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Project: East and West Boundary Fence Plant Line Intercepts - percent cover, Jornada Basin
East and west boundary fence plant line intercepts - percent cover for 9 species, Jornada Basin, 1982 to 1992
Investigate the response of vegetation with respect to landscape characteristics as well as release from grazing through vegetation monitoring across fence lines of grazed and ungrazed pastures.
In the spring of 1982, as part of the establishment of the Jornada Long-Term Ecological Research site in southern New Mexico, a 135 ha portion of a 1500 ha, internally drained, watershed was exclosed from grazing by domestic livestock. Prior to exclosure the watershed, as well as the rest of the Jornada basin, had been moderately to heavily grazed for the past 100 years. Concurrent with grazing, the vegetation had undergone a dramatic change from desert grassland, with an almost continuous cover of C4 perennial grasses, to isolated patches of the original grassland in a mosaic with desert shrub dominated plant communities (Buffington and Herbel, 1965).
The exclosure lies along a northeast facing piedmont slope at the base of a steep isolated mountain peak, and covers a variety of component landforms from the foot of the mountain to the basin floor. This provided the opportunity to investigate the response of vegetation with respect to landscape characteristics as well as release from grazing. The exclosure is located immediately upslope from College Playa near NMSU College Ranch Headquarters.
1. Permanently marked 30m line transects were located at 50m intervals along the two parallel long axis fences of the exclosure. These fences run from north to south up the slope of the watershed, traversing the component landforms characteristic of the bajadas of the Basin and Range Province. The transects are thus perpendicular to the direction of major drainage flow. Plant line intercepts were measured for all perennial plant October of 1982, 1986, 1992, and 1998. These were converted to Mean Percent Cover per line transect per station.
2. Vegetation communities along each fence were defined using sliding window boundary analysis (Ludwig and Cornelius) and canonical discriminant analysis.
3. T tests were performed on the changes in cover of individual species within the exclosure (ungrazed) versus without (grazed), by vegetation community. Tests were also performed on groups of high grazing preference species as opposed to low grazing preference species.
After five years of exclosure, the grass dominated communities within the exclosure appear to have a lush cover of grass compared to the grazed areas outside. This is a reflection of the increased cover of Bouteloua eriopoda in Community B (going from 3.3% cover in 1982 to 20.4% in 1987), and the combined increase of Aristida and Sporobolus species in Community C (going from 2.5% cover to 17.6%). All these genera are highly palatable grasses that were important components of the original grassland. The remaining species and communities showed little or no response to release from grazing.
The degree of response is closely tied to landscape position. Those communities (B & C) which are in a positions to receive high additions of water and nutrients from runon from upper parts of the watershed showed the greatest response (if the original grass species had not been completely lost, as in Community A). In contrast, we have not yet observed a significant response in communities which are still grass dominated (D & E), but are in landscape positions that receive less runon. Thus they are not sinks for materials transported along the watershed. Shrub dominated communities (F) on erosional landforms with little or no runon showed no response for any species.
Recovery of perennial grass cover in desert grasslands of the northern Chihuahuan Desert appears to be possible, though a slow process that is dependent on landscape position.