Examining the Impacts of Wildfire on Throughfall and Stemflow Chemistry and Flux at Plot and Catchment Scales

Abstract:

This study investigates the effects of fire on the chemistry and flux 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 of New Mexico. The loss of canopy cover from 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. In order to track these changes throughfall and stemflow collectors were installed beneath burned and unburned canopies in two catchments impacted by the Thompson Ridge fire. Throughfall, stemflow, and open precipitation samples were analyzed for major cations, anions, organic matter, trace metals, and rare earth elements to determine how fire affects the chemical composition of the precipitation that interacts with burned canopies.

Precipitation samples collected from both burned and unburned sites during the 2014 summer monsoon season show variations across burn severity, specifically in calcium and strontium concentrations, and collector type with stemflow concentrations generally higher than throughfall and open precipitation concentrations. Precipitation samples collected from burned sites have distinct rare earth element concentrations, positive europium anomalies, and titanium/zirconium ratios as compared to those of unburned sites. Aqueous extracts of ash and charred bark were also analyzed to determine the origin of these signatures. A stem count model was used to determine tree density for individual plots and catchments from orthophotos taken before and after the 2013 fire. Upscaling these plot scale concentrations and fluxes to catchment scale allows this study to represent changes to an entire catchment and quantify effects of wildfire on chemical load and water chemistry.