2.2 Stream
sampling
To capture a gradient of disturbance and quantify the spatial
variability of post-fire stream Na+ and
NO3-, we sampled stream water roughly
every 800 meters along the mainstems of our study watersheds (Figure 1).
This distance was selected to ensure a consistent sampling interval that
maximized the number of samples collected per watershed but would allow
us to complete watershed sampling within one day. Low-flow conditions
were stable and there were no precipitation events during our sampling
period (6/1/2018-6/7/2018). Previous research at the Hayman Fire
demonstrated that patterns of elevated stream
NO3- in severely burned watersheds
persist across flow conditions (Rhoades et al., 2019) so our June
sampling date should be broadly representative. All stream samples from
a given watershed were collected within a single day in pre-washed 1 L
high-density polyethylene bottles moving in the upstream direction.
Samples were immediately filtered with 0.45 µm polyvinyl diethylene
filters (MilliporeSigma, Burlington, MA) and analyzed for concentrations
of stream Na+ and
NO3- using ion chromatography (Dionex
ICS-3000, Waltham, MA and Waters 580, Sunnyvale, CA). Detection limits
for both Na+ and
NO3- were 0.01 mg/L; any
concentrations below that were replaced with ½ the detection limit.
2.3 Geospatial
analysis
We conducted a terrain analysis to characterize the underlying watershed
structure. First, flow direction was derived from a 10-m digital
elevation model (DEM) (U.S. Geological Survey, 2018) using the multiple
triangular flow direction algorithm (Seibert & McGlynn, 2007).
Watershed contributing areas were delineated and upslope accumulated
area (UAA) was calculated for all sampling points (0.32 – 35.4
km2) using the openSTARS package (Peterson & Ver
Hoef, 2014) in R Studio. We summarized topographic, vegetation, and fire
variables as means and proportional extents within the contributing
areas for each sampling location (Table 3).
Table 3: Watershed predictor variables that were summarized for
the contributing area to each sampling point. Pearson correlation
coefficients were calculated between each predictor variable and
Na+ or NO3-.
Vegetation metrics represent current conditions (i.e., June 2018)
whereas fire metrics represent immediate post-fire condition (i.e.,
August 2003). Variables marked with a ˣ were removed prior to linear
mixed model selection due to strong correlation (>0.90)
with another predictor variable. Coefficients depicted in grey identify
variables removed during linear mixed model selection; those in black
were retained for subsequent modeling.