The effectiveness of quadrats for measuring vascular plant diversity

Quadrats are widely used for measuring characteristics of vascular plant communities. It is well recognized that quadrat size affects measurements of frequency and cover. The ability of quadrats of varying sizes to adequately measure diversity has not been established. An exhaustive search of sixteen 40 by 40 m old field plots documented presence or absence of 88 vascular plant species (mean = 58.8 per plot). Against these benchmarks, measurements of species richness were obtained from 50 samples in each plot and compared among three different sized quadrats (0.2 by 0.5 m, 0.5 by 0.5 m and 0.5 by 1.0 m). These quadrats differed in the number of species measured, with an average of 33.6, 38.7 and 41.4 species respectively (P<0.001; Least Significant Difference test critical difference = 0.7 species). Quadrats are relatively ineffective for documenting the actual number of vascular plant species present on old field plots (at least 30% of species undetected). However, under the intensity of sampling used in this study (n=50) a difference of less than one species per plot was detected at P<0.05. Thus, quadrat sampling regardless of quadrat size can provide a precise index of vascular plant species richness.