From 2011 to 2013, California researchers thinned crowded forest stands near Pinecrest to help trees withstand drought; mortality fell from 34% to 11%

When California's drought hit in 2012 and lasted through 2015, it did not just dry up reservoirs and lawns. It turned many Sierra Nevada forests into tinderboxes and killed trees in large numbers. But in one patch of woodland near Pinecrest, something different happened. Research carried out at the Stanislaus-Tuolumne Experimental Forest near Pinecrest, California, and published in Forest Ecology and Management titled Variable thinning and prescribed fire influence tree mortality and growth during and after a severe drought, examined mixed-conifer stands that had been experimentally treated between 2011 and 2013. Averaged across prescribed-burn treatments, the researchers found that thinning reduced overall mortality between 2014 and 2018 from 34% in unthinned control stands to 11% in thinned stands at the Pinecrest study site.

The idea of cutting down trees to protect a forest can feel counterintuitive at first, especially to many people who associate a dense stand of trees with a healthier forest. That assumption helped leave many western U.S. forests densely packed. After more than a century of suppressing wildfires as quickly as possible, forests that would once have been kept naturally open by frequent, low-intensity fires grew thick and crowded instead. Without periodic fire, smaller trees were not thinned out, and the forest became increasingly crowded and competitive for water.

In a serious drought, trees packed tightly together are essentially rationing water between far more mouths than the soil can comfortably support, and rising temperatures only sharpen that strain. Weakened, thirsty trees also become easy targets for bark beetles, insects that can finish off a tree already struggling to defend itself.


How thinning changed the odds for these trees

The Forest Ecology and Management paper tested how thinning and prescribed burning affected the fate of trees during and after the severe drought. The experimental treatments were carried out between 2011 and 2013 in mixed-conifer stands containing white fir, sugar pine, incense cedar, ponderosa pine and Jeffrey pine.

When the severe drought struck between 2012 and 2015, the difference was stark. According to the study's authors, over a third of the trees in the untreated, crowded areas died between 2014 and 2018, and 23% of basal area was lost. In the thinned areas, growth in the surviving trees offset almost all of the losses, so overall wood volume changed little. The researchers also noted that it made little difference whether the thinning left trees in an even pattern or a more irregular one; both approaches worked about as well as each other, which may be useful for land managers.

Not every species responded the same way. Between 2014 and 2018, mortality was highest among white fir at 42%, followed by sugar pine at 18%, incense cedar at 12%, and ponderosa and Jeffrey pines at 10%. The researchers also found that overall mortality was higher in the prescribed-burn treatments, particularly among smaller trees, while thinning reduced mortality and helped limit the additional mortality associated with burning.

What the results suggest beyond one forest

Prescribed burning increased overall tree mortality, with the effect most pronounced among smaller trees. However, trees in stands that had been thinned before burning were less likely to die, suggesting that reducing competition beforehand helped limit some of the additional mortality associated with prescribed fire.

For California and other drought-prone regions, the Pinecrest study suggests that thinning crowded forests can help improve resilience to drought stress. In the experimental stands, thinning was associated with substantially lower tree mortality during the study period, indicating that reducing competition for resources may help remaining trees withstand severe drought conditions.