New research from scientists at the University of Montana found that slow-growing ponderosa pines may have a better chance of surviving longer than fast-growing ones, especially as climate change increases the frequency and intensity of drought,
This new research is important because the timber industry and U.S. Forest Service often justify “thinning” in ponderosa pine forests on public lands to increase the growth rate and “restore” resilience. But what if in their zeal to “restore” the forest and increase the growth rate of the remaining ponderosa pines, the timber industry and Forest Service might actually be destroying the very forest they are trying to create?
As George Wuerthner (who has degrees in wildlife biology and botany, and has worked as a biologist for the federal government) wrote in an email when he shared this new research, “Here’s another example of how foresters are ignorant and damaging our forests when they suggest that they are thinning to ‘restore’ resilience. When they thin the forest they have no idea of the genetic make up of the trees. But some trees naturally grow slowly and are more resistant to drought. This has been documented in other species like lodgepole pine as well. Foresters with a paint gun have no idea which trees are genetically resistant to bark beetles, drought, cold, etc. but they often take out 50% of the trees in thinning projects. In their efforts to ‘restore’ the forest they may destroy it.”
According to the press release from the University of Montana:
The study, led by UM alumna Beth Roskilly and Professor Anna Sala, was published in the Proceedings of the National Academy of Sciences this week. The researchers sampled growth rates of ponderosa pine trees of varying ages at two remote sites in Idaho. They also studied structural traits of the trees’ xylem — vascular tissue that transports water and minerals through the wood and provides structural support.
Their findings reveal that some young trees grow quickly while others grow slowly. But old ponderosa pine trees — those older than 350 years — are slow growers compared to younger trees, and these individual trees have always been slow growing, even when they were young.
In contrast to predictions, slow-growing trees, whether old or young, did not produce denser, tougher wood, which might have made the trees more resistant to disease or decay. Instead, a key difference between fast and slow growers resides in a microscopic valve-like structure between the cells that transport water in the wood, called the pit membrane. The unique shape of this valve in slow-growing trees provides greater safety against drought, but it slows down water transport, limiting growth rate.
“Ponderosa pines, like people, cannot have it all,” said Roskilly, the paper’s lead author. “Drought resistance contributes to longevity but also to slow growth. In other words, there is a fundamental tradeoff based on xylem structure. Our study suggests that trees with fast growth become large quickly, which can be beneficial for young trees competing for resources, but they are more vulnerable to drought and can die at earlier ages. On the other hand, trees that grow slowly are more drought resistant, which enhances longevity.”
Roskilly earned her UM master’s degree in organismal biology, ecology and evolution in 2018, and the study is a result of her degree work in UM’s College of Humanities and Sciences.
“Ancient trees are special for many reasons,” said Sala, a professor in UM’s Division of Biological Sciences and an adjunct professor in the W.A. Franke College of Forestry and Conservation. “They are beautiful, they make the highest quality musical instruments, they help maintain diversity, and they store atmospheric carbon in wood for a long time. But the results of this research also suggest they are special because forest managers cannot make just any ponderosa pine tree live for centuries no matter how hard they try. For ponderosa pines to become centennials, their wood must possess this unique structure.”