Devan Allen McGranahan

Connecting Soil Organic Carbon and Root Biomass with Land-Use and Vegetation in Temperate Grassland

Soils contain much of Earths terrestrial organic carbon but are sensitive to land-use. Rangelands are important to carbon dynamics and are among ecosystems most widely impacted by land-use. While common practices like grazing, fire, and tillage affect soil properties directly related to soil carbon dynamics, their magnitude and direction of change vary among ecosystems and with intensity of disturbance. We describe variability in soil organic carbon (SOC) and root biomass—sampled from 0–170 cm and 0–100 cm, respectively—in terms of soil properties, land-use history, current management, and plant community composition using linear regression and multivariate ordination. Despite consistency in average values of SOC and root biomass between our data and data from rangelands worldwide, broad ranges in root biomass and SOC in our data suggest these variables are affected by other site-specific factors. Pastures with a recent history of severe grazing had reduced root biomass and greater bulk density. Ordination suggests greater exotic species richness is associated with lower root biomass but the relationship was not apparent when an invasive species of management concern was specifically tested. We discuss how unexplained variability in belowground properties can complicate measurement and prediction of ecosystem processes such as carbon sequestration.

A cross-taxonomic comparison of insect responses to grassland management and land-use legacies

Many species of plants and animals associated with grasslands are rare or declining due to habitat loss and degradation. Although grassland plants and insects evolved in the context of both grazing and fire, the appropriate use of grazing and fire has been debated among those concerned with protecting insect communities. We established an experiment to test insect responses to three grassland management treatments: (1) patch-burn graze (burning of spatially distinct patches and free access by cattle), (2) graze- and-burn (burning of entire tract with free access by cattle), and (3) burn-only. Because we expected that land-use legacies could also affect insect abundance and diversity, we evaluated effects of time since fire, grazing history, remnant history (remnant or reconstructed grassland) and pre-treatment vegetation characteristics, which were assumed to be a legacy of prior land-use. Butterflies (Lepidoptera), ants (Hymenoptera: Formicidae), and leaf beetles (Coleoptera: Chrysomelidae) were surveyed for three years to compare their responses to each of these treatments as measured by abundance, richness and species diversity. Each of these taxa is relatively diverse and was expected to have the potential to have strong negative responses to grazing and burning, but we predicted more positive responses to patch-burn grazing. Our results showed that land-use legacies affected insect abundance, richness and diversity, but treatments did not. Ant abundance was lower in tracts with a history of heavy grazing. Ant species richness was positively associated with pre-treatment time since fire and vegetation height and negatively associated with pre-treatment proportion native plant cover. Butterfly abundance was positively associated with pre-treatment litter cover. Leaf beetle diversity was positively associated with pre-treatment native plant cover, and leaf beetle abundance was negatively associated with time since fire. Our results indicate that land-use legacies can exert more influence on grassland insect community composition than current management, but the particular aspects of these land-use legacies that are important vary across insect taxa. The implications of these finding are that (1) land-use legacies should garner more attention in grassland management and (2) conservation of grassland insect communities will be improved by taxon- specific analysis of land-use legacy variables.

An Invasive Cool-Season Grass Complicates Prescribed Fire Management in a Native Warm-Season Grassland

ABSTRACT: Invasive species challenge managers of natural areas. In many ecosystems, restoring and maintaining pre-historic disturbance regimes promotes native biodiversity and controls invasive plant species. We report challenges in applying patch-burn grazing to restore the ecological interaction between fire and herbivores in eastern North American tallgrass prairie. We use United States Forest Service fire behavior software and fuel models to simulate the effect of a cool-season grass invasion into tallgrass prairie. By introducing a high-moisture fuel type into the native fuelbed, tall fescue (Lolium arundinaceum Schreb. S.J. Darbyshire) creates a heterogeneous fuelbed that limits fire spread rate. Our results indicate that a high-moisture fuel type invading a high proportion of a fuelbed requires up to four times the wind speed to achieve a rate of spread similar to that in an uninvaded fuelbed. Reduced fire spread undermines restoring the natural fire regime, putting invaded grassland systems at a higher risk of succession to a woodland state. To mitigate against negative effects of high-moisture invaders on prescribed burning and ensure that fires maintain the intensity required for management goals, managers should consider the following when planning burns: Accumulate higher fuel loads by reducing herbivory or reducing fire frequency; burn earlier in the spring or later in the fall, during the dormant season of the invader; and consider alternative fire weather such as lower relative humidity, perhaps with lower wind speeds to maintain an acceptable level of complexity.

An invasive grass increases live fuel proportion and reduces fire spread in a simulated grassland.

Fire is a globally important ecosystem process, and invasive grass species generally increase fire spread by increasing the fuel load and continuity of native grassland fuelbeds. We suggest that invasive grasses that are photosynthetically active, while the native plant community is dormant reduce fire spread by introducing high-moisture, live vegetation gaps in the fuelbed. We describe the invasion pattern of a high-moisture, cool-season grass, tall fescue (Schedonorus phoenix (Scop.) Holub), in tallgrass prairie, and use spatially explicit fire behavior models to simulate fire spread under several combinations of fuel load, invasion, and fire weather scenarios. Reduced fuel load and increased extent of tall fescue invasion reduced fire spread, but high wind speed and low relative humidity can partially mitigate these effects. We attribute reduced fire spread to asynchrony in the growing seasons of the exotic, cool-season grass, tall fescue, and the native, warm-season tallgrass prairie community in this model system. Reduced fire spread under low fuel load scenarios indicate that fuel load is an important factor in fire spread, especially in invaded fuel beds. These results present a novel connection between fire behavior and asynchronous phenology between invasive grasses and native plant communities in pyrogenic ecosystems.

Managing private, commercial rangelands for agricultural production and wildlife diversity in Namibia and Zambia

The private game industry has grown across Africa since the mid-20th century. While considerable research has documented wildlife production on commercial land in many eastern and southern African countries, few studies have focused specifically on the integration of livestock and game production in Namibia and Zambia. This paper reports a survey of 43 commercial conservancy members in Namibia and 23 game farmers in Zambia conducted between September 2004 and June 2005. The survey was based on inductive sampling theory and queried farmers on how they have integrated wildlife production into their management practices. Farmers in each country reported considerable integration of wildlife conservation and agricultural production. Namibian farmers reported substantial problems with bush encroachment, whereas none of their Zambian counterparts raised similar complaints. This paper describes the state of rangeland management on commercial farms in Namibia and Zambia and identifies important areas where further research can contribute to the enhancement of this conservation-production system.

Temporal variability in aboveground plant biomass decreases as spatial variability increases

Ecological theory predicts that diversity decreases variability in ecosystem function. We predict that, at the landscape scale, spatial variability created by a mosaic of contrasting patches that differ in time since disturbance will decrease temporal variability in aboveground plant biomass. Using data from a multi-year study of seven grazed tallgrass prairie landscapes, each experimentally managed for one to eight patches, we show that increased spatial variability driven by spatially patchy fire and herbivory reduces temporal variability in aboveground plant biomass. This pattern is associated with statistical evidence for the portfolio effect and a positive relationship between temporal variability and functional group synchrony as predicted by metacommunity variability theory. As disturbance from fire and grazing interact to create a shifting mosaic of spatially heterogeneous patches within a landscape, temporal variability in aboveground plant biomass can be dampened. These results suggest that spatially heterogeneous disturbance regimes contribute to a portfolio of ecosystem functions provided by biodiversity, including wildlife habitat, fuel, and forage. We discuss how spatial patterns of disturbance drive variability within and among patches.

Identifying ecological sustainability assessment factors for ecotourism and trophy hunting operations on private rangeland in Namibia

Trophy hunting and ecotourism are important forms of sustainable utilisation in the human-impacted, working landscapes outside of formal parks and reserves. Research on the sustainability of such tourism operations, however, has focused on the financial viability of tourism operators, rather than environmental effects. This paper examines the sustainability strength of wildlife utilisation on private rangelands in Namibia in terms of ecological impact. Using grounded theory, 43 members of commercial conservancies were surveyed to identify themes in the practice and perception of sustainable utilisation. While basic tourism was infrequent, trophy hunting was a common source of revenue from wildlife. Three emergent themes from the data included differences in tourist versus hunting operations; attitudes and perceptions of the administration of conservation efforts; and the co-management of livestock and wildlife, especially farm economics and game-proof fencing. Overall, increases in the proportion of income derived from game (above 20% of farm revenue) were associated with concurrent reductions in domestic livestock, but did not increase the use of game-proof fences. Other factors delineating “weak” and “strong” sustainability on commercial Namibian rangelands are discussed.

A historical primer on the US farm bill: Supply management and conservation policy

While Americans are accustomed to US Congress revisiting the farm bill every five to seven years, less familiar is the breadth of influence agricultural policy has in todays society. For example, as 2012 waned without a new farm bill, surprised consumers learned milk prices could nearly double if agricultural supply management programs reverted to default policies established in 1949 (Editorial Board 2012). Furthermore, nutrition programs received over two-thirds of the US

Heterogeneity as the Basis for Rangeland Management

400+ billion allocated by the 2008 farm bill (figure 1), indicating that agricultural policy extends well beyond the farm. Farm bills are particularly relevant to soil and water conservation, as their conservation programs have become the dominant source of public funds for private lands conservation (Burger et al. 2006). The number and complexity of farm bills, however, make it difficult to comprehend the scope and impact of this legislation. Here we provide a historical primer on US agricultural policy as a foundation for understanding current food, energy, and environmental policy discussions. This retrospective marks the 80th anniversary of the first comprehensive farm bill—the Agricultural Adjustment Act of 1933 (table 1). Given the emphasis of this journal, we focus on agricultural policies with direct, environmental impact; specifically, we focus on commodity…

Patch Burning Improves Forage Quality and Creates Grass-Bank in Old-Field Pasture: Results of a Demonstration Trial

Rangeland management, like most disciplines of natural resource management, has been characterized by human efforts to reduce variability and increase predictability in natural systems (steady-state management often applied through a command-and-control paradigm). Examples of applications of traditional command and control in natural resource management include wildfire suppression, fences to control large ungulate movements, predator elimination programs, and watershed engineering for flood control and irrigation. Recently, a robust theoretical foundation has been developed that focuses on our understanding of the importance of variability in nature. This understanding is built upon the concept of heterogeneity, which originated from influential calls to consider spatial and temporal scaling in ecological research. Understanding rangeland ecosystems from a resilience perspective where we recognize that these systems are highly variable in space and time cannot be achieved without a focus on heterogeneity across multiple scales. We highlight the broad importance of heterogeneity to rangelands and focus more specifically on (1) animal populations and production, (2) fire behavior and management, and (3) biodiversity and ecosystem function. Rangelands are complex, dynamic, and depend on the variability that humans often attempt to control to ensure long-term productivity and ecosystem health. We present an ecological perspective that targets variation in rangeland properties—including multiple ecosystem services—as an alternative to the myopic focus on maximizing agricultural output, which may expose managers to greater risk. Globally, rangeland science indicates that heterogeneity and diversity increase stability in ecosystem properties from fine to broad spatial scales and through time.