Linking fine root diversity with ecosystem processes in models and the real world: Allocation of NPP belowground and fine root phenology
Fine roots are an important link between plants and soils and account for a significant fraction of net primary production (NPP). At the same time, temporal patterns of root production and activity mediate the capacity of a plant to obtain soil resources across the growing season. However, efforts to accurately describe belowground productivity have been hindered by simplistic definitions of fine roots and limited observation. Here, we explore the topics of fine root NPP and root phenology to better understand their links and contributions to ecosystem processes. First, fine roots have traditionally been defined as all roots below 2 mm in diameter yet recent observations highlight clear functional division between lower order and higher order fine roots. Based on this discrepancy, we determine how estimates of fine root NPP are altered by separating fine root biomass into functional classes of absorptive and structural fine roots using an established global dataset of root biomass as well as new data collected from a mixed temperate forest in Changbaishan, Jilin Province, China. Second, we foster an open discussion of fine root phenology in relation to ecosystem processes and debate whether root phenology should be included in future ecosystem and global models.
By separating fine root biomass into distinct pools of absorptive and structural fine roots estimates of global NPP allocated to fine root biomass were reduced from the original estimate of 33% to roughly 23% (likely range between 14% and 28%). The reduction was more striking in woody biomes than in biomes dominated by herbaceous species due to the higher proportion of long-lived, structural fine roots in woody root systems. Here, NPP allocation would be reduced to roughly15% (range of 8% to 26%) while in herbaceous systems the estimated NPP allocated to fine root biomass averaged 28%. Results from Changbaishan largely mirror those reported in the global analysis. Together, these results support a growing consensus that fine roots should not be treated as a homogeneous biomass pool but rather as a mixture of absorptive and structural fine roots, each marked by its own demographic patterns and turnover rates. On the other hand, the role of root phenology in models remains largely unresolved. Its importance likely depends on 1) how pools of carbohydrates are allocated among different plant components and 2) whether soil resources are variable through time and if soil resource acquisition is tightly coupled to fine root biomass.