Fourth of a five-part series.
Roy Renkin, a vegetation management specialist at Yellowstone National Park, measures the circumference of a lodgepole pine.
Roy Renkin, a vegetation management specialist at Yellowstone National Park, measures the circumference of a lodgepole pine. Laura Krantz/NPR
Roy Renkin points out a serotinous pine cone. This cone can remain in the tree's branches for decades, until the heat of a passing fire melts the resin that seals it and allows the cone to open, dropping its seeds.
Roy Renkin points out a serotinous pine cone. This cone can remain in the tree's branches for decades, until the heat of a passing fire melts the resin that seals it and allows the cone to open, dropping its seeds. Laura Krantz/NPR
To the left is a mature lodgepole pine forest. The canopy has closed, blocking out sunlight and preventing new trees from growing. But on the edge of the road, where sunlight is plentiful, younger trees have sprouted up.
To the left is a mature lodgepole pine forest. The canopy has closed, blocking out sunlight and preventing new trees from growing. But on the edge of the road, where sunlight is plentiful, younger trees have sprouted up. Laura Krantz/NPR
After decades of working at Yellowstone National Park, Roy Renkin knows the terrain like the back of his hand.
The vegetation management specialist has vivid memories of the 1988 fires that burned 1.2 million acres in the greater Yellowstone area. But Renkin has discovered that these were not the first huge fires to scorch the park — carbon dating shows that similar blazes occurred in the 1700s and in the early 1850s.
"Piecing together that kind of evidence," Renkin says, "one might be able to conclude that in an area the size of Yellowstone, roughly once every century, that we have a very large fire event."
Fire ecologists now know just how important these fires are to Yellowstone's vegetation. The forests of Yellowstone are dominated by lodgepole pines, which thrive despite the poor quality of the soil. Their tall, skinny, limbless trunks are thought to have been favored by American Indians for building lodges and teepees — hence the name lodgepole pine.
These trees produce what scientists call a serotinous pine cone. Resins hold the scales of the cones tightly closed with the seeds inside, and they can remain in the crowns of the trees for 30 to 50 years. Without fire, the seeds would likely never be released.
"What's necessary for those cones to open up and release those seeds is the heat that's generated from a passing fire," Renkin says. "Once the fire burns through those resins that hold them together, the cone scales open up and the seeds fall out."
After the seeds fall to the forest floor, the germination process begins quickly. The heat from the passing fires does not penetrate more than a few centimeters into the earth, which allows the material below ground — nutrients and soils — to give life to the next generation of trees.
"It's not as though the fire burns and [the] place is nuked," Renkin says. "What we see above ground may look burned very badly, but there's a lot of activity below ground."
After the 1988 fires, Renkin and his colleagues dispersed to five sites around the park to figure out how many lodgepole pine seeds were on the ground. They counted between 15,000 and 2 million seeds per acre. A year later, when they went back to see how many of those seeds had germinated, they found 2,000 to 12,000 seedlings per acre on the same sites.
Views over the Yellowstone valleys show that the forest has definitely grown back in previously burned areas. Ninety-five percent of these trees germinated in the first year after the fires.
There are a few areas where the lodgepole pines and other plant communities have struggled to return, however, as a result of human fire-management techniques that occasionally do more harm than the fires themselves. Renkin cites an area burned in 1953 where firefighters used a bulldozer to scrape out a fire line. The forest that burned around the line has recovered, but the line itself remains almost barren. Renkin says that the plant communities haven't adapted to deal with these kinds of firefighting practices.
"We scrape away the topsoil — we scrape away that seed source — the plants can't respond to it," he says. "This scar will be evident for centuries, for 200 years."
There are now growing efforts to repair these human-made disturbances, as replacing the topsoil gives the plant communities a good jump start into healing. It is one important lesson that the park service has learned from its experiences dealing with fire, and it has given fire ecologists a greater appreciation of how this ecosystem has adapted.
Written and produced by Laura Krantz.