Post-Fire Nitrogen Fixation by Shrubs
Collaborators:	Stephanie Yelenik, David Hibbs, Jeff Shatford (OSU)
Funding:	USGS, Oregon BLM, and Oregon Department of Forestry

Nitrogen (N) fixing shrubs are frequent components of early successional, post-fire forest communities. Functionally, these shrubs can contribute to ecosystem recovery after fire by replenishing soil nitrogen and carbon that is lost to combustion. High rates of N-fixation by shrubs can restore lost nitrogen in as little as 10 years, in comparison to roughly 1000 years from precipitation alone. The absence of N-fixing shrubs after fire (whether by management, or by chance) can therefore result in a loss of long-term soil fertility and forest productivity.

To date, no studies have evaluated shrub N-fixation in the Klamath-Siskiyou Douglas-fir forest zone of southwestern Oregon and northwestern California. Fire is frequent and sometimes intense in this region, and Douglas-fir forests growing here are strongly N-limited. Post-fire forests can also have significant cover of N-fixing shrubs, yet field rates of N-fixation by shrubs are unknown, which makes it difficult to evaluate their role in aiding forest recovery after fire.

We are characterizing the rates and significance of N-fixation by shrubs, primarily Ceanothus integerrimus (deerbrush), in forests burned by intense wildfire 10-20 years ago. N-fixation will be determined using stable isotope ¹⁵N tracers, and the method will be applied to evaluate the roles of site moisture and soil phosphorus in regulating N-fixation. Site treatments have been in place since 2007, with measurements beginning in 2009. Results of this work will enable improved decision-making of how to manage fire-prone forests to maintain long-term forest fertility and productivity.

photos courtesy of Jeff Shatford

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