Abstract
Abstract. As a key component of the Earth system, roots play a key role in linking Earth's lithosphere, hydrosphere, biosphere and atmosphere. Here we combine 10 307 field measurements of forest root biomass worldwide with global observations of forest structure, climatic conditions, topography, land management and soil characteristics to derive a spatially explicit global high-resolution (∼ 1 km) root biomass dataset, including fine and coarse roots. In total, 142 ± 25 (95 % CI) Pg of live dry-matter biomass is stored belowground, representing a global average root : shoot biomass ratio of 0.25 ± 0.10. Earlier studies (Jackson et al., 1997; Robinson, 2007; Saugier et al., 2001) are 44 %–226 % larger than our estimations of the total root biomass in tropical, temperate and boreal forests. The total global forest root biomass from a recent estimate (Spawn et al., 2020) is 24 % larger than this study. The smaller estimation from this study is attributable to the updated forest area, spatially explicit aboveground biomass density used to predict the patterns of root biomass, new root measurements and the upscaling methodology. We show specifically that the root shoot allometry is one underlying driver that has led to methodological overestimation of root biomass in previous estimations. Raw datasets and global maps generated in this study are deposited at the open-access repository Figshare (https://doi.org/10.6084/m9.figshare.12199637.v1; Huang et al., 2020).
Highlights
Roots act as a hub that connects complex feedbacks among biomes, soil, water, air, rocks and nutrients
We investigated the performance of the allometric scaling and three non-parametric models: the random forest (RF), artificial neural networks (ANNs) and multiple adaptive regression splines (MARSs)
Our lower estimation of root biomass compared to earlier studies is attributable to differences in forest area (Supplement, Table S5), aboveground biomass density (Supplement, Table S5), root biomass measurement and upscaling methodology
Summary
Roots act as a hub that connects complex feedbacks among biomes, soil, water, air, rocks and nutrients. More than 20 years ago, Jackson et al (1996, 1997) provided estimates of the average biomass density (weight per unit area) and vertical distribution of roots for 10 terrestrial biomes. Robinson (2007) further suggested that R : S was underestimated by 60 %, which translated into an even higher global root biomass of 540–560 Pg. With an updated R : S and broader vegetation classes, Mokany et al (2006) gave a higher global root biomass of 482 Pg. Robinson (2007) further suggested that R : S was underestimated by 60 %, which translated into an even higher global root biomass of 540–560 Pg These studies provided a first-order estimation of the root biomass for different biomes but not of its spatial details. It is worth noting that estimations of the total global root biomass have increased with time, likely associated with improved methods in excavating roots that reduce under-sampling
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