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.