Should we estimate plant cover in percent or on ordinal scales?

Abstract

Question: We explored the error resulting from different methods for recording the cover of plants in vegetation plots, specifically the direct estimation of percent cover vs. the use of ordinal cover scales (7-step Braun-Blanquet and 5-step Hult-Sernander-Du Rietz). Methods: We simulated 121 plant species of different cover, sampled with 13 different levels of estimation precision. Estimation precision was either based on a constant coefficient of variation (0.1–1.0) across all cover values or on empirical data from Hatton et al. (1986, Journal of Range Management 39: 91–92) (× 0.5, × 1.0, × 1.5). Each sampling was repeated 10 times. Subsequently, we determined the mean relative and absolute errors that occurred in the data used for ensuing numerical analyses. Results: Except for few cases with unrealistic settings (very high estimation error and ignorance of species with lower cover values), direct estimation in percent yielded better results than the use of ordinal scales. Based on the empirical values of estimation accuracy, the use of ordinal scales inflated the mean absolute and relative errors nearly 2-fold in case of the 7-step Braun-Blanquet scale and about 1.5-fold in case of the Hult-Sernander-Du Rietz scale if only considering cover values above 1%. Conclusions: From our personal experience, the careful application of an ordinal scale is not faster than the direct estimation of percent cover. For this reason, we see no plausible argument supporting the use of ordinal cover scales when essentially all subsequent analyses are numeric. Abbreviations: Br.-Bl. = 7-step variant of the Braun-Blanquet scale and its numerical replacement as in Table 2; CV = coefficient of variation; H.-S. = Hult-Sernander-Du Rietz scale and its numerical replacement as shown in Table 1.