4.3 Nitrate and DOC responses relate more to local watershed
characteristics than climate guild or burn extent
Previous literature has observed high levels of variability of post-fire
streamflow response across North America, due to differences in climate
and burn extent
(Hallema et al.,
2017). Water supplies in regions with warm summer continental climates
are susceptible to wildfire effects due to the high severity of
infrequent fires
(Hallema et al.,
2017). Runoff on wildfire-affected soils are generally controlled by
infiltration capacity, impacting solute chemistry post-fire
(Rhoades et al.,
2019). The variation and magnitude in wildfire impacts on DOC may be
regionally dependent, linked to post-fire hydrological cycles that vary
across climate regimes
(Bartels et al., 2016;
Hallema et al., 2017; Richardson et al., 2024). As previous studies
have suggested that DOC dynamics are driven by local hydrologic
processes and precipitation regimes
(Wampler et al.,
2023) and given that there are large differences in precipitation
regimes, terrestrial species composition, and soil properties across
climates, we anticipated that there would be differences in nitrate and
DOC across climates. Somewhat surprisingly, we found no significant
differences for the means of effect size between climates over the first
five years post-fire for both nitrate and DOC (Fig. 3). However, we did
observe significant differences in the variability within climate
classifications for both nitrate and DOC, suggesting that while we
observe no consistent difference in the average response of water
quality of fire climate, more localized catchment characteristics may
still act as primary controls on their responses (Fig. S11, S12). For
instance, for nitrate we observed a significant difference between two
climate guilds with no difference in mean annual precipitation across
(Subarctic, subtropical highland) (Fig. 3). Conversely, other climate
classifications with no significant effect have variable precipitation
throughout the year (Mediterranean, Warm-humid, Warm-Mediterranean,
Hot-Mediterranean, Humid subtropical, cold semi-arid)
(Zepner et al.,
2021). This finding builds and strengthens the emerging view that
regional heterogeneity in antecedent conditions, local catchment
characteristics, and post wildfire climate contribute to substantial
variability in water-quality response to a wildfire, thus muting
significant effects when observing trends across climates
(Guo et al., 2023;
Richardson et al., 2024; Santos et al., 2019a; Wampler et al., 2023;
Wine et al., 2018).
DOC dynamics are better explained by key watershed characteristics
compared to nitrate, indicating that such characteristics strongly
control DOC responses post-wildfire (Fig. 5). Elevation was consistently
the most significant predictor for both nitrate and DOC, potentially
related to the role that elevation plays in residence time of solutes
and vegetation composition
(Chiriboga & Borges,
2023). Catchment elevation and slope are inter-related physical
controls on biogeochemical responses across watersheds because higher
elevation catchments generally have steeper slopes which result in lower
residence time, thus altering hyporheic exchange fluxes, and associated
biotic processes
(Lintern et al., 2018;
Son et al., 2015). Likewise, catchment area can be associated with
residence time because larger catchments are typically associated with
higher order channels that are relatively flatter than smaller headwater
catchments (Connolly
et al., 2018). Vegetation composition also influences vegetation
recovery rates, thereby influencing the sources and the mobilization of
solutes throughout watersheds
(Gustine et al.,
2022). Accordingly, we found that nitrate dynamics are more influenced
by the specific climate classifications and burn characteristics,
suggesting that nitrate responses post-wildfire are driven primarily by
biotic factors. Cumulatively, our results highlight the importance of
considering catchment context when interpreting biogeochemical responses
to disturbance (Smith
et al., 2011).
We did not identify a relationship between burn extent and solute
response to wildfire (Fig. 4), although burn severity was a relatively
important predictor of nitrate effect sizes (Fig. 5). Previous studies
have reported that burn extent determines the magnitude of wildfire
effects on watershed processes, however, the interaction between climate
and wildfire is complex. For example, Rhoades et al (2019) found that
nitrate was an order of magnitude higher in streams draining catchments
that burned > 60% of their areas, compared to unburned
catchments. Furthermore, multiple studies have observed annual river
flow typically changes when greater than 19% of the watershed burns
(Guo et al., 2023;
Hallema et al., 2018). Our results do not indicate a clear threshold in
nitrate or DOC based on burn extent. Therefore, we postulate that
varying catchment characteristics and post-burn climate conditions lead
to hydrologic conditions that control the observed variability of solute
responses to wildfire. However, these relationships are difficult to
disentangle, due to the multitude of confounding variables
(Hampton et al.,
2022). Alternatively, this could indicate that other burn attributes,
such as burn severity play a more significant role compared to burn
extent (Crandall et
al., 2021; Santos et al., 2019; Tshering et al., 2023). Further work is
needed to better understand the relationships between nitrate, DOC, and
burn characteristics such as area burned and burn severity.