For decades, it’s been assumed that trees die from forest fires because they lose their leaves or the ability to move water within their trunks. In theory, they die of dehydration or starvation. But a new study co-authored by a professor at the University of Georgia challenges these conventional—yet unproven—ideas, determining that it’s the cells just under the bark that fail, causing the plant to die. The study was recently published in the journal New Phytologist. The research is the first to document, using controlled laboratory burns, the fire-induced death of the phloem and cambium cells, which produce new wood layers from under the tree bark. The study also found the xylem, which transports water throughout the tree, was largely unaffected by intense fires. “People were thinking it was total hydraulic failure in the xylem, but the phloem and cambium are the only living parts. If the fire kills the phloem and cambium, the tree will die—and we can quantify that these trees are dead,” said Dan Johnson, an associate professor at the UGA Warnell School of Forestry and Natural Resources. “We can do a kind of autopsy on the saplings and measure electrolyte leakage to see if the cells are dead.” The study was conducted at the University of Idaho’s burn lab using Ponderosa pine saplings. These trees are known for their fire tolerance, but even so, the trees have their limits. The research team set fires around small, 2-year-old trees that, based on previous research, would almost certainly kill them. After the fire, they sliced thin layers of the stem and examined them under high-powered microscopes to look at individual cells. Even without testing the phloem and cambium, said Johnson, it was clear the cells were dead—but other parts of the tree remained intact. Raquel Partelli-Feltrin, the lead author of the study who is now a postdoctoral researcher at the University of British Columbia in Vancouver, Canada, found traditional theories on tree mortality just weren’t true. These theories included that the loss of leaves or needles would inhibit photosynthesis; that fire would irreversibly damage the xylem, leaving the tree to dry up; or that the fire would kill the roots. But soil is an excellent insulator, said Johnson, and it wasn’t the loss of needles that resulted in mortality. “What Raquel has found is that xylem is 100% intact in these trees burned with a fire that we know is going to kill them,” said Johnson. But while those cells were intact, the phloem and cambium cells were a different story. “It turned out that after these plants were burned, there were very little sugars in the lower portion of the plants, indicating dysfunction in the phloem.” In trees, phloem helps move sugars, or food, to stems, leaves and roots, while cambium makes new phloem and xylem cells. “So, if you kill phloem and cambium cells, the tree is done,” added Johnson. The study is the first to document this process in Ponderosa pine saplings, and Johnson said the process translates to adult trees. From here, researchers are turning to other trees growing in fire-prone areas, such as Douglas fir, loblolly pine and longleaf pine. As more is known about this process, it can help forest landowners better plan for controlled burns and other management techniques to keep trees safe and thriving. “We are also incorporating the effects from drought, which is affecting pine forests across the American South and West,” added Johnson. “Using burn labs, we can now better understand why some trees withstand fire and the mechanics on a cellular level.” Read More: New Phytologist
