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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. 

Abstract Wildfires have the potential to dramatically alter the carbon (C) storage potential, ecological function, and the fundamental mechanisms that control the C balance of Pacific Northwest (PNW) forested ecosystems. In this study, we explored how wildfire influences processes that control soil C stabilization and the consequent soil C persistence, and the role of previous fire history in determining soil C fire response dynamics. We collected mineral soils at four depth increments from burned (low, moderate, and high soil burn severity classes) and unburned areas and surveyed coarse woody debris (CWD) in sites within the footprint of the 2020 Holiday Farm Fire and in surrounding Willamette National Forest and the H.J. Andrews Experimental Forest. We found few changes in overall soil C pools as a function of fire severity; we instead found that unburned sites contained high levels of pyrogenic C (PyC) that were commensurate with PyC concentrations in the high severity burn sites—pointing to the high background rate of fire in these ecosystems. An analysis of historical fire events lends additional support, where increasing fire count is loosely correlated with increasing PyC concentration. An unexpected finding was that PyC concentration was lower in low soil burn severity sites than in control sites, which we attribute to fundamental ecological differences in regions that repeatedly burn at high severity compared with those that burn at low severity. Our CWD analysis showed that high mean fire return interval (decades between fire events) was strongly correlated with low annual CWD accumulation rate; whereas areas that burn frequently had a high annual CWD accumulation rate. Within the first year postfire, trends in soil density fractions demonstrated no significant response to fire for the mineral-associated organic matter pool but slight increases in the particulate pool with increasing soil burn severity—likely a function of increased charcoal additions. Overall, our results suggest that these PNW forest soils display complex responses to wildfire with feedbacks between CWD pools that provide varying fuel loads and a mosaic fire regime across the landscape. Microclimate and historic fire events are likely important determinants of soil C persistence in these systems.