Changes in pinyon-juniper woodlands in western Utah's Pine Valley between 1933-1989.

Abstract

Changes in woodland vegetation integrate the consequences of livestock grazing intensity, the alteration of fire regimes, and possible climate alterution, as well as other factors. Quantitative measurements of these changes, if taken over sufficient intervals, can allow evaluation of conservation management strategies. In 1933, vegetation along a 37-km transect in southern Pine Valley, Utah was described from circular 19-m2 plots located every 42 m. The major intermediate management treatment has been reduction of grazing pressure by introduced animals, although a fraction of the area was chained and burned in 1977. During a period climatically and phenologically similar to the original study, we reexamined representative segments of this transect by a more detailed updating of the original "square-foot-density" method. Significantly greater shrub and perennial grass covers (more than threefold increases) were found in 1989, even where overall dominance is still by pinyon-juniper [Pinus monophylla (Torrey & Fremont) and Juniperus osteosperma (Torrey) Little]. This change is more obvious on steeper slopes away from roads and water, where both human and livestock disturbances would be expected to be minimized. Except in the chained portion, the observed shifts in dominance/diversity are contrary to widely accepted expectations.