Individual study: The effect of grazing on grassland vegetation in the Chaco Culture National Historic Park, New Mexico, USA
Floyd M.L., Fleischner T.L., Hanna D. & Whitefield P. (2003) Effects of historical livestock grazing on vegetation at Chaco Culture National Historic Park, New Mexico. Conservation Biology, 17, 1703-1711
Cattle and sheep grazing have been widespread and intensive across the semiarid shrublands and grasslands of western USA. The ecological consequences of such grazing are suggested to be severe, including reduction in biodiversity and aiding the spread of alien species. Of particular concern is the role of livestock in removing microbiotic (microphytic or cryptogamic) soil crusts, which are carbon sources on sparsely vegetated areas. These soil crusts provide nitrogen, increase soil stability (preventing erosion), retain soil moisture, and are seed-germination sites, thus keeping land between plants fertile and with a good microbial community. However, grazing has been so extensive in the western USA that long-term studies on the effects of grazing have been limited. In this study, the effect of grassland exclosures (no grazing) on the plant community and the microbiotic soil crust is investigated.
Study sites: In 1907, the Chaco Culture National Monument was established, and between 1936 and 1948 the boundaries were fenced creating a 8,600 ha exclosure. In 1980, an expanded Chaco Culture National Historic Park was designated, and from 1995 to 1999 four new parcels (totalling 5,000 ha) were also fenced. The surrounding areas have been grazed continually over this period, although the livestock type, grazing density, duration and frequency are unknown. Six study areas were selected, each with three adjacent (where possible) grazing treatments (>50 yr exclosure, <5 yr exclosure, and continually grazed land). In each of the six study areas, slope, aspect, topography and substrate were similar, allowing grazing to be considered as the only experimental variable. Within each grazing treatment at a site, 6-10 sample points spaced by a minimum of 100 m were randomly selected.
Monitoring: At each of the 130 sample points in spring 1999 and 2000, the relative frequency of shrubs, grasses, bare soil and microbiotic soil crust (black crust: consisting of lichen Collema spp., several species of cyanobacteria, and the mosses Tortula canineruis and Didymodon tophaceus) was estimated in 10, 0.5 x 0.5 m quadrats. Additionally, all plant species were identified, and their relative abundance and cover was estimated (no further details of methods given).
Black biotic soil crust: At each of the six sites, black biotic soil crust was greatest in areas with >50 yr exclosures. Furthermore, <5 yr exclosures were also consistently higher than the currently grazed control areas (see Table 1, numbers estimated from original figure).
Number of plant species: At each of the six sites, the number of plant species was greatest in areas with >50 yr exclosures. Furthermore, the number of plant species in the <5 yr exclosures was higher than the currently grazed control areas in five of the six sites (see Table 2 attached; numbers estimated from original figure). Between 20% and 33% of all plant species at a site were alien, but there was no difference in the proportion of exotic species among treatments.
Bare soil: Currently grazed control areas had more bare soil than exclosure areas in five of six sites (see Table 3, numbers estimated from the original figure).
Shrub & grasses: There were significant differences between the three grazing treatments, with shrub and grass cover increasing in exclosures at four and three sites respectively. However, the differences did not follow a general pattern across sites, which suggests that local conditions have a large effect on the pattern of their growth.
Conclusions: The creation of long-term, large exclosures consistently benefited the plant community by increasing species richness and maintaining microbiotic soil crusts. Furthermore, there was a more general trend of increased shrub and grass cover, and less bare soil, in exclosures. Overall, the creation of exclosures appears to have been a successful management strategy.
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