Scale-dependent variation in visual estimates of grassland plant cover

Plant cover was visually estimated by five observers, independent of each other, in a species-rich grassland in the Bile Karpaty Mts., southeastern Czech Republic, in seven plots ranging from 0.001 to 4 m(2). Variation of total plant cover among the observers was high at small scales: 0.001 - 0.016 m(2); coefficient of variation, CV = 35 to 45%, but much lower at larger scales: 0.06 - 4 1112 CV = 7 to 15%. Differences between visual estimates of plant cover of individual species made by different observers were affected by plot size, total cover and morphology of particular plants. CV of the cover of individual species ranged front 0 to 225% and decreased with increasing plot size. For abundant plants the CV attained ca. 50%, independent of plot size. In spite of a very high number of sterile plants with similar leaf morphology and colour, the observed variation in cover estimates in the studied grassland was comparable with results reported front other vegetation types. Differences between estimates by individual observers were often larger than Usual year to year changes in undisturbed grasslands. Therefore, I Suggest that to avoid difficulties in the interpretation of results based on plant cover data obtained front visual estimates, several observers should always work together, adjusting their extreme estimates.