UNIVERSITY PARK, Pa. — Restocking forest patches after fires is important but relies on landscape and local factors, making successful tree replacement and forest recovery difficult. Now, a team of geographers analyzed forest recovery to develop aids for managers in maintaining forests.
"We used to think forests were resilient, but we are increasingly seeing that the pace of fire is alarming," said Erica A.H. Smithwick, distinguished professor of geography, Penn State. "We found that managers need tools to decide how to manage post-fire reconstruction."
Climate change is producing more frequent and more severe forest fires, not only during the traditional fire seasons in expected places, but year-round, across the country. "In 2020, California faced six of its 10 largest fires on record, while Colorado fought its largest, second-largest, and third-largest fires in modern history," the researchers reported in Landscape Ecology. The recent fire in Colorado, north of Denver, on Dec. 30, 2021, indicates that even winter is not free from the specter of fires.
In nature, serotinous trees such as lodgepole pine recover from fire prolifically because their cones are primed to open and release seeds from the fire's heat. Other trees reseed at different rates.
"In different places, there are likely to be different trees," said Jamie L. Peeler, a recent Penn State doctoral graduate currently doing a fellowship for The Nature Conservancy at the University of Montana. "For serotinous trees it doesn't matter. Their cones drop seeds directly on the ground and they will recover. However, non-serotinous trees are going to be vulnerable, and where the treeless plot is on the landscape matters."
The researchers note that if a burned-out area is on the very top of the landscape, the chances of that area naturally recovering are incredibly slim because there is nowhere for seeds from living trees to migrate into the burn zone. Also, very large, burned areas will take a long time to fill in completely because of the distances that seeds need to travel to reach the center of the burn zone.
Foresters follow different rules depending on the type of forest. Wilderness areas receive the least replanting, while other forests might be heavily reforested. The budgets to replace lost trees are, of course, limited and so it would be wasteful to plant an area that will naturally recover, but it would also be unfortunate to skip an area that cannot recover, and create a permanent bare area.
"It is very normal to replant forests," said Peeler. "How the surrounding area plays into replanting, where trees should be planted and where foresters should let nature take its course, is important. We hope that the science can narrow it down to where to plant to best use resources."
Although the foresters know the forest, they do not always have access to spatial tools and information on how hot the fire was, the local climate and topography, said Smithwick.
The researchers looked at 71 areas near Jackson, Wyoming, and used computer analysis to look at landscape components that affect tree recovery. They looked at seed type and where seed would come from for natural regrowth, soil types and nutrients, and fire severity. They found that across post-fire plots, landscape factors indicate that recovery will differ depending on the location, tree type, understory and size of the burned area.
Although foresters evaluating post-fire forests must consider the purpose assigned to the area — wilderness, logging, wildlife habitat — the researchers developed some basic guidelines to evaluate where to replant and where to let the forests naturally recover. For burn sites that are serotinous trees — such as lodgepole — or resprouting trees — such as aspen — seeds for reforesting are not the primary concern, but local site conditions are important. For burn sites above seed sites, natural recovery is not possible.
The researchers also suggest that knowing the distance to seeding sources can predict recovery, but that the species of the seed sources are also important. They also note that about a 330-foot buffer with 40% of that as a seed source is necessary to reach the desired stocking levels.
"They (foresters) have to decide where they are going to prioritize and be the most effective," said Smithwick. "They are trying to anticipate what is going to happen so having information that allows them to estimate what will happen in 50 years is important."
The National Science Foundation, Joint Fire Science Program, National Geographic, and NASA Space Grant Consortium supported this work.