A nested-intensity design for surveying plant diversity

Managers of natural landscapes need cost-efficient, accurate, and precise systems to inventory plant diversity. We investigated a nested-intensity sampling design to assess local and landscape-scale heterogeneity of plant species richness in aspen stands in southern Colorado, USA. The nested-intensity design used three vegetation sampling techniques: the Modified-Whittaker, a 1000-m(2) multiple-scale plot (n = 8); a 100-m(2) multiple-scale Intensive plot (n = 15); and a 100-m(2) single-scale Extensive plot (n = 28). The large Modified-Whittaker plot (1000 m(2)) recorded greater species richness per plot than the other two sampling techniques (P< 0.001), estimated cover of a greater number of species in 1-m(2) subplots (P< 0.018), and captured 32 species missed by the smaller, more numerous 100-m(2) plots of the other designs. The Intensive plots extended the environmental gradient sampled, capturing 17 species missed by the other techniques, and improved species-area calculations. The greater number of Extensive plots further expanded the gradient sampled, and captured 18 additional species. The multiscale Modified-Whittaker and Intensive designs allowed quantification of the slopes of species-area curves in the single-scale Extensive plots. Multiple linear regressions were able to predict the slope of species-area curves (adj R-2 = 0.64, P< 0.001) at each Extensive plot, allowing comparison of species richness at each sample location. Comparison of species - accumulation curves generated with each technique suggested that small, single-scale plot techniques might be very misleading because they underestimate species richness by missing locally rare species at every site. A combination of large and small multi-scale and single-scale plots greatly improves our understanding of native and exotic plant diversity patterns.