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Abstract Forest plantations are widespread globally. Young forest plantations (hereafter young forests) differ from natural old‐growth forests (hereafter old forests) in above‐ and below‐ground structures, shaping water and carbon cycling processes. While above‐ground differences are well studied, below‐ground hydrology and biogeochemical processes remain poorly understood. Here we asked:How do hydrological flow paths and dissolved carbon processes belowground differ between young and old forests?Using a process‐based hydro‐biogeochemical model (BioRT‐HBV) constrained by streamflow and dissolved organic and inorganic carbon (DOC and DIC) data, we analyzed three pairs of young‐old forests at the H.J. Andrews Experimental Forest, Oregon, USA. Detailed simulations for a 57‐year‐old plantation (WS01) and a naturally regenerated ∼500‐year‐old forest (WS02) showed that the young forest had lower streamflow and smaller deep groundwater contributions (20%) than the old forest (30%). DOC was mainly produced in shallow soil but diverged with depth: transformed into DIC in the young forest and further produced in the old forest, yielding contrasting export patterns of flushing (DOC increases with discharge) and dilution (DOC decreases with discharge). These differences likely stem from variations in subsurface structures, supported by deeper, denser roots in old forest. Extending the analysis to two additional pairs showed (a) higher DOC and DIC concentrations in all old forests; (b) consistent DIC dilution patterns but variable DOC patterns. Numerical experiments indicate that these diverse DOC behaviors result from interactions among forest age, geology, and hydrological connectivity, and other factors, highlighting the overlooked role of forest development in subsurface carbon cycling.
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Carbon‐Water Tradeoffs in Old‐Growth and Young Forests of the Pacific Northwest
Abstract Despite much interest in relationships among carbon and water in forests, few studies assess how carbon accumulation scales with water use in forested watersheds with varied histories. This study quantified tree growth, water use efficiency, and carbon‐water tradeoffs of young versus mature/old‐growth forest in three small (13–22 ha) watersheds in the H.J. Andrews Experimental Forest, Oregon, USA. To quantify and scale carbon‐water tradeoffs from trees to watersheds, tree‐ring records and greenness and wetness indices from remote sensing were combined with long‐term vegetation, climate, and streamflow data from young forest watersheds (trees ∼45 years of age) and from a mature/old‐growth forest watershed (trees 150–500 years of age). Biomass production was closely related to water use; water use efficiency (basal area increment per unit of evapotranspiration) was lower; and carbon‐water tradeoffs were steeper in young forest plantations compared with old‐growth forest for which the tree growth record begins in the 1850s. Greenness and wetness indices from Landsat imagery were not significant predictors of streamflow or tree growth over the period 1984 to 2017, and soil C and N did not differ significantly among watersheds. Multiple lines of evidence show that mature and old‐growth forest watersheds store and accumulate more carbon, are more drought resistant, and better sustain water availability compared to young forests. These results provide a basis for reconstructions and predictions that are potentially broadly applicable, because first‐order watersheds occupy 80%–90% of large river basins and study watersheds are representative of forest history in the Pacific Northwest region.
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- PAR ID:
- 10554081
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- AGU Advances
- Volume:
- 5
- Issue:
- 4
- ISSN:
- 2576-604X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2024AV001188
