Regional constraints to N2-fixation in post-fire forest succession

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.