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Restoration of riparian forest cover increases carbon stocks in the Pacific Northwest

Published Article

Oregon

Publication date: July 1, 2025

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Reforesting degraded riparian zones offers meaningful natural climate‑solution benefits, yet carbon gains vary across ecosystems. Using a chronosequence of riparian plantings across a 430‑km gradient in western Oregon, this study quantified carbon storage in trees, shrubs and soils. Woody and soil carbon increased with planting age—averaging 24% and 1% per year, respectively—with tree carbon strongly influenced by stem density. Understory carbon rose mainly with age, while soil carbon gains were tied to precipitation. Results show that riparian reforestation substantially boosts carbon storage, especially when both trees and shrubs are included. Because carbon accumulation is gradual but compounding, projects initiated today may take a decade or more to deliver high climate‑mitigation benefits, underscoring the urgency of implementation.

Subject Tags

  • Forest
  • Carbon storage
  • Natural climate solutions

Abstract

Reforestation of degraded riparian areas provides climate mitigation benefits through increased carbon (C) storage. However, the magnitude of this potential natural climate solution (NCS) remains uncertain across ecoregions. Few studies have evaluated riparian planting C sequestration and storage, particularly in highly productive wet riparian ecosystems. In recent decades, riparian reforestation has accelerated in the Pacific Northwest (PNW) of the United States, primarily aiming to restore ecosystem functions and associated benefits. Using these plantings as a ‘natural experiment’, we assessed C storage in woody vegetation (trees and shrubs) and soils across a chronosequence of PNW riparian reforestation sites. Our study evaluated changes in C storage with planting age and identified key covariates affecting C storage in plants and soils and their relationship with planting age across a ∼430 km latitudinal gradient in western Oregon, USA. We found that woody and soil C stocks increase with planting age, averaging 24% and 1% per year, respectively. Increases in tree C were strongly driven by increasing planting age and tree stem density. Understory C was weakly related to stand characteristics and geomorphic properties, and strongly related to planting age. Soil C gains were positively driven by precipitation. We find that riparian reforestation can result in increased C storage, with woody vegetation comprising most of the increase. Our results highlight the importance of including both trees and shrubs in plantings to realize C accumulation gains in the earlier years. Because C accumulation is gradual, yet compounding (i.e. 10+ and 15+ years for total C stocks to increase by 1.95, and 19.2 Mg C ha−1, respectively), riparian reforestation projects implemented today could take over a decade to deliver high NCS benefits, emphasizing the urgency to implement these projects to limit the worst of climate change impacts.

Citation

O’Kelley, R., Graves, R. A., Amer, H., & Silva, L. C. (2025). Restoration of riparian forest cover increases carbon stocks in the Pacific Northwest. Environmental Research Letters, 20(8), 084003. https://doi.org/10.1088/1748-9326/ade5bb

TNC Authors

  • Rose A. Graves
    Conservation and Climate Scientist, Oregon
    The Nature Conservancy
    Email: rose.graves@tnc.org