COS 73-2 - Regional constraints to N2-fixation in post-fire forest succession

Wednesday, August 10, 2011: 1:50 PM
6B, Austin Convention Center
Stephanie G. Yelenik, Pacific Island Ecosystems Research Center, U.S. Geological Survey, Hawaii National Park, HI, Steven S. Perakis, Forest and Rangeland Ecosystem Science Center, US Geological Survey, Corvallis, OR and David E. Hibbs, Forest Ecosystems and Society, Oregon State University, Corvallis, OR
Background/Question/Methods

Temperate forests often include a nitrogen (N)-fixing component in early post-fire succession, which can serve to replenish soil N, the principle nutrient lost in fire events.  Therefore, N-fixing plants stand to play an important role in forest productivity.  Temperate forests remain largely N-limited, however, making understanding the constraints on N-fixation an important facet of forest ecology.  While ecologists have started to understand the various abiotic and biotic factors that regulate N-fixation, understanding the relative importance of these factors in spatially heterogeneous landscapes has been difficult to pin down.  We explored the regional climatic, nutrient, and competitive constraints on N-fixation by an early successional shrub (Ceanothus integerrimus) in the mixed conifer/hardwood forests of the Klamath National Forest, Northern California.  We used 15N isotope dilution techniques, coupled with detailed surveys of plant biomass, to quantify Ceanothus N-fixation rates at 6 sites across the region.  We also used a P addition experiment to specifically ask if N-fixation in the region is P-limited.  We then asked whether climate, biomass of overtopping trees, or P-limitation drove regional variation in N-fixation rates.

Results/Conclusions

Variation in N-fixation rates can be driven by (1) the degree to which plants are fixing atmospheric N2 versus using soil N sources (“%Ndiff”) as well as (2) the biomass of fixers on the landscape.  Our preliminary results show that Ceanothus %Ndiff was loosely driven by climate and soil P.  In these forests, however, the main driver of N-fixation rates (on a per m2 basis) was shrub biomass.  Although the Klamath region experiences summer drought conditions, sites with lower rainfall surprisingly supported higher shrub biomass and thus, overall N-fixation rates.  Structural equation models show a large, negative effect of tree biomass on shrub biomass, and thus, regional variation in N-inputs.  Preliminary results thus suggest that resprouting hardwood species quickly (10-20 years) overtop Ceanothus in higher rainfall/productivity sites, effectively lowering N-fixation rates in the zones of highest tree growth.  This decoupling of N-inputs and forest N-demand suggests that regional variation in climate, and how it affects plant community processes, plays a large role in influencing N-fixation rates and maintaining N-limitation in temperate forests.

Copyright © . All rights reserved.
Banner photo by Flickr user greg westfall.