Engaging FRED (Fine-Root Ecology Database): Leveraging variation in root functional traits within and among plant functional types to better understand and model above- and belowground ecosystem processes

Background/Question/Methods

Terrestrial biosphere models capture the fundamental complexity and variation in plant structure and function across the globe by aggregating plant species into groups, or Plant Functional Types (PFTs). While fine plant roots are an important driver of belowground processes (i.e., carbon, water, and nutrient cycling) the distribution and dynamics of fine roots are poorly represented in the current generation of terrestrial biosphere models. This adds uncertainty to model simulations of current and future ecosystem processes and environmental conditions. A newly-developed Fine-Root Ecology Database (FRED) gathers and organizes key root functional traits (e.g., morphology, chemistry, distribution, lifespan, water and nutrient acquisition) to inform our understanding and modeling of fine-root distribution and dynamics across the globe. So far, FRED houses nearly 7000 observations of root functional traits spanning boreal, temperate, tundra and tropical biomes. Leveraging FRED, our objectives were three-fold: (1) Identify global patterns and trends in root functional traits within and among PFTs, (2) Compare empirical observations with the treatment of fine roots in current terrestrial biosphere models, and assess the implications for simulating below- and aboveground ecosystem function, and (3) Determine where additional measurements are needed to fully represent the diversity in root form and function across the globe. 

Results/Conclusions

Two major themes emerged in our investigation of key root functional traits in FRED: (1) Root traits differ within and among PFTs. For example, root N, P, C (mg g^-1), and C:N ratio are significantly different in broadleaf deciduous tropical versus broadleaf deciduous temperate tree biomes. Rooting depth varies significantly between broadleaf deciduous tropical, temperate and boreal tree biomes. In addition, there are significant differences in P and N storage between root orders 1-3 and 4+. These trends suggest that root form and function vary under differing environmental conditions, such as nutrient availability. (2) More data are needed to accurately quantify the variation among many important root traits. In particular, fine root trait data are limited in tropical and boreal evergreen and deciduous tree biomes. Despite this, current trait data can improve belowground modeling efforts. For example, root C:N ratio is static (42) for all PFTs in the Community Land Model v4.5. However, FRED suggests actual root C:N ratio varies (from 14 to 63) across all PFTs, with substantial implications for soil C:N cycling. This study identifies global root trait trends linking above- and belowground dynamics to predict trait values in poorly represented biomes.