A comparison of long-term vegetation changes was made between a protected (non livestock grazed) and heavily livestock grazed desert grassland range at Page-Trowbridge Experimental Ranch in southeastern Arizona (southwest USA).
The study sites comprised two adjacent 320 acre (129.5 ha) areas of desert grassland. One was conservatively grazed from 1923 to 1941 and protected from livestock grazing thereafter. The other was part of a heavily grazed range.
Each area was divided into eight 40-acre (16.2-ha) blocks and a 50 foot (15.24 m) line transect established in each. Ten samples were systematically taken (from a random start point) giving 80 samples per area. Using the line intercept method, perennial grasses and forbs (basal cover and species frequency of occurrence) were measured at ground level, and trees and shrubs at the crown, in 1941 and 1969.
Basal cover of perennial forbs changed little in both areas (<0.3% in1941 and 1969). Grasses increased significantly, although remaining at low overall cover values (grazed: 1941 - <0.3% 1969 - 1.2%; protected: 1941 - 1.7% 1969 - 3.1%).
Crown cover of trees and shrubs increased significantly (primarily due to increases in velvet mesquite Prosopis juliflora var. velutina), the increase being most marked in the protected area (grazed: 1941 - 8.8% 1969 - 9.8%; protected: 1941 - 3.9% 1969 - 8.0%)
Two grasses, Arizona cottontop Digitaria californica and sideoats grama Bouteloua curtipendula, and the herbaceous perennial Wright buckwheat Eriogonum wrightii were dominant in the understory on the protected range, and these, along with other species indicated improving and ‘intermediate’ range condition.
On the grazed range, Rothrock grama Bouteloua rothrockii, poverty threeawns Aristida divaricnta and A.hamulosa (indicating an improving trend), burroweed Haplopappus tenuisectus (poisonous to mammals) and annuals dominated the understory; the area was classed as in ‘low’ range condition,
Without a change in management, the authors suggest that it is probable that mesquite will continue to increase on both areas.