| Ward W. Brady, John E. Mitchell, Charles D. Bonham and John W. Cook |
| Authors are professor, Dept. of Planning (Environmental Resources), Arizona State University, Tempe 85287-2005; range scientist, USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colo. 80524; professor, Rangeland Ecosystem Science, Colorado State University, Fort Collins 80523; and research assoc., Dept. of Planning (Environmental Resources), Arizona State University, Tempe 85287-2005. |
Abstract |
| To assess the power of point data (collected systematically at each meter along a permanently-situated, 100-m line transect) to detect actual changes in plant basal cover, we developed a computational approach whereby a simplified shortgrass steppe community was spatially simulated on a computer screen. Cover was then reduced using a random disturbance pattern. One transect could detect an actual decrease in cover from 12% to 8% with less than 20% probability, while 5 transects increased this power to about 80% (P<.05). A reduction in cover from 12 to 6% could be detected with 80% probability with only 2 transects, while a cover reduction to 10% could only be detected with 40% probability using 10 transects (P<.05). Artificial populations provide a valuable mechanism for quantitatively evaluating field sampling designs. |
| Key Words: short-grass steppe, vegetation measurement, Type II error, sample size, hypothesis testing, basal cover, disturbance |