MODELING POST-FIRE NITRATE EXPORT IN A WET FORESTED WATERSHED: INFLUENCE OF FIRE SEVERITY AND CLIMATE VARIABILITY DURING VEGETATION REGROWTH

ankit rawal

Wildfires are intensifying in the Pacific Northwest, affecting both forest ecosystems and downstream water quality. In wet temperate forests, fire-driven combustion of vegetation can increase soil nitrate accumulation, thereby increasing its export to streams. Research indicates that the timing and rate of nitrate exports can vary with fire severity and post-fire climate, rendering predictions challenging. This study examined how fire severity and interannual climate variability affect nitrate export in a wet watershed using the Regional Hydro-Ecological Simulation System (RHESSys). Simulations included evenly distributed low, moderate, high-severity fire scenarios, as well as the historical fire (i.e., Holiday Farm Fire occurred in 2020). In addition, dry and wet climate scenarios were defined based on precipitation and evapotranspiration (ET) averaged over a moving five-year water period from historical climate data. These simulations were used to examine the temporal dynamics of nitrate export and vegetation regrowth following fire. Simulations of the historical fire-historical climate, during which time the Holiday Farm Fire occurred with mixed severity (7th September 2020, 42.8% at high, 27.1% at moderate, 22.4% at low severity burn, and 7.7% remained unburned) reproduced both early and delayed nitrate peaks observed in heterogeneous burn landscapes compared to baseline (no fire). This variation in nitrate response reflects differences in fire severity. Due to partial biomass loss, increased mineralization, and decreased plant nitrogen uptake, low and moderate-severity fires caused immediate and noticeable nitrate pulses during the first post fire year. High-severity fires, on the other hand, caused delayed nitrate responses where exports were comparatively lower than low and moderate-severity fires during the first year but significantly increased in the third and fourth years as mineralization remained high, with more soil nitrate accumulation than plant nitrogen uptake. Vegetation recovery (Leaf Area Index (LAI) and plant biomass) and soil and litter organic matter are strongly controlled by fire severity. Low- and moderate-severity fires show relatively rapid recovery, with LAI returning within one to three years after fire. In contrast, high-severity areas recover more slowly and exhibit incomplete LAI recovery even five years after fire. Similarly, dry conditions (during low precipitation and low ET years) increased soil nitrate accumulation and led to greater annual nitrate exports, whereas wetter conditions (during high precipitation and high ET) reduced exports through increased plant uptake. However, this difference was not significant because the historical climate data for this relatively wet watershed exhibit limited interannual variability. Recovery of vegetation was also slightly faster under high precipitation and high ET conditions. These findings show that post fire nitrate exports are driven by fire severity and climate variability. High-severity fires pose prolonged threats to water quality, highlighting the necessity of long-term monitoring following a fire. As wildfire frequency and intensity increase under future climate change, early severity assessment, prescribed fires to reduce fuel load, targeted reforestation, and fuel-reduction treatment will be essential for reducing nitrate export and protecting drinking water resources in forested watersheds.

MODELING POST-FIRE NITRATE EXPORT IN A WET FORESTED WATERSHED: INFLUENCE OF FIRE SEVERITY AND CLIMATE VARIABILITY DURING VEGETATION REGROWTH

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