Confusion among three common plant cover definitions may result in data unsuited for comparison

Question: Does measurement of plant cover have consistent and comparable definitions in its applications for vegetation response in monitoring and research? Methods: A survey of the sources of definitions of cover was completed to determine common definitions and evaluate the comparability of the resulting cover methods between 1950 and 2007. Results: Methods for estimating and defining cover have varied, and relatively few citations often form the core of widely used sampling methods. Three common definitions were derived: Aerial cover - the proportion of each species at the uppermost surface of the vegetation (e.g., the aerial view), Species cover - the cover of the upper layer of each plant species independent of overhanging cover of other species, and Leaf cover - all the layers of each species from the uppermost surface to the surface of the soil (related to leaf area index). Aerial cover is the least time consuming and most easily linked to imagery, but emphasizes the dominant plants. Species cover better expresses the response of individual species but can be substantially more time consuming than aerial cover. Leaf cover correlates well to plant volume, biomass, and physiology, but can be prohibitively time consuming to collect. Conclusions: For common monitoring goals, such as species immigration (invasion) and emigration (loss of desired species), species cover can be a better choice. Publications often do not distinguish the type of cover being reported and this can lead to difficulty because the three cover methods do not result in directly comparable data, except in some unusual situations.