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δ15N patterns of Douglas-fir and red alder riparian forests in the Oregon Coast Range

Forest Science
By: , , and 

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Abstract

We used naturally occurring stable isotopes of N to compare N dynamics in near-stream and upslope environments along riparian catenas in N-fixing red alder (Alnus rubra) and Douglas-fir (Pseudotsuga menziesii) forests in the Coast Range of western Oregon. Based on the existing literature, we expected soil δ15N to be enriched closer to streams owing to inputs of isotopically heavy, marine-derived N by spawning salmon, higher rates of denitrification near the stream, or both. However, it has been unclear what effect red alder might have on soil δ15N patterns near streams. We found a consistent −1‰ δ15N signature in red alder foliage, and δ15N of total N in soils under red alder averaged 2.2‰ along sampling transects extending 20 m upslope from the stream. Surprisingly, δ15N of total N in soil under Douglas-fir was progressively depleted nearer to streams, opposite from the pattern expected from N losses by denitrification or N inputs from anadromous salmon. Instead, δ15N of total N in soil under Douglas-fir converged toward soil δ15N values typical of red alder sites. We consider that the historic presence of red alder may have contributed a legacy of lower soil δ15N nearer to streams on sites that are currently dominated by young Douglas-fir forest.

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Publication type Article
Publication Subtype Journal Article
Title δ15N patterns of Douglas-fir and red alder riparian forests in the Oregon Coast Range
Series title Forest Science
Volume 54
Issue 2
Year Published 2008
Language English
Publisher Society of American Foresters
Contributing office(s) Forest and Rangeland Ecosystem Science Center
Description 8 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Forest Science
First page 140
Last page 147
Country United States
State Oregon
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