Overview

The central hypothesis of this project stated that any direct, negative effects on ecosystem properties will recover over the subsequent growing season, even under cattle grazing.

Results of this project support the hypothesis: Livestock grazing appears to be a sustainable option for post-wildfire rangeland. Forage nutritive value was higher on burned areas, which adequately recovered their productivity, and there was little evidence for negative effects of grazing on measures related to soil health.

Broad objectives

  • Compare ecosystem impacts of wildfire to pre-burn/unburned rangeland

    • We accomplished this objective by measuring soil nutrient pools, soil microbes, available forage biomass (herbaceous standing crop), and forage nutritive value after the fire event in both burned and unburned areas.
    • Forage data were compared in a Control-Impact design.
    • Soil data were compared to identical samples collected prior to the fire event, in a Before-After-Control-Impact design.

  • Examine interactive effects of wildfire and post-wildfire livestock grazing

    • We accomplished this objective by constructing grazing exclosures in both the burned and unburned areas, allowing a full-factorial comparisons of soil and vegetation responses after exposure to both fire and grazing
    • Cattle were stocked in the pasture in the first season after the fire.

  • Develop new cattle grazing strategies that will help sustain rangelands, range livestock agriculture, and rural ranching communities challenged by wildfire.

    • Work on this objective remains ongoing, but the above-mentioned data suggest that grazing in the first season after wildfire does not have immediate negative effects on soil and vegetation.
    • In fact, the data suggest fire has a positive effect on soil health and forage nutritive value, and grazing can take advantage of this higher-quality forage.
    • Continued research is assessing longer-term effects on how different defoliation/deferment patterns affect productivity after several years of post-fire management.

Specific objectives

  • Describe how soil C, N, and microbial communities respond immediately to wildfire
    • Soil nutrients generally had a seasonal decline in the absence of fire, which was modulated by initial burn severity at burned sample points: points with Very High and High tended to resist the decline, while lower-severity areas tended not to decline as much as unburned areas.
    • Soil microbes also showed a seasonal decline. All but points with Very High initial severity had soil microbial abundance slightly less than unburned points. Bacteria were least affected, while fungi were slightly more affected by fire.
  • Investigate resilience of soil and forage resources over the subsequent grazing season
    • Patterns in soil nutrients persisted into the next grazing season across all measures. Total P and nitrate at points with higher initial severity remained higher through the second season after fire, whereas there was no apparent differences in ammonium in the second season after fire.
    • Little evidence that soil nutrients were affected by grazing in either burned or unburned areas.
    • There were no differences in soil microbial abundance across the sample points in subsequent seasons, regardless of initial burn severity.
    • In the full factorial comparison of fire and grazing, soil microbial abundance had considerable variability, with trends for seasonal declines in soil microbes in burned areas and higher soil microbial abundance in grazing exclosures. But little evidence for meaningful differences all around.
    • Forage nutritive value was better in burned areas in May, June, and July, but the difference disappeared later in the grazing season.
    • After May, when substantial growth began, herbaceous standing crop available as forage in the burned area was consistently 75% of that in the unburned area, due to the removal of standing dead and litter material.
  • Explore feasibility of livestock grazing as an opportunity for post-wildfire management
    • Livestock grazing appears to be a sustainable option for post-wildfire rangeland.
    • Forage nutritive value was high, available forage was adequate, and there was little evidence for negative effects of grazing on measures related to soil health.

Results

Soil

Soil nutrient and microbial responses were variable, with considerable inter-annual and seasonal variation. Initial burn severity had a stronger and longer effect on nutrients than microbes. Microbes might be more sensitive to grazing.

Soil nutrient pools

  • Overall little difference in soil nutrients across burned and unburned areas in general, due to wide variability.
  • Specific fire responses appear correlated with initial burn severity, with higher levels of severity associated with higher C, P, total N, and plant-available N immediately after the fire event and into the next spring.
  • Although levels of P and plant-available N were generally lower in the second season after the fire, the correlation between initial burn severity and higher P and nitrate levels appears to persist.
Soil nutrient pools by burn severity. In 2024, under the no-cost extension, analysis funds were focused on P and plant-available N as fire effects on total C and N have been reported elsewhere.

Soil nutrient pools by burn severity. In 2024, under the no-cost extension, analysis funds were focused on P and plant-available N as fire effects on total C and N have been reported elsewhere.

Soil nutrients by wildfire and grazing exposure in the season after a wildfire. However, these data are from early June, after cattle had only been on the pasture for a few weeks. Data from July forthcoming.

Soil nutrients by wildfire and grazing exposure in the season after a wildfire. However, these data are from early June, after cattle had only been on the pasture for a few weeks. Data from July forthcoming.

Soil microbes

  • Overall few substantial changes in total bacteria, total fungi, and total microbial abundance with respect to objectives—fire and grazing. Most variability apparently attributable to inter-annual and seasonal differences.
  • With respect to burn severity:
    • Soil microbial abundance generally declined over the seven weeks between sampling events in the year of the fire, when unburned sample points are compared to pre-burn samples. Only sample points with very high burn severity appeared to resist this seasonal decline; other burn severity classes were lower but not different from unburned points 1 week after the fire.
    • Soil microbial abundance was overall lower at both the beginning and end of the next subsequent seasons, perhaps due to lower soil moisture. Little evidence of any difference between burned and unburned points.
  • In the full-factorial comparison of fire and grazing exposure:
    • No clear evidence of differences.
    • Soil microbe abundance appears to decline from early to mid summer on burned plots
    • Burned plots open to grazing tended to have lower microbial abundance than burned plots without grazing, but these values did not appear to be substantially different from grazed and unburned plots.
Summed soil microbial abundance (PLFA concentration) by burn severity.

Summed soil microbial abundance (PLFA concentration) by burn severity.

Soil microbial abundance (PLFA) by wildfire and grazing exposure in the season after a wildfire.

Soil microbial abundance (PLFA) by wildfire and grazing exposure in the season after a wildfire.

Vegetation

Rangeland vegetation appears very resilient to wildfire, with higher nutrient value post-burn and substantial recovery of productivity after June in the season after the fire.

Standing crop

Clipping from plots open to cattle grazing—cattle were stocked from June through September—shows there was little difference in standing crop between burned and unburned areas across all ecological sites.

By the time cattle were on the pasture in June, standing crop on the burned area had recovered to just over three-quarters of the available biomass in unburned areas.

Average sample mass in burned and unburned plots, and the % difference in standing crop over the grazing season, June-September, in burned vs. unburned plots by site.
site burned unburned Burned v. unburned (%) Overall mean
Sandy 9.2 14.8 62 77
Shallow 5.96 11.6 51
Silty 12.4 15.7 79
Steep 18.5 15.9 116

Forage quality

Forage nutritive value in burned plots exceeded that of unburned plots in the first part of the grazing season following fire:

  • Crude protein, total digestible nutrients, and net energy for both maintenance and lactation were higher in burned vs. unburned plots in May, June, and July.
  • Fiber was lower in burned plots vs. unburned plots in May, June, and July.
  • No measures of forage nutritive value differed between burned and unburned plots in August and September.
  • Concentrations of most minerals (excluding Zinc and Manganese) were higher in forage collected from burned vs. unburned plots May-July.
Average values for burned and unburned plots, and the % difference across burned vs. unburned plots, early in the grazing season (May-July).
response Burned Unburned Burned v. unburned (%)
Crude protein (%) 9.55 6.97 137
Fiber (ADF, %) 35.9 40.1 90
Total Digestable Nutrients (%) 54.2 49.8 109
Net energy, maintenance (Mcal/lbs) 0.53 0.477 111
Net energy, lactaction (Mcal/lbs) 0.552 0.503 110
Calcium 0.411 0.337 122
Copper 4.5 3.49 129
Magnesium 0.148 0.113 131
Potassium 1.55 1.03 150
Sulfur 0.157 0.11 143
Phosphorus 0.182 0.154 118
Iron 160 138 116
Manganese 37.4 38 98
Zinc 24.3 23.8 102
Forage nutritive value on an as-fed basis from the sub-samples analyzed via wet chemistry for calibration of the NIRS equation. These samples were collected from areas outside exclosures open to cattle grazing.

Forage nutritive value on an as-fed basis from the sub-samples analyzed via wet chemistry for calibration of the NIRS equation. These samples were collected from areas outside exclosures open to cattle grazing.

Forage mineral content on an as-fed basis from the sub-samples analyzed via wet chemistry for calibration of the NIRS equation. These samples were collected from areas outside exclosures open to cattle grazing.

Forage mineral content on an as-fed basis from the sub-samples analyzed via wet chemistry for calibration of the NIRS equation. These samples were collected from areas outside exclosures open to cattle grazing.

Products

Data from this project featured prominently in talks at these events:

  • Society for Range Management, February 2024, Sparks, NV
  • Grassland Society of Southern Africa, July 2024, Gariepdam, Free State, RSA (invited keynote)
  • Montana Prescribed Fire Council Annual Workshop, November 2024, Dillon, MT

Working titles and target journals for peer-reviewed papers in preparation:

  • ``Rangeland soil and vegetation are resilient to wildfire in the Northern Great Plains’’, Science of the Total Environment
  • ``Soil microbe community responses as indicators of post-wildfire recovery in Northern Great Plains rangeland’’, Environmental Management
  • ``Remotely-sensed burn severity and recovery indices as indicators of post-wildfire grazing sustainability’’, Ecological Indicators
  • ``Long term effects of post-fire grazing management on rangeland productivity’’, Rangeland Ecology & Management

Outreach events (field tours) included:

  • US Fish and Wildlife Service Northern Great Plains Joint Venture Fall meeting, October 2024
  • Montana Range Days, June 2024
  • Montana State Extension annual meeting (can’t recall or find specifics)

Methods

Study area

An escaped prescribed fire on Fort Keogh that spread through a pasture in which soil and vegetation data had already been collected ahead of planned prescribed burns, and could be stocked with cattle in the season after the fire, provided an opportunity to study how wildfire and cattle grazing affect ecosystem properties in mixed-grass prairie.

L: X marks Fort Keogh within the Northwestern Great Plains. R: The Lemonade Fire of 22 Sept 2022 covered about 900 acres on Fort Keogh. Severity was determined from LandSat imagery collected just a few days before the burn, and just a few days after.

L: X marks Fort Keogh within the Northwestern Great Plains. R: The Lemonade Fire of 22 Sept 2022 covered about 900 acres on Fort Keogh. Severity was determined from LandSat imagery collected just a few days before the burn, and just a few days after.

Data collection

Based on focus group with stakeholders, we designed a study to measure the following responses:

  • Soil
    • Total elemental nutrients and plant-available nitrogen
    • Microbial communities
    • 1 week post-fire and summer after
    • By severity and ecological site
  • Vegetation
    • Total aboveground herbaceous (grass + forb) standing crop
    • Forage nutritive value
    • Paired burned/Unburned plots nested by ecological sites
    • Clipped monthly summer after