Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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 (<span class="inline-formula">∼</span> 1 km) root biomass dataset, including fine and coarse roots. In total, 142 <span class="inline-formula">±</span> 25 (95 % CI) Pg of live dry-matter biomass is stored belowground, representing a global average root : shoot biomass ratio of 0.25 <span class="inline-formula">±</span> 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 (<a href="https://doi.org/10.6084/m9.figshare.12199637.v1">https://doi.org/10.6084/m9.figshare.12199637.v1</a>; Huang et al., 2020).

Highlights

  • Roots act as a hub that connects complex feedbacks among biomes, soil, water, air, rocks and nutrients

  • 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

  • Our lower values of root biomass compared to Saugier et al (2001), Mokany et al (2006) and Robinson (2007) are caused mainly by our lower aboveground biomass density and R : S (Supplement, Table S5)

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Summary

Introduction

Roots act as a hub that connects complex feedbacks among biomes, soil, water, air, rocks and nutrients. While Earth observation data combined with field data enable the derivation of spatially explicit estimates of aboveground biomass with a spatial resolution of up to 30 m over the whole globe (Santoro, 2018b), the global carbon stock and spatial details of the distribution of belowground root biomass (fine + coarse) have so far relied on sparse measurements and coarse extrapolation, remaining highly uncertain. 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. Multiplying their average root biomass density with the area of each biome results in a global root biomass pool of 292 Pg, with forests accounting for ∼ 68 %. 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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