Improving the representation of roots in terrestrial models

Abstract

Root biomass, root production and lifespan, and root-mycorrhizal interactions govern soil carbon fluxes and resource uptake and are critical components of terrestrial models. However, limitations in data and confusions over terminology, together with a strong dependence on a small set of conceptual frameworks, have limited the exploration of root function in terrestrial models. We review the key root processes of interest to both field ecologists and modelers including root classification, production, turnover, biomass, resource uptake, and depth distribution to ask (1) what are contemporary approaches for modeling roots in terrestrial models? and (2) can these approaches be improved via recent advancements in field research methods? We isolate several emerging themes that are ready for collaboration among field scientists and modelers: (1) alternatives to size-class based root classifications based on function and the inclusion of fungal symbioses, (2) dynamic root allocation and phenology as a function of root environment, rather than leaf demand alone, (3) improved understanding of the treatment of root turnover in models, including the role of root tissue chemistry on root lifespan, (4) better estimates of root stocks across sites and species to parameterize or validate models, and (5) dynamic interplay among rooting depth, resource availability and resource uptake. Greater attention to model parameterization and structural representation of roots will lead to greater appreciation for belowground processes in terrestrial models and improve estimates of ecosystem resilience to global change drivers.

Sensitivity of four ecological models to adjustments in fine root turnover rate

“…Together, these models attempt to forecast the responses of terrestrial systems to changes in local environments and global climate and may inform better management and policy decisions (Stocker et al, 2013). However, due in large part to a lack of empirical data, many belowground processes are poorly represented in ecological models (Ostle et al, 2009;Iversen, 2010;Smithwick et al, 2014), with complex belowground behaviors (e.g., autotrophic and heterotrophic respiration, C allocation, decomposition) represented by a small set of critical parameters. Of these, fine root turnover is one of the most common parameters included in terrestrial process models and is often used to tune belowground C fluxes.…”

Earth System Model Needs for Including the Interactive Representation of Nitrogen Deposition and Drought Effects on Forested Ecosystems

“…Although models tend to have homogeneous soil horizontally (in a grid cell), the vertical structure of resources is dynamic. Allowing roots to proliferate in soil layers where resources are concentrated gives PFTs the chance to adapt to changes in environment and can further change the vertical distribution of C and N. Baker et al [63] improved the modeled Net Ecosystem Exchange cycle in the Simple Biosphere Model compared with observations in the Amazon by adding hydraulic redistribution and soil depth to 10 m. Other elements of root systems that should be included in models are root order and classification (which will differ in respiration, uptake, turnover, and storage capacity), root phenology and turnover, and resource uptake response to heterogeneity of resources [133]. Warren et al [121] provided additional suggestions for improving root representation in models, including scaling root function across temporal and spatial scales and including root traits that inform function and hydraulic redistribution.…”

Non-invasive Acoustic Sensing of Belowground Wooden Tissues: Possible Application to Spatial Mapping of Soil Usage by Tree Roots

“…However, limitations in data and some confusions over terminology, together with a strong dependence on a small set of conceptual frameworks, have limited the exploration of root function in terrestrial models (Smithwick et al, 2014). Relying on traditional sampling and observation techniques for such insights can be costly, time consuming, and infeasible, especially if the spatial scales involved are large (Jayawickreme et al, 2014).…”

“…In ecological point of view, root production and lifespan, root biomass, and rootmycorrhizal interactions govern the soil carbon fluxes and resource uptake, and are critical components of terrestrial models (Smithwick et al, 2014). However, increasing areas of impermeable surface in the urbanized environment can increase the stresses placed upon street trees and urban forests (Mullaney et al, 2015).…”