Functional groups differ in trait means, but not in trait plasticity to species richness in local grassland communities

Abstract

Despite growing interest in incorporating intraspecific variation of functional traits in community-level studies, it remains unclear whether species classified into functional groups based on interspecific trait differences are similar regarding their variation in trait expression in response to varying plant diversity and composition in local communities. In a large biodiversity experiment (Jena Experiment) designed on a trait-based a priori definition of functional groups (grasses, legumes, small herbs, tall herbs), we studied means, extent of variation (coefficient of variation across communities) and plasticity to increased plant diversity (slopes over a logarithmic species richness ranging from 1, 2, 4, 8 and 16 to 60 species) for nine functional traits. Species means and extent of variation in traits related to nitrogen (N) acquisition and N use differed among functional groups and were more similar in phylogenetically closely related species than expected by chance. Species in the same functional group showed a weak phylogenetic signal and varied widely in means and extent of variation in traits related to shoot architecture and to a smaller extent in leaf traits related to carbon acquisition. This indicated that functional groups were less distinguishable in light than in nitrogen acquisition strategies. The direction and degree of trait plasticity to increasing species richness did not show a phylogenetic signal and were not different among functional groups, but varied largely among species within functional groups. Correlation structures in trait means, extent of trait variation and trait plasticity revealed functional tradeoffs in the acquisition of nitrogen and light across species. While correlations between trait means and extent of trait variation varied from trait to trait (positive, negative or unrelated), trait means and trait plasticity were mostly unrelated. Our results suggest that the concept of functional groups is viable, but context-specific trait measurements are required to improve our understanding about the functional significance of intraspecific trait variation and interspecific trait differences in local plant communities.

On the use of the coefficient of variation to quantify and compare trait variation

“…For example, in the study by Roscher et al. (2018), differences in CVs between traits related to gas exchange and growth on the one hand and traits related with leaf morphology, anatomy, and photochemistry on the other hand were interpreted as due to differences in the “level of organization,” whereas the two groups of traits markedly differed in their dimensions and statistical distribution (see Ramírez‐Valiente et al. 2018 for a similar issue).…”

“…Recently, the CV or related statistics expressing variation in relation to the mean (e.g., CV 2 ) has been used to measure and compare both types of variation across traits (Fajardo and Piper 2011; Roscher et al. 2018; Acasuso‐Rivero et al. 2019, for phenotypic plasticity, and Houle 1992; Hansen et al.…”

Plant diversity effects on plant longevity and their relationships to population stability in experimental grasslands

“…The major difference among species assigned to the functional groups of ‘small herbs’ and ‘tall herbs’ in the Jena Experiment was growth height. On average, small herbs are <20 cm in height, while growth height of tall herbs is mostly more than 30 cm at peak canopy development (Roscher et al., 2018). Marbà et al.…”

“…Furthermore, it has been suggested that plant longevity is related to traits of the plant economics spectrum, such as SLA (Klimešová et al., 2016). In situ measurements have shown that SLA decreases from small herbs to legumes to tall herbs in the Jena Experiment (Roscher et al., 2018). This observation provides further evidence for demographic differences between slow‐ and fast‐growing plants (Klimešová et al., 2016), because mean and maximum age decreased in our study from tall herbs to legumes to small herbs.…”

“…On average, small herbs are <20 cm in height, while growth height of tall herbs is mostly more than 30 cm at peak canopy development (Roscher et al, 2018). Marbà et al (2007) tested for allometric scaling in plants, as it is well known for animals, that is, the size dependency of 'birth' and mortality rates, across species spanning from phytoplankton to trees.…”