Study Of 33 Different Western Ponderosa, Douglas Fir Sites: Most Not Regenerating After Fire

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Davis and her colleagues looked at growth rings of nearly 3,000 young trees in 33 fire-damaged areas of California, Colorado, the Northern Rockies and the southwestern United States to see when the forests recovered after fires over the past 30 years. Analyzing climate data over the same period, they found certain thresholds involving summer humidity for ponderosa pine, surface temperature for Douglas fir, and soil moisture for both species, beyond which there was a sharp decline in forest regrowth. The warmer, drier air isn't harming mature trees, but it is preventing future generations from growing, Davis said. "There could be a lot of areas where there is currently forest but if we have a fire we might not see regeneration," she said.

Davis and her colleagues found that most of the sites they looked at had crossed the temperature and humidity threshold at some point in the last 20 years. They targeted the driest and warmest sites in the region to see if climate change was already beginning to affect forests. The researchers now plan to assess the extent to which regeneration is affected in relatively cooler, wetter sites in the region.

Several factors influence a forest's regrowth after a wildfire, such as the severity of the fire, regional drought and how the trees produce seeds. The researchers noted that as the region sees fewer years with climate conditions suitable for seedlings to grow, the nature of the trees' seed production, with heavy crops of cones only every few years, will further limit new growth.

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The inability of forests to bounce back from such fires is cause for concern, said Joe Fargione, science director for the Nature Conservancy's North America region. "The thresholds identified here show how much is at stake—losing forests because trees can't grow back—if we don't accelerate the switch to clean energy and invest more in natural climate solutions," Fargione said. "This study will help land managers identify forests at the greatest risk of not regenerating post-fire."

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https://insideclimatenews.org/news/11032019/forest-wildfire-climate-change-tipping-point-study-douglas-fir-ponderosa-pines-west