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High altitudes no longer protect pine trees from disease

Researchers from UC Davis have gathered some of the first scientific evidence that climate change can affect the distribution of pathogens. The team studied the incidence of white pine blister rust in the forests of the Sequoia and Kings Canyon National Parks. White pine blister rust is caused by a fungus Cronartium ribicola that infects several types of pine trees, including whitebark pine, and has caused serious damage to white pine populations in the United States. 

The pathogen was introduced by accident in 1900 and is an invasive species that inevitably causes tree mortality. Part of the life cycle of C. ribicola is completed in secondary hosts, namely currant and gooseberry plants.

In the past, blister rust infections did not occur in high-altitude forests because the pathogen prefers warmer, milder conditions. Consequently, the forests above the Sequoia and Kings Canyon National Parks acted as a refuge from this disease. However, with changing climatic conditions, the pathogen has begun to infect trees that are higher up the slopes. 

“Because pathogens have thermal tolerances, we are seeing expansions and contractions in this disease’s range,” said study lead author Joan Dudney, a postdoctoral researcher at UC Davis in the lab of Professor Andrew Latimer. “Climate change isn’t so much leading to widespread increases in this disease but rather shifting where it is emerging.”

The researchers used data from long-term monitoring plots in the National Park forests in the southern Sierra Nevada mountains. Data spanned the period between 1996 and 2016, which is considered to be a warmer, drier period than normal. Observations from over 7,800 potential host trees were included and, in addition, the scientists measured stable isotope ratios in pine needles. 

They found that the optimal climatic conditions for blister rust moved, during the 20-year study period, from lower to higher elevations. The incidence of blister rust decreased by 5.5 percent in the more arid, lower-elevation forests and increased by 7 percent in forests at cooler, higher elevations.

“Our study clearly demonstrates that infectious plant diseases are moving upslope, and they’re moving fast,” said Dudney. “Few pines are resistant to what is basically a Northern Hemisphere white pine pandemic.”

The high-elevation refuges where pine trees were protected from pathogens by the inhospitable conditions, are now under threat because of climate warming; conditions there are becoming tolerable to the diseases and pests. Pathogens are expanding their ranges into these higher elevation areas while contracting in the lower areas where the climate is now too warm and dry for their survival.

“It’s kind of a race between evolution and climate change,” said Professor Latimer. “So far, climate change is winning.”

Although it seems inevitable that blister rust will impact severely on white pine populations as the climate becomes warmer, Dudney stated that employing disease prevention methods could help to slow the spread of the disease. 

The study is published recently in the journal Nature Communications.

By Alison Bosman, Staff Writer

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