# Satellite data show increased biomass carbon stocks in northern young forests

Hui Yang, Philippe Ciais, Wigneron J.-P.

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## To cite this version:

Hui Yang, Philippe Ciais, Wigneron J.-P.. Satellite data show increased biomass carbon stocks in northern young forests. Nature Geoscience, 2023, 16 (10), pp.843-844. 10.1038/s41561-023-01276-2. hal-04235252

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# Satellite data show increased biomass carbon stocks in northern young forests

Accurate estimates of the land carbon sink are vital for informing climate projections and net-zero policies. Application of a strict filtering method to microwave satellite data enabled the evaluation of global vegetation biomass carbon dynamics for 2010–2019. The results highlight the role of demography in driving forest carbon gains and losses.

**This is a summary of:**

Yang, H. et al. Global increase in biomass carbon stock dominated by growth of northern young forests over past decade. *Nat. Geosci.* https://doi.org/10.1038/s41561-023-01274-4 (2023).

**Publisher's note**

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Published online: 2 October 2023

## The problem

Understanding changes in terrestrial carbon storage owing to environmental and land use changes is vital for improving estimates of regional and global carbon budgets. Existing approaches and data sources for estimating biomass carbon budgets have led to varying predictions and, thus, limited consensus<fnref n="1" />. Low-frequency L-band vegetation optical depth (L-VOD) from daily Soil Moisture and Ocean Salinity (SMOS) satellite passive microwave observations, which probe live woody biomass, can be used to monitor vegetation carbon changes at regional scales at a spatial resolution of 25 × 25 km (ref. 2). However, the application of L-VOD data globally has been hindered by signal disruptions caused by radio frequency interference (RFI) from human activities, particularly in the densely populated countries within the Northern Hemisphere, and sensitivity to vegetative water content³.

## The solution

To overcome the limitations that restrict L-VOD usage, we developed a strict filtering method to remove the effects of RFI and vegetative water content dynamics from the satellite observations. We subsequently used our L-VOD data to derive the annual total live biomass carbon (above- and below-ground carbon) and to assess its spatial distributions and temporal dynamics for 2010–2019.

We found that global biomass carbon stocks have been increasing at a rate of 0.51 ± 0.17 PgC yr⁻<fnref n="1" /> from 2010 to 2019 (Fig. 1a). Notably, boreal and temperate forests emerged as the primary contributors to the global carbon sink, with contributions of +0.37 ± 0.02 PgC yr⁻<fnref n="1" /> and +0.13 ± 0.05 PgC yr⁻<fnref n="1" />, respectively, whereas tropical forests were a net carbon source. Moreover, young (<50-years old) and middle-aged (50–140-years old) temperate and boreal forests are the largest biomass carbon sinks (Fig. 1b), owing to their higher carbon uptake rates compared with old-growth forests. Conversely, old-growth forests (aged >140 years) showed a net biomass loss over the past decade. Old-growth forests that are losing biomass are mainly located in the tropics (Fig. 1b), where anthropogenic disturbances (such as deforestation and agriculture) and degradation are causing the losses.

Our results, derived from L-VOD data, contrast with estimates from state-of-the-art dynamic global vegetation models (DGVMs) from the TRENDY project (Fig. 1b). These DGVMs used in the Global Carbon Budget

and Intergovernmental Panel on Climate Change assessments find that the wet tropics are a carbon sink and predict that the largest carbon sinks are old-growth forests, whereas young forests are small carbon sources. The discrepancy between the DGVM and L-VOD data might result from the fact that the models do not typically account for variations in carbon sequestration rates between different forest-age classes. Moreover, the models do not fully account for light limitation in dense forest stands; drought-induced mortality, which increases the turnover of biomass; limitations of emerging nutrients (that is, phosphorus) on growth; tropical forest degradation; and small fires⁴. Simplified assumptions used in DGVMs lead them to mainly locate carbon sinks in old-growth forests in response to rising CO₂ levels and climate effects⁵.

## The implications

This work provides an independent and spatio-temporally explicit estimate of the global carbon balance, based on satellite data. Such estimates are essential to establishing climate mitigation goals for the implementation of the pledges of the Paris Agreement.

Furthermore, this work underscores the significance of forest demography in influencing carbon gains and losses. The incomplete representation of forest management and demography in DGVMs could hinder accurate predictions of future carbon sink dynamics under changing climatic conditions.

However, our strict filtering process to eliminate RFI-affected data results in a reduction of available data in regions including southern China and the Middle East. This reduction is particularly noticeable during the early 2010s, potentially leading to uncertainty in the estimation of decadal changes in biomass. Furthermore, when the observation footprint covers peatland areas, the L-VOD measurements might have a high uncertainty, leading to potential overestimation of the carbon sink.

In future work, our continuous data on carbon dynamics in vegetation derived from L-VOD observations could be used to understand the underlying processes behind short- and long-term variations in the global and regional carbon budget.

Hui Yang<fnref n="1" />, Philippe Ciais<fnref n="1" /> & Jean-Pierre Wigneron<fnref n="2" />

<footnote n="1">¹Laboratoire des Sciences du Climat et de l'Environnement, Gif-sur-Yvette, France.</footnote>

<footnote n="2">²INRA Nouvelle-Aquitaine, Villenave d'Ornon, France.</footnote>
## EXPERT OPINION

"The focus of the work is the attribution of carbon stock changes to forest ages, providing scientific evidence of the influence of forest demography (in terms of age) on the global carbon sinks and

sources. This information is a notable step forward in better understanding biomass responses to climate change. **David Chaparro, German Aerospace Center, Wessling, Germany.**

## FIGURE

<figure><img src="image_4.png" /><figcaption>**Fig. 1** | Total live biomass changes during 2010–2019 and the relationship with forest age. a, Global spatial pattern of mean annual changes in total live biomass (ΔTB) carbon density derived from L-VOD data. b, ΔTB derived from L-VOD and TRENDY DGVMs partitioned according to forest-age classes for tropical and arid, as well as temperate and boreal, biomes. The ΔTB of intact forests are shown as white bars within L-VOD bars. The error bars for TRENDY data denote the s.d. across 15 TRENDY DGVMs of total biomass changes. The L-VOD data show that tropical old-growth forests (>140 years) are carbon sources whereas temperate and boreal young (<50 years) and middle-aged forests (50–140 years) are the largest sinks. Oppositely, the TRENDY DGVMs indicate that tropical old-growth forests are large sinks. © 2023, Yang, H. et al.</figcaption></figure>

## BEHIND THE PAPER

The magnitude and distribution of the terrestrial carbon sink are subjects of ongoing debate, with changes in vegetation carbon storage owing to environmental impacts and human disturbances posing a considerable challenge to understanding the terrestrial carbon cycle. A satellite dataset, L-VOD, holds promise for monitoring terrestrial carbon dynamics. However, its reliability is susceptible to variations in vegetation water content and human activities.

To address this issue, we developed a stringent filtering methodology. Furthermore, we isolated the carbon change signal by deducting seasonal fluctuations and used L-VOD data from the wettest periods. These periods exhibit vegetation water content saturation with less year-to-year variability. It is very exciting that we are finally able to provide an autonomous, geographically explicit and global coverage map of carbon dynamics over the past decade. **H.Y.**

## REFERENCES

1. Bastos, A. et al. Sources of uncertainty in regional and global terrestrial CO₂ exchange estimates. *Global Biogeochem. Cycles* **34**, e2019GB006393 (2020).

**This paper presents the uncertainties in global and regional carbon budgets using 'bottom-up' and 'top-down' approaches.**

2. Fan, L. et al. Satellite-observed pantropical carbon dynamics. *Nat. Plants* **5**, 944–951 (2019).

**This paper reports the tropical carbon dynamics derived from microwave L-band vegetation optical depth observations.**

3. Oliva, R. et al. Status of radio frequency interference (RFI) in the 1400–1427 MHz passive band based on six years of SMOS mission. *Remote Sens. Environ.* **180**, 64–75 (2016).

**This paper reports the issue of RFI in microwave observations from the SMOS satellite.**

4. Yang, H. et al. Comparison of forest above-ground biomass from dynamic global vegetation models with spatially explicit remotely sensed observation-based estimates. *Glob. Change Biol.* **26**, 3997–4012 (2020).

**This paper reports the origin of the biomass discrepancy between DGVM and satellite observations.**

5. Pugh, T. A., Arneth, A., Kautz, M., Poulter, B. & Smith, B. Important role of forest disturbances in the global biomass turnover and carbon sinks. *Nat. Geosci.* **12**, 730–735 (2019).

**This paper reports the higher carbon sink capacity of regrowth forests compared with that of old-growth forests.**

## FROM THE EDITOR

"The authors' estimates of yearly changes in the amount of carbon stored in living terrestrial vegetation over the past decade represent much needed information for forming policies focused on achieving carbon neutrality. The results also greatly advance our understanding of carbon dynamics in growing forests." **Xujia Jiang, Senior Editor, Nature Geoscience.**