The Lasting Impacts of Wildfire

Re-posted from Water Current News, WSU Extension

Hillsolope with trees in the foreground, and rockier slopes with a forested patch in the background
The Entiat Experimental Forest 40 years after a wildfire. Photo Marketa McGuire.

In 1970, when a large lightening caused wildfire started in the Entiat Experimental Forest in north-central Washington, researchers had already collected 12 years of baseline data on three watersheds. Weather and streamflow (including quantity, quality, and timing of water discharge) had all been recorded. This provided a unique opportunity to study the long-term effects of post-fire recovery efforts on hydrology.

Decades after the fire occurred, Ryan Niemeyer, an adjunct professor in Washington State University’s Center for Sustaining Agriculture and Natural Resources, along with his colleagues Kevin Bladon and Rick Woodsmith, began examining the long-term impacts the fire had on the hydrologic system. His findings, recently published in the journal Hydrological Processes, reveal that fire can affect annual stream discharge, peak flows, low flows, and evapotranspiration even 40 years after the burn.

Charred tree trunks standing on bare soil showing signs of erosion
Example of post-fire landscape in the region. This photo was taken after the Carlton Complex in North Central Washington in 2014. Photo: Ryan Niemeyer.

Researchers sampled the three catchment areas over the years. Salvage logging, aerially seeding, and fertilization occurred in two of the study areas after the fire, while the third area had no treatment. In the first seven years after the blaze, all three areas had much higher peak water stream flows, driven by snowmelt, and much greater low flows in the summer months when compared to the baseline data. There were increases in annual stream discharge (150–202% of baseline) and peak flows (234–283%), along with increases in low flows (42–81%) and evapotranspiration (34–45%).  Niemeyer describes the increase in evapotranspiration this way, “Trees are like straws in the soil pulling water from it (like you suck the juices out of a shaved ice). If trees are removed from the landscape by fire, less water will be pulled out of the soil and more flow is returned to the stream.”

Fast forward 35 to 41 years after the fire, Niemeyer and his colleagues found that the hydrologic cycle had returned to pre‐fire levels in the two salvaged logged and seeded areas (Figure 1). However, in the untreated area, the annual discharge and runoff ratios remained elevated, with evapotranspiration rates still at lower levels (Figure 1).

Bar graph comparing discharge for three different catchments. Each one has a bar (with range) for post fire, the first few years after fire, and after 40 years.
Figure 1. Discharge in the three catchments, and how it changed from pre-fire values in the immediate post-fire (Post-Fire, Short), and long-term post-fire (Post-Fire, Long) periods. Values presented are median discharge values in mm/year, and the error bars represent the interquartile range during the hydrologic year. Data source: Niemeyer et al. 2020.

Niemeyer’s team theorizes that the differences in long‐term hydrologic recovery across these catchment areas were caused by delayed vegetation recovery in the unmanaged catchment.  The trees in the re-seeded sections are much bigger than those in the untreated area.

This research demonstrates that how land managers treat landscapes after a fire can have significant impacts on the recovery of natural processes and streamflow. Few studies have investigated long‐term effects of post‐fire land management on catchment hydrology.  The salvage logging, seeding and fertilizing regime resulted in a quicker return to the baseline hydrological condition. Depending on the land management objectives this may be desirable. Post-fire recovery will likely vary based on the type of tree, soil, or slope aspect.

As we anticipate a future with an increased number of larger and hotter fires due to climate change, post-incident management of landscapes grows more important. And as drought becomes more prevalent, understanding the long-term impacts of fire on hydrology is essential.

References:

Niemeyer, Ryan J., Kevin D. Bladon, Richard D. Woodsmith. Long‐term hydrologic recovery after wildfire and post‐fire forest management in the interior Pacific Northwest. Hydrological Processes. January 2020. Online Access.

Science Daily. Response to fire impacts water levels 40 years into future. January 10, 2020. Online Access.

Washington Department of Ecology. Drought preparedness & response webpage: Online Access.

Weybright, Scott. Response to fire impacts water levels 40 years into future. January 9, 2020. Online Access.

This article originally appeared in Water Currents News, WSU Extension in Spring 2020 – Issue 3. It has been modified, with permission, for AgClimate.net.