by Bison bison in a tallgrass prairie

Abstract Question: To what extent do bison serve as seed dispersers via epizoochory and endozoochory in tallgrass prairie, and does this differ by age-sex class of bison and by month? Location: Tallgrass Prairie Preserve, Osage County, Oklahoma, USA. Methods: We collected bison hair from 19 bulls, 45 cows, and 47 juveniles in fall and we collected bison dung monthly for a year. We recovered and identified seeds from hair and dung samples, and classified seeds by size and diaspore characteristics. Results: Bison hair samples contained 2768 seeds from at least 76 plant species. Several species found in hair did not feature specialized appendages for adhesive dispersal. Seed species composition differed in hair collected from bulls, cows and juveniles, possibly due to differential habitat use. Dung samples contained 7418 seeds from at least 70 species, from which graminoids accounted for 3936 seeds from 27 species. Species composition of seeds contained in dung samples differed among months, corresponding to availability. Grasses constituted about half the seeds in both dung and hair samples, but Asteraceae were more strongly represented in hair, while Scrophulariaceae and Solanaceae were more strongly represented in dung. Conclusions: We conclude that bison are potentially important dispersers of forbs as well as graminoids. A high abundance and wide diversity of seeds were found in both bison hair and dung. The great majority of seeds found undamaged in bison dung were small seeds, which agrees with the ‘foliage is the fruit’ hypothesis. Dispersal by both epizoochory and endozoochory may play an important role in life history of many species in tallgrass prairie landscapes. Nomenclature: Kartesz & Kartesz (1980).

Digital Photography: Reduced Investigator Variation in Visual Obstruction Measurements for Southern Tallgrass Prairie

Abstract Landscapes with structural heterogeneity or patchiness can support diverse and stable wildlife populations. Visual obstruction methods (i.e., Robel pole and Nudds coverboard) are common and useful techniques for quantifying vegetation structure; however, both rely on ocular estimations, which can be highly variable between observers. Our objectives were to 1) compare measurement and observer variation for visual obstruction among the two standard methods and the digital image method we developed using a digital camera; and 2) compare the performance of the Robel pole and digital image to estimate standing crop. The mean variation across the five observers using the digital image method (6.8%) was significantly lower (Pu200a<u200a0.05) than both the Nudds coverboard (32.1%) and the Robel pole (52.2). There were no significant differences among locations for the digital image method; however, there were for both the Robel pole and Nudds cover board (Pu200a<u200a0.05). The digital image method provided a better estimate of standing crop (r2u200au200a=u200au200a0.89) compared to the Robel pole (r2u200au200a=u200au200a0.68), accounting for 21% more of the observed variation in biomass. Long-term research programs that utilize seasonal field technicians to quantify habitat structure with a visual obstruction method could benefit from implementing use of the digital image method we developed. The low measurement error observed with this technique relative to the more traditional methods compared in this study might limit year-to-year and within-year variability of habitat structure data collected by numerous technicians with a high annual turnover.