2 blazes in 16 years set up for­est study

Ar­eas tagged Dense­town, Stump­town


YEL­LOW­STONE NA­TIONAL PARK, Wyo. — This is a tale of two forests, Dense­town and Stump­town, whose paths di­verged after a suc­ces­sion of wild­fires. One il­lus­trates the his­toric re­silience of the dense Yel­low­stone pinelands. The other por­tends a much sparser fu­ture for th­ese forests un­der cli­mate change.

Seven­teen years ago, fire swept through this area north of Jack­son Lake and Grand Te­ton’s glaciered peaks, burn­ing a for­est of old-growth lodge­pole pines that had tow­ered for more than 200 years. In its af­ter­math, a new gen­er­a­tion of pines sprouted, healthy and dense.

Then last year, wild­fire again roared through. Thanks to the va­garies of wind and ter­rain, one thick sec­tion of the for­est was un­touched. A neigh­bor­ing plot of trees was re­duced to singed stumps and ashen pine skele­tons. A crew of sci­en­tists there this sum­mer study­ing the for­est’s re­growth nick­named the sec­tions Dense­town and Stump­town.

Their find­ings on how forests re­spond to fire will help guide for­est man­age­ment and fire­fight­ing poli­cies as a warm­ing cli­mate con­trib­utes to more fre­quent wild­fires. Fires are now rag­ing across the West, from South­ern Cal­i­for­nia to Glacier Na­tional Park in Mon­tana, with ex­perts pre­dict­ing that 2017 will go down as one of the worst wild­fire sea­sons in decades.

Al­though light­ning-sparked fires are a nat­u­ral part of the forests’ life cy­cles, forests re­burn­ing at short in­ter­vals is a rel­a­tively rare phe­nom­e­non. For the past 10,000 years, th­ese woods have burned ap­prox­i­mately ev­ery 100 to 300 years, mean­ing fires typ­i­cally scorched old trees.

But as cli­mate change leads to longer and hot­ter dry sea­sons, younger forests through­out the Yel­low­stone re­gion may start burn­ing more fre­quently. (The jury is still out on how cli­mate change will af­fect wild­fires in other West­ern conifer forests.)

“If that be­comes the norm, where there’s no time for th­ese forests to take a break, to grow for 150 years or so with­out burn­ing, you could see some wide­spread changes to the forests,” said Richard Hutto, an ecol­o­gist at the Univer­sity of Mon­tana.

Th­ese changes could play out in a cou­ple of ways.

First, short-in­ter­val fires could over­whelm an evo­lu­tion­ary adap­ta­tion that in the past al­lowed burned lodge­pole forests to re­grow just as thickly as be­fore. Many of the lodge­poles in the area are seroti­nous, mean­ing they grow pine cones sealed with a sappy resin that pro­tects their seeds from flames. Dur­ing a fire, the cones open and the seeds are re­leased. Only ma­ture lodge­poles pro­duce th­ese resinous cones, while younger ones yield un­pro­tected cones that re­lease their seeds as soon as they’re fin­ished grow­ing.

When fires are in­fre­quent, the for­est has time to ma­ture and build up a stock of seroti­nous cones that will re­launch the next gen­er­a­tion: hence Dense­town. But when part of the young for­est burned again just 16 years into its re­growth, cre­at­ing Stump­town, it had not yet pro­duced many seroti­nous cones. Its seed stock was oblit­er­ated.

Sec­ond, the fires could burn up larger sec­tions of for­est. Small is­lands of for­est of­ten sur­vive even within other­wise burned ar­eas, said Brian Har­vey, an ecol­o­gist at the Univer­sity of Wash­ing­ton, and seeds from th­ese pre­served ar­eas of­ten blow into the sur­round­ing burned forests or are car­ried there by an­i­mals. This re­seed­ing method is es­pe­cially im­por­tant at higher al­ti­tudes where lodge­poles don’t pro­duce seroti­nous cones.

“But what we’re see­ing now is more ho­mo­ge­neous burn­ing through­out the forests, with fewer is­lands of un­burned ar­eas,” Har­vey said. “When that hap­pens, there are fewer seed sources to re­place the stands.”

That’s im­por­tant in the area be­cause lodge­poles make up 80 per­cent of the trees in the heav­ily forested re­gion, which in­cludes Yel­low­stone and Grand Te­ton Na­tional Parks, five na­tional forests, and a hand­ful of out­ly­ing wilder­nesses and wildlife refuges. What will hap­pen to the forests if a chang­ing cli­mate means not only old forests burn, but young ones, too?

That’s what Har­vey and his col­league, Mon­ica Turner, an ecol­o­gist at the Univer­sity of Wis­con­sin, are in­ves­ti­gat­ing. Yel­low­stone’s re­cent fires of­fer a rare nat­u­ral ex­per­i­ment to see how forests re­gen­er­ate after burn­ing and re­burn­ing at short in­ter­vals.

On a blis­ter­ing sum­mer day in Stump­town, sur­rounded by black­ened dirt and leaf­less, life­less trees that of­fer no re­lief from the sun, a dozen stu­dents and re­search as­sis­tants fanned out on hands and knees. Min­utes later, one shouts “Found one!” and the oth­ers con­verge on her dis­cov­ery: an inch-high baby lodge­pole pine peek­ing up in the shadow of a charred log.

By count­ing seedlings a year after the most re­cent fire, the team can cal­cu­late how densely new lodge­poles might re­grow in Stump­town. Al­though there could be some late-sprout­ing seeds and an­i­mals might carry in new seeds in the years to come, based on their ini­tial count, the re­searchers pre­dict that there will be around 400 trees per acre in the area. In Dense­town nearby, there are some 32,000 trees per acre.

In ad­di­tion to be­com­ing sparser, Stump­town’s tree pop­u­la­tion seems to be di­ver­si­fy­ing. Aspens came in after the 2000 fire and re­sprouted after the 2016 fire. Now they are es­tab­lish­ing along­side the lodge­poles.

Through­out Yel­low­stone’s long his­tory of fire and re­growth, forests have tended to come back like Dense­town. But cli­mate change may be push­ing even those hardy forests past their break­ing point, said Har­vey, and how trees re­grow in Stump­town could be a sign of things to come. Taken to­gether, their pre­lim­i­nary find­ings sug­gest many of Yel­low­stone’s dense, lodge­pole-dom­i­nated forests will give way to sparser, more di­verse wood­lands and mead­ows.

“When fires burn at short in­ter­vals, we have a lot fewer trees com­ing back,” Turner said. “It’s still enough for a for­est, but it will be sparser than be­fore.”

There are ad­van­tages to sparser forests. More preva­lent grasses and aspens pro­vide food for elk and deer, and bird di­ver­sity of­ten ex­plodes after a burn.

One prob­lem with younger forests burn­ing in­stead of older ones, Hutto said, is that some birds are picky when it comes to charred habi­tats, pre­fer­ring burned ma­ture forests.

“If your for­est doesn’t get very big be­fore it burns, you don’t get the coolest stuff,” he said.

Roy Renkin, a veg­e­ta­tion man­age­ment spe­cial­ist at Yel­low­stone Na­tional Park, said he is skep­ti­cal that young forests will burn more fre­quently be­cause they do not pro­duce enough fuel.

“It’s an ex­am­ple of lin­ear think­ing to say that warmer and drier equals more and big­ger fires,” he said. “There are feed­backs and in­ter­ac­tions that change the lin­ear­ity.”

Cli­mate change might be al­ter­ing th­ese fun­da­men­tal for­est dy­nam­ics, said Wil­liam Romme, a for­est fire re­searcher at Colorado State Univer­sity.

“The big ques­tion we’re ask­ing now is, ‘What does the fu­ture hold for th­ese forests?’” Romme said. “Are we en­ter­ing an era in which things aren’t go­ing to be­have like they did be­fore?”

Stump­town it­self, Turner noted, is ev­i­dence that given the right cli­mate con­di­tions, young forests in Yel­low­stone can and will re­burn.

Ul­ti­mately, she says, so long as wild­fires are not en­dan­ger­ing hu­man lives, it is best to let them take their course. Rather than try to pre­serve forests in their cur­rent state, of­fi­cials should fo­cus on things they can con­trol or mit­i­gate, like pol­lu­tion, habi­tat frag­men­ta­tion and in­va­sive species, she said.

“Change is go­ing to hap­pen,” Turner said. “But we’ll still have forests. We’ll still have a wide va­ri­ety of na­tive species. It will still be Yel­low­stone.”

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