Abstract
Forested watersheds provide many ecosystem services that have become
increasingly threatened by wildfire. Stream nitrate
(NO3-) concentrations often increase
following wildfire and can remain elevated for decades. We investigated
the drivers of persistent elevated stream
NO3- in nine watersheds that were
burned to varying degrees 16 years prior by the Hayman fire, Colorado,
USA. We evaluated the ability of multiple linear regression and spatial
stream network modeling approaches to predict observed concentrations of
the biologically active solute NO3-and the conservative solute sodium (Na+).
Specifically, we quantified the degree to which landscape and stream
network characteristics predict stream solute concentrations. No
landscape variables were strong predictors of stream
Na+. Rather, stream Na+ variability
was largely attributed to flow-connected spatial autocorrelation,
indicating that downstream hydrologic transport was the primary driver
of spatially distributed Na+ concentrations. In
contrast, vegetation cover, measured as mean normalized differenced
moisture index (NDMI), was the strongest predictor of spatially
distributed stream NO3-concentrations. Furthermore, stream
NO3- concentrations had weak
flow-connected spatial autocorrelation and high spatial variability.
This pattern is likely the result of spatially heterogeneous wildfire
behavior that leaves intact forest patches interspersed with high burn
severity patches that are dominated by shrubs and grasses. Post-fire
vegetation also interacts with watershed structure to influence stream
NO3- patterns. For example, severely
burned convergent hillslopes in headwaters positions were associated
with the highest stream NO3‑concentrations due to the high proportional influence of hillslope water
in these locations. Our findings suggest that reforestation is critical
for the recovery of stream NO3-concentrations to pre-fire levels and targeted planting in severely
burned convergent hillslopes in headwater positions will likely have a
large impact on stream NO3-concentrations.