Young Versus Old: Does Forest Age Regulate Water and Dissolved Carbon Processes Belowground?

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Water Resources Research

Key Points:

· Higher vertical connectivity in an old forest promoted deep flow contributions and sustained more baseflow compared to a young forest

Supporting Information:

Young Versus Old: Does Forest Age Regulate Water and Dissolved Carbon Processes Belowground?

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 constrained by streamflow and dissolved organic and inorganic carbon data, we analyzed three pairs of young-old forests at the H.J. Andrews Experimental Forest, Oregon, USA. Detailed simulations for a fifty-seven-year-old plantation and a naturally regenerated approximately five hundred-year-old forest showed that the young forest had lower streamflow and smaller deep groundwater contributions (twenty percent) than the old forest (thirty percent). Dissolved organic carbon was mainly produced in shallow soil but diverged with depth: transformed into dissolved inorganic carbon in the young forest and further produced in the old forest, yielding contrasting export patterns of flushing (dissolved organic carbon increases with discharge) and dilution (dissolved organic carbon 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 dissolved organic carbon and dissolved inorganic carbon concentrations in all old forests; (b) consistent dissolved inorganic carbon dilution patterns but variable dissolved organic carbon patterns. Numerical experiments indicate that these diverse dissolved organic carbon 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.

Plain Language Summary Clear-cutting followed by replanting has created many forest plantations around the world. These forests differ from older, naturally regenerated forests above and below the surface, where water and carbon move through soil and rock. However, we still know relatively little about how these belowground processes differ between younger and older forests and how they influence stream water chemistry. In this study, we used a computer model with streamflow and water chemistry data from the H.J. Andrews Experimental Forest in Oregon, USA, focusing on two important water quality components, dissolved organic carbon and dissolved inorganic carbon, which play key roles in carbon cycling and ecosystem health. We compared a younger, fifty-seven-year-old plantation with an older, approximately five hundred-year-old forest to examine how water movement and dissolved carbon processes vary with forest age. We found that the older forest had stronger vertical connections, allowing more water and carbon to move through deep soil layers and sustaining streamflow from groundwater. In contrast, the younger forest had shallower flow paths and less deep carbon production. Streams draining older forests contained higher dissolved carbon overall, showing how differences in subsurface structure shape water quality and carbon cycling in changing forests.

One. Introduction

Two. Methods

Two point two. Data

Two point three. BioRT-HBV Overview

Two point three point two. BioRT Model

Two point four. Reactions

Two point four point two. Rate Laws

Two point five. Model Calibrations

Two point five point two. BioRT

Two point six. Model Uncertainty Analysis

Two point seven. Catchment Scale Production and Export Rates

Two point eight. Root Abundance Data

Two point nine. Numerical Thought Experiment for Other Young and Old Catchments

Three. Results

Three point three. Subsurface Depth Profiles: Modeled Dissolved Carbon Consistent With Root Data

Three point four. Production and Export Rates

Three point five. Drivers of Dissolved Carbon Production and Export

Three point six. Numerical Experiments and Diverse Carbon Dynamics Across Young and Old Forests

Four. Discussion

Four point two. Higher Contribution of Deep Groundwater Flow Indicates Higher Vertical Connectivity in Old Forests

Four point three. Higher Dissolved Carbon Indicates Higher Carbon Production Versus Export in Old Forests

Four point four. Consistent Dilution Patterns Indicate Deep Respiration at Depth

Four point five. Model Limitations

Five. Conclusion

Conflict of Interest

Data Availability Statement

Overview

The study explores hydrological and biogeochemical differences between young and old forests, highlighting that older forests have stronger vertical connectivity which supports deeper flow and higher dissolved carbon levels. The findings suggest significant implications for water resource management and carbon cycling.

Key Points

1Older forests exhibit higher vertical connectivity, leading to more substantial contributions to baseflow
2Dissolved carbon concentrations are greater in old forests primarily produced in shallow subsurface layers
3Differences in forest age significantly influence dissolved organic carbon (DOC) export patterns
4The study used a hydro-biogeochemical model to analyze forest age effects on water quality and carbon processes
5Forest management practices should consider age-related differences in hydrological and biogeochemical functions.

Details

Authors: Fiona S. Liu, Devon Kerins, Shreya Ramesh, Pamela L. Sullivan, Sharon A. Billings, Daniel R. Hirmas, Hoori Ajami, Alejandro Flores, Catalina Segura, Li Li
Category: Environmental Studies

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