Impacts of Wildfire on Throughfall and Stemflow Precipitation Chemistry

Abstract:

The occurrence of large, stand replacing wildfires is more frequent in the western United States now than ever before. The loss of canopy cover due to wildfire drastically modifies landscapes and alters ecosystems as high intensity burns replace canopies with charred branches and trunks, change soil composition and erosion processes, and affect hydrologic flow paths and water chemistry. Precipitation that is not intercepted by the forest canopy makes its way to the forest floor as throughfall or stemflow. Tracking variations in the amount and chemistry of precipitation that interacts with burned versus unburned forest stands, as well as open precipitation, will help to quantify changes in hydrologic routing and catchment water chemistry caused by wildfire. This study investigates the effects of fire on the volume and chemical composition of precipitation diverted to the forest floor as stemflow and throughfall by observing the impact of the June 2013 Thompson Ridge wildfire in the Jemez River Basin Critical Zone Observatory field site in the Valles Caldera National Preserve of New Mexico.

Throughfall and stemflow collectors were installed beneath both burned and unburned canopies and open areas in two catchments impacted by the Thompson Ridge fire. Initial results of field parameters, including electrical conductivity, pH and volume of precipitation collected from both burned and unburned sites, show variations across collector type (stemflow, throughfall and open precipitation), site location as the two catchments differ in aspect and gradient, and burn severity. Throughfall, stemflow and open precipitation samples were analyzed for trace metals, major cations, anions, nutrients and organic matter to determine how fire affects the chemical composition of the precipitation that interacts with burned canopies. This study is one of the first to quantify the relationship between wildfire and the chemistry and flux of stemflow and throughfall in conjunction with a full suite of pre and post fire precipitation, soil and surface water chemistry.