δ¹⁵N Patterns of Douglas-Fir and Red Alder Riparian Forests in the Oregon Coast Range

Author： Scott Emily E. Perakis Steven S. Hibbs David E.

Publisher： Society of American Foresters

ISSN： 0015-749X

Source： Forest Science, Vol.54, Iss.2, 2008-04, pp. : 140-147

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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.