Application of the fire-grazing interaction to restore a shifting mosaic on tallgrass prairie

Summary

In this study, the interactions between fire–grazing prairie grassland management and its potential as a land-management approach investigated. Whether patch burning followed by focal grazing of Great Plains grasslands creates a shifting mosaic pattern of vegetation structure and composition was assessed. A heterogeneity-based approach, in which fire was applied to discrete patches, was compared with typical homogeneity-based management in the North American Great Plains, to determine if patch burning followed by focused grazing creates a shifting mosaic pattern of vegetation structure and composition.

Study area: The study area was located in an area of grassland at the Oklahoma State University Research Range, 21 km south-west of Stillwater, Oklahoma, USA.

Experimental desighn: A completely randomized design was used with two treatments, replicated three times (a total of six pastures), established. Treatment units varied in size from 45 to 65 ha. The treatments were:

1) burning distinct patches within a treatment unit with free access by moderately stocked cattle (patch treatment)

2) no burning with free access by moderately stocked cattle (traditional treatment)

In the patch treatment, one-sixth of the experimental unit was burned each spring (March to April) and one-sixth each summer (July to October). Application of fires within each patch treatment pasture was in sequential to give a 3-year return interval. No patches were burned in the traditional treatment. The first year (1999) of recording represented pretreatment data.

Cattle performance: Pastures were moderately grazed with yearling cattle from about 1 December to 1 September, at 3 ha/animal. This stocking rate was based on long-term research to optimize sustainable production. Cattle were weighed on electronic scales at the beginning (1 December), middle (1 April) and end (1 September) of the grazing season each year. Gain per hectare and average daily gain (ADG) were analysed to identify differences between patch and traditional treatment pastures.

From 13 April to 1 September 2000, cattle were observed three times a day, once a week (54 observations) to determine patch preference and grazing activity.

Vegetation composition and structure: Vegetation was sampled in late August–early September of 1999, 2000, 2001 and 2002. For each patch within each pasture, canopy cover for plant functional groups, amount of bare ground and cover by litter was recorded within 30, 10 x 10 cm randomly located quadrats (180/pasture).
The angle of obstruction (AOB) was used to measure vegetation structure simultaneous to measures of vegetation composition in 1999, 2000 and 2001. Thirty randomly located AOB points were measured in each patch (180 in each pasture).

Vegetation composition and structure: Forb cover and bare ground increased in the patch treatment in 2000 and remained higher than the traditional treatment for the duration of the study. Litter increased in the traditional treatment and remained more abundant than the patch treatment. Tallgrasses were more abundant in the traditional treatment, including pretreatment (1999), indicating that treatment had little effect on these grasses at the pasture scale. Annual grasses and little bluestem Schizachyrium scoparium were variable across years, and did not differ between treatments.

The invasive chinese lespedeza Lespedeza cuneata occurred in all treatment units and increased overall during the study. Its cover was greatest in the traditional treatment throughout the study, where the rate of increase over 4 years was also greatest. L.cuneata fluctuated at low values in the patch treatment, the amount being dependent upon which patch served as the focal fire-grazing patch.

The differences in structure of these grasslands were best described by the treatment–year interaction. AOB of the traditional treatment was near 90° every year, indicating a dense, continuous herbaceous canopy resulting from uniform moderate to light disturbance. Initiation of patch treatment in 2000 reduced the overall AOB to about 80°, indicating a more open structure. This effect was due primarily to the effect of fire and focal grazing on the most recently burned patches.

Heterogeneity among patches: Variability of litter, bare ground and forbs increased among patches. By 2001, heterogeneity of the patch treatment had increased by three- to fivefold over the 1999 pretreatment measurements. Structural heterogeneity, based on AOB, followed patterns similar to the cover variables. By 2001, heterogeneity of structure had increased more than fourfold in the patch treatment. These findings were supported by Principal Components Analysis (PCA).

Resilience under focal fire-grazing disturbance: Results of the PCA suggested that focal patches within the patch treatment differed little from the traditional treatment after 3 years since fire. This was confirmed by regression analysis of months since disturbance and differences in cover between patch and traditional treatments. Cover of grass on focal patches matched that of traditional treatment pastures after about 2 years and generally exceeded it after 3 years. Rapid increase of grass cover was probably due to limited production in the traditional treatment, because of high litter accumulation. Forbs were highly variable from year to year, but cover was highest during the first year to 18 months after the focal disturbance and consistently more abundant in the patch treatment than the traditional treatment.

Cattle response: Grazing behaviour differed between treatments largely because cattle preferentially selected grazing patches in the patch treatment pastures, while in the traditional pastures the cattle grazed fairly evenly, with only one patch grazed more than 20% of the time (26%) and one patch less than 10% of the time (8.6%). In the patch treatment pastures, 75% of grazing time was in the two patches that were most recently burned, and no more than 8% of grazing time was spent in any single patch not burned within the past year. The weight gain of grazing animals differed among years but did not between treatments

Conclusions: This study demonstrates that the fire-grazing management technique used may be useful for generating heterogeneity in grasslands. Fires applied to patches, and patchy grazing by the cattle promoted a shifting vegetation mosaic across the pastures.


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