Drought-stressed trees more likely to die from wildfires, study finds

A first-of-its-kind project highlights perils faced by drier western forests

Drought and wildfires often, unfortunately, go hand in hand. While some fires can be planned and controlled, dry conditions create a tinderbox for much of American’s forests.

And while it’s possible for trees to survive the damage wrought by these large fires, the stress caused by lack of water can make death inevitable, according to two recent studies published in the journals Fire and Plant, Cell and Environment.

These papers highlight the perils faced by saplings of ponderosa pine trees subjected to both drought and fire. The studies, led by Raquel Partelli-Feltrin, a postdoctoral researcher at the University of British Columbia, also challenge previous methods used to predict the effects that fire has on water transport in trees.

The papers can be found in Plant, Cell and Environment (published online earlier this year, focusing on physiological responses) and in Plant, Cell and Environment from fall of 2020 (focusing on stem hydraulics following a fire). 

For generations, fire ecologists and land managers have worked under the assumption that intense heat causes irreversible damage to a tree’s vascular system. Although, that assumption is sometimes correct, the water status of the plant also plays a major role in its survival, depending on the intensity of the fire it’s exposed to. Partelli-Feltrin’s study found 70% of the well-watered saplings survived a low-intensity fire—compared with none of the drought-affected trees. And even though none of the trees in the study survived high-intensity fires, the research raises questions about large-scale survival of trees facing the double-whammy of drought and fire.

“This work is truly pioneering; nobody has done anything else like this before,” said Dan Johnson, an assistant professor in the University of Georgia Warnell School of Forestry and Natural Resources who served as Partelli-Feltrin’s advisor while she was undertaking the research at the University of Idaho. The papers are part of her doctoral thesis. “People have been trying to look at what fire does to plants by putting them in a microwave and getting them hot, or putting them in a water bath and getting them hot, but that is very different from fire.”

Partelli-Feltrin is the first to test the hypothesis of fire-induced hydraulic failure using actual fire, as well as the first to test drought and fire in a burn lab. Working with another team of researchers who were measuring the amount of energy released by the fire, Partelli-Feltrin collected pine needles from a ponderosa pine stand, sorted, dried and weighed them, and then spread them in a 1-square-meter area at the base of a 1-year-old ponderosa pine sapling. This allowed researchers to begin to understand the amount of energy a plant could survive—and the amount that might lead to their demise.

“In this way, we can dissect out the mechanisms with which these plants were dying,” she said.

In one study, she dried out some trees before introducing them to fire. She and her team monitored the trees to see how dry they became, measuring the pressure inside the trees’ vascular systems to understand when they reached a deficit of water similar to values obtained in the field during a severe drought in Idaho in 2015.

Inside each tree is a “water column,” or a system that transports water from its roots to its branches. As a tree begins to dry out, this column comes under an increasing amount of negative pressure—an indicator of moisture stress. If the tree goes long enough without water, the stress becomes too great and the column of water breaks—an embolism, similar to a vascular condition in people.

Then, Partelli-Feltrin subjected the trees—whether or not they were stressed by drought—to various “doses” of fire, using pine straw measurements to determine the intensity of the heat.

“Breaking that column is one way a plant can die from fire, and that was the hypothesis people had put out before Raquel started her Ph.D.,” said Johnson. “People believed that when fire came through, the water in the water column gets heated up and because that warmer water has reduced surface tension, that’s what’s breaking the rope. But nobody ever tested it.”

Drought stress can cause any number of system failures in a plant, including how well it processes nutrients or the ability to generates new leaves. But as fire is introduced to the equation, Partelli-Feltrin said issues are compounded and become more complex. “It seems like a simple question—when people say, ‘How does fire kill trees?’ they are going to say, ‘Oh, it burns the plants and they die.’ Yes, it does. But some of them survive and some of them don’t. So, when you start thinking about that question, it starts being very complex,” she said.

Although the post-fire tree mortality can be driven by a set of injuries that can lead to different physiological responses, the first steps to better understand how fire kills trees is to understand each of them in isolation. In one of her studies, she specifically measured the effect of fire on water transport to test the hypothesis that trees die after fire due to the hydraulic failure caused by the heat.

The study points to new tactics and red flags for land managers concerned about how drought might complicate forest recovery following a wildfire. It also shows the potential for large-scale changes across forests affected by high-intensity wildfires, with the potential for large amounts of tree losses.

Current tools used to predict tree mortality focus on post-fire assessments, such as tree crown and bark scorch height. Partelli-Feltrin said these models overlook interior systems and tree health. This study, she added, can help create a foundation to build better models—something that’s needed as drought becomes a bigger issue and wildfires become an annual threat.

There are forests where land managers plan for fires, and weather and fuel moisture are a part of that process. But no matter how well a forest may be managed, issues can still arise whenever an area experiences some level of drought.

“Today, even fire-dependent forests—forests that depend on periodic fires to keep their structure and health—are subjected to lose all their trees,” she said. “This is because these forests have been suppressed from fire for decades and over the years fuels were building up and the density of plants in understory and overstory increased. And now with longer fire seasons and higher temperatures driven by climate change, when fire burns in these forests, it burns very hot, different from the fire that the trees are adapted to survive, thus the chances of tree survival are very low.”