Belowground considerations in defining plant functional types: Can we characterize and represent root functional types?

Background/Question/Methods

Plant functional type (PFT) is a designation with a long history in ecological research. Plant species are generally grouped into PFTs defined based on aboveground characteristics, including physiological and morphological traits, in order to represent the dynamic responses of plants and plant communities to changing environmental conditions as projected by climatic models at large, global scales. Belowground plant characteristics, and their similarities and differences among plant species and traditionally-defined PFTs, have received less attention, and are not currently represented in most large-scale models. In particular, new characteristics are needed in global models to represent the plant communities that thrive under the relatively extreme conditions (e.g., cold, anoxic) typified by organic soils in wetlands and in the tundra, where a literature survey indicates that roots and belowground stems can average as much as six times the mass of aboveground plant parts. Species that colonize boreal and arctic ecosystems have a variety of belowground adaptations to cope with harsh environmental conditions. Shrubs and trees rely on shallow rooting distributions that remain above the average water-table level, as well as mycorrhizal symbioses, while sedges and rushes tend to rely on aerenchyma to oxygenate deeper rooting distributions, and often lack mycorrhizal symbioses.

Results/Conclusions

The representation of plant species in models may be improved by characterizing belowground plant properties as ‘root functional types’ (RFTs). RFTs can then be associated with aboveground characteristics traditionally represented in PFTs. Definitions of RFTs should include root characteristics that can strongly influence ecosystem CO₂ and CH₄ fluxes, such as aerenchyma and root exudation, or ecosystem water and nutrient fluxes, such as specific root length and rooting depth distribution. These root characteristics will not necessarily be captured within traditionally-defined PFTs. For example, the PFT encompassing ‘grasses’ or ‘sedges’ will group species which differ in aerenchyma cross-section, nutrient content, root radial O₂ loss, and root exudation rates.

Databases are being increasingly populated by information which facilitates the qualitative separation of plant species based on aboveground characteristics, including specific leaf area and leaf longevity. However, more data are needed on relevant belowground plant characteristics, including linkages between root anatomy and root function, as well as species-specific characterization of rooting depth distribution, mycorrhizal colonization, exudation rates, root porosity, specific root length, and root lifespan. These data are necessary to determine which, if any, RFTs will improve dynamic vegetation representation in models at the global scale.