Functional traits and trait diversity of leaves: palaeoecological perspectives

Functional traits were originally defined as any characteristic of an organism that contributes to fitness. From this autecological perspective, trait-based research has considerably expanded into approaches of ecosystem analysis that also have high potential for palaeoecological research. In the ecosystem context, the meaning of trait has become much broader, encompassing all sorts of measurable quantities carrying ecological information that are themselves categorized into different trait classes. For instance, response traits are organismal traits responding to environmental parameters whereas effect traits act upon the environment. As primary producers, plants represent a crucial part of ecosystem functioning. Basic ecophysiological processes of plants, particularly gas exchange and photosynthesis, are key elements in the carbon and water cycle and can thereby be understood as effect traits. Fossil anatomical traits, such as from fossil leaves, allow for deriving basic ecophysiological parameters from physical laws (such as calculating leaf gas conductance from the diffusion equation). Biochemical parameters, however, are not provided by fossil material and require therefore estimation based on extant plants (such as kinetic properties of the enzyme apparatus of photosynthesis) which adds a certain error margin to the results. Nevertheless, these mixed approaches to fossil plant ecophysiology allow for obtaining crucial benchmark data on various ecosystem characteristics, such as primary productivity or evapotranspiration. Another branch of trait-based ecosystem research is the study of functional diversity which can be roughly described as the richness and distribution of functions expressed by organisms coexisting within a habitat. Functional diversity is less frequently considered for fossil vegetation compared to the study of autecological effect traits. One reason may be that various approaches for studying extant functional diversity are difficult or even impossible to apply to fossil plants, requiring the development of novel methods suitable for fossil remains. As a recent example, the Shannon Diversity of leaf architecture based on functional leaf traits identifiable from fossil leaf material was shown to be related to environmental parameters for extant as well as fossil angiosperms. Devising trait-based approaches to functional diversity suitable for fossil organisms can offer additional fruitful research perspectives for studying environments of the past.