Alien plant invasion in relation to site characteristics and disturbance: Eragrostic lehmanniana on the Santa Rita Experimental Range, Arizona, 1937-1989

Abstract

Presence and composition of Eragrostis Lehmanniana was measured on 75 permanent transects every 3-8 years between 1958-1989 on the Santa Rita Experimental Range, Arizona. The number of transects occupied and average composition increased rapidly over this period. A repeated measures analysis of two soil groups with different water holding capacity and permeability revealed that the rate of increase of E. Lehmanniana was different between the groups. Repeated measures analysis of five grazing intensities revealed that the rate of E. Lehmanniana increase was different among intensities. It appears that the highest intensity had the highest rate of increase. The influence of grazing on E. Lehmanniana spread was also expressed by the lack of difference in relative composition measured in 1989 between ten livestock enclosures and adjacent grazed areas. This research suggests that E. Lehmanniana will spread and dominate with an average of 89 mm of summer precipitation in 40 days; a lower regime than estimated by other research.