Moisture within Ponderosa pine trees is a significant indicator of future extreme wildfire forecasts, according to new study published in the Fire Ecology journal.
Researchers with the Los Alamos National Laboratory and the University of Texas at Austin recently studied New Mexico’s Bandelier National Monument and found that moisture stress within the trees is projected to extend fire seasons annually in the area.
The national monument was the site of the 2011 Las Conchas Fire, which became the state’s largest wildfire before being surpassed by the Whitewater-Baldy Fire in 2012, the Black Fire in 2022, and the Calf Canyon-Hermits Peak Fire in 2022. The Las Conchas Fire burned over 156,000 acres after it was ignited by a tree falling onto a power line above extremely flammable vegetation, according to the National Park Service.
Live fuel moisture content, or the amount of water in living plant tissues, is crucial to the prediction of climate impacts on fire regimes, according to the researchers. Higher moisture content typically corresponds to lower flammability, due to the energy required to evaporate the moisture before the plants can combust.
“Traditional fire risk indices are calculated through empirical relationships derived exclusively from weather and seasonal timing,” the researchers said. “While these have been used to estimate the change in future fire risk, they do so without considering the crucial thresholds that plants may cross. Key to predicting fire behavior is an understanding of how the plant carbon and water cycles alter fuel availability (live or dead vegetation available to burn) under non-analog climate conditions.”
The researchers used Ponderosa pine trees as their focus because they’re widespread throughout the Rocky Mountains, ranging from Montana to northern Mexico and east to Texas. They then built a model to simulate how future moisture stress in the trees would be affected by projected climate changes.
They determined that the number of days that the trees experience stress is projected to significantly increase under multiple climate scenarios. The increase is expected to extend the state’s usual fire season into later in the year.
“The overall increase in plant water stress (as represented by loss of hydraulic conductivity and leaf water potential) and lower live fuel moisture appear to be driven by reduced precipitation during late summer monsoons typical of the region,” the researchers said. “We predict increasing variability in the P. ponderosa wildfire regime under both climate projections driven by changing productivity, rising mortality, and an overall decrease in live fuel moisture.”
