The convex relationship between plant cover and biomass: Implications for assessing species and community properties

Abstract

Questions

Cover and biomass serve as common measures of species abundance in plant ecology. However, the underlying relationship between these two measures and its implications remain poorly understood. This makes results based on cover and biomass difficult to compare.

Locations

Wet meadow, southeast of České Budějovice, Czech Republic (48°57′ N, 14°36′ E).

Method

We developed theoretical expectations for systematic differences in characterizing vegetation using cover and biomass for species and community characteristics, including species diversity, temporal dynamics, and responses to experimental manipulations. We then tested these expectations using cover and biomass data from an experimental study of fertilization and dominant removal spanning 14 years (2001–2014).

Results

Consistent with our expectations, on average, species biomass corresponded to the power of species cover, with a power coefficient slightly below 3/2. Community diversity indices calculated using cover and biomass were tightly correlated but were higher for cover. Temporal variabilities based on cover and biomass for individual species were also correlated, but higher for biomass than cover. Though strongly correlated, cover data show much stronger asynchrony, suggesting higher importance of compensatory dynamics. However, using the sum of individual species' cover values as a measure of total community abundance or productivity is problematic. Such a measure is nearly independent of total biomass and leads to contradictory results when used to characterize temporal variability. Species- and community-level responses to treatments were congruent between the measures.

Conclusions

Our study provides theoretical background for a convex relationship between plant cover and biomass. The data analysis confirms the relationship and its consequences for describing species- and community-level properties. Most characteristics are well correlated between cover and biomass, but with one metric systematically shifted higher in many cases. Total abundance is the most sensitive measure and is well characterized by sum of biomass, but not by sum of cover. Understanding these systematic differences allows meaningful comparison of studies based on biomass and cover.