Forskere blæst væk af orkansimulering

For næsten 30 år siden, Harvard Forest forskere begyndte en unik, langvarigt eksperiment på en 2-acre skovstrækning ved hjælp af et langt stålkabel, et spil, og et kraftigt tømmerhøstkøretøj kaldet en skider.

En efter en, de trak kablet til 279 udvalgte træer, fastgjorde den højt på bagagerummet, og radioerede derefter fyren ved skideren, parkeret uden for banen for at undgå at forstyrre skovbunden, at udløse spillet. Og, en efter en, træerne faldt ned.

Da de var færdige, de havde tilnærmet skaden, der blev påført i New England af den store orkan i 1938, at skabe en eksperimentel orkan, der ramte 50 procent af de store baldakintræer, åbnede undervisningen for nyt lys, startede en årtier lang genoprettelsesproces, og skabte et mareridtlandskab af nedfaldne stammer og krydsede grene, der lettere kunne gennemgås af en parkour -ekspert end forskere, der er ansvarlige for regelmæssigt at registrere de ændringer, der kommer.

I marts 2019, mareridtet havde stort set lagt sig. Skovbunden var stadig oversået med nedbrydelige stammer, men deres langsomme tilbagevenden til jorden var kommet så langt, at de let kunne krydses af omkring to dusin forskere og besøgende, der samlede sig i det snedækkede forsøgsgrund, i Harvard Forest's Tom Swamp Research Tract i Petersham, Massachusetts.

Omkring dem var træer, der mens de er yngre og tyndere end dem, de udskiftede, havde for længst lukket hullerne i skovens baldakin. De lignede sminkede dem før stormen - en overraskelse for forskere, der forventer, at flere pionerarter vil tage fat. Det var også bemærkelsesværdigt, hvor ubemærket traktaten så ud. Det var som mange andre løvskove i New England, bar og afventer forårsblade på en kølig senvintermorgen.

Faktisk, den almindelighed - et udtryk for stabiliteten i skovøkosystemet i New England, selv i kølvandet på en katastrofe engang i et århundrede-var en anden vigtig lektion, sammen med opdagelsen af, at skove, der administreres som naturlige miljøer, bedst er i stand til at genoprette sig selv frem for at blive hjulpet sammen med "bjærgningslogning", der er udbredt efter stormen i 1938 og stadig er almindelig efter nedblæsning, brande, og trædræbende insektangreb i dag.

Forskerne, stammer fra Harvard Forest og tilhørende institutioner, samlet den morgen for at reflektere over gennemførelsen af ​​eksperimentet, store fund, og betydningen af ​​sådan forskning, som kræver en patient, lang tids tilsagn fra finansieringskilder, fra værtsinstitutioner som Harvard Forest, og fra forskerne selv, en udsigt, der bliver sjældnere i en utålmodig, resultater-nu verden.

Taler til forsamlingen den morgen, Harvard Forest Director David Foster sagde, at orkanforsøget var vigtigt ikke kun på grund af den videnskab, det muliggjorde, men også fordi det var en af ​​de første og mest slående efter Harvard Forest's udpegning som Long Term Ecological Research site (LTER) af National Science Foundation i 1988.

Den betegnelse, fornyet med seks års mellemrum, har givet et fundament for økonomisk støtte - omkring 1 million dollars årligt - til arbejde som den eksperimentelle orkan, og udnytter mellem fem og 10 gange det i finansiering fra andre agenturer. LTER -projekterne er også et omdrejningspunkt for uddannelsesprogrammer i skoven, herunder både K ‒ 12 og college-niveau programmer. Tusinder af Harvard -studerende har besøgt under ekskursioner, arbejdede på lokaliteterne under sommerforskningsprogrammet, og studerede deres resultater på Fosters førsteårsseminar i global forandringsbiologi.

Den 18. og 19. marts, Harvard Forest var vært for en to-dages begivenhed, der markerede 30-årsdagen for dens LTER-udpegning. Den første dag var dedikeret til besøg på stedet og den anden til et dagligt videnskabeligt symposium, med detaljerede præsentationer af resultater hidtil til 125 deltagere. I dag, Harvard Forest er et af 28 LTER -steder i hele landet, del af et netværk, der måske er lidt kendt for offentligheden, men som økologer ærbødiger.

"Det er store steder for økologer, "sagde seniorøkolog i Harvard Forest, Jonathan Thompson, der for nylig overtog efter Foster som hovedforsker for skoven LTER -bevilling. "Der er intet som dem."

Efter at have set orkanens udblæsning, forskere lastet i varevogne på vej mod andre forsøg langs de smalle snavspor, der krydser de 4, 000 hektar stor skov. Et stop var et standpunkt med tårnhøje stedsegrønne - hemlocks overvåges af seniorøkolog David Orwig. Hundredvis af år gammel og aldrig logget, hemlocks 'dage er ikke desto mindre talte på grund af angreb fra den invasive uldne adelgid, hvis indtrængning så langt nord er blevet lettere af regionens stadigt varmere vintre.

Gruppen besøgte miljøovervågningsstedet, hvor verdens første forskningstårn blev bygget til at måle indtag og udstrømning af gasser, når skoven ånder. Forskning ved tårnet, pioner af Steven Wofsy, Harvards Abbott Lawrence Rotch professor i atmosfærisk og miljøvidenskab, og nu overvåget af seniorforsker i atmosfærisk kemi J. William Munger, viste, at genoprettende skove som New Englands-klare i kolonitiden-hjælper med at bekæmpe klimaforandringer ved at låse atmosfærisk kulstof i deres træ, når træerne vokser tykkere og skubber højere.

Dagen sluttede ved et tern af brunt på den sneklædte skovbund. De snøløse grunde er kendetegnende for et langvarigt eksperiment i jordopvarmning. Ved hjælp af opvarmede kabler begravet under jorden, den næsten 30-årige indsats søger at forstå, hvordan jordmikrober og vejrtrækning fra trærødder kan reagere på en opvarmende verden.

Vedligeholdt ved 5 grader Celsius over den omgivende jord for at afspejle den høje ende af opvarmningsestimaterne for slutningen af ​​århundredet, plots har vist, at de opvarmede mikrober og rødder sparker i højt gear, hurtigt øget mængden af ​​frigivet kulstof, der havde været låst i jorden. Efter at have nået et højdepunkt, emissionerne faldt, stabiliseret i flere år, og derefter - i endnu en eksperimentel overraskelse - steg til en anden top.

"Vi regnede med, at vi har et trefaset fænomen, og vi er fortsat med at foretage målinger, "sagde Jerry Melillo, fornem videnskabsmand ved University of Chicago's Marine Biological Laboratory og en seniorforsker ved skoven. "Vi er nu i en anden hvileperiode. Yderligere 20 eller 30 år kommer vi nok tæt på et svar."

Melillo sagde, at LTER -finansieringen var afgørende for forsøget, først fordi det gav vigtige grundlæggende penge, der blev forstærket af finansiering fra andre kilder som det amerikanske energiministerium. Derefter, da interessen faldt, efter at den første top af kulstofemissioner faldt, LTER -finansieringen holdt eksperimentet kørende. Uden det, Melillo sagde, det andet udbrud af kulstofemissioner - og den forbedrede forståelse af opvarmningens virkninger på skovjord - ville være gået ubemærket hen.

LTER and a 'signature experiment'

Melillo, who has conducted research at Harvard Forest for 40 years, played an important role in getting the initial LTER experiments up and running, Foster said. Not long after the LTER designation, Melillo counseled that the forest needed something exciting to help it stand out.

"'What we need is a signature experiment, '" Foster recalled Melillo saying. "'We need something that they'll talk about at NSF, that'll be unlike something that anybody else has done.'

"I said, 'Jerry, what is that?' And he said, 'I don't know, but we gotta have it.'"

Foster eventually seized on replicating one of the most devastating forces that mold the New England forest landscape:major hurricanes that blast ashore every 100 to 150 years. But how to do it? He rejected the idea of bulldozing trees because it would tear up the forest floor and disrupt the invisible but nonetheless critical cycling of nutrients and gases between the floor and the atmosphere.

When Foster suggested winching trees down instead, the idea was ridiculed as unworkable by a visiting scientist:The roots were far too strong. Foster chewed over the problem until he mentioned it to John Wisnewski, a Harvard Forest staffer with experience logging.

"'I'd just pull them over, '" Foster recalled Wisnewski saying. "'We do it all the time.'"

Wisnewski, today Harvard Forest's woods crew supervisor, told Foster that loggers need a flat area to stage removal of logs from the forest. So instead of cutting trees, which would leave stumps behind, they simply pull them down with a winch and cable, lop off the trunks, and pile the roots to one side.

An experimental path cleared, Foster turned to the forest's archive to plan the simulated storm In 1938, graduate student Willett Rowland recorded the Great Hurricane's damage at the forest, showing that about half of the large trees came down and that some species, such as white pine, were more susceptible to wind damage.

With that knowledge in hand, Foster laid out an east-west plot 50 meters by 160 meters in the Tom Swamp tract and marked the trees to come down. Preparations complete, they drove in the skidder and hauled the cable into the forest, pulling down tree after tree, all oriented so their crowns pointed northwest, as if felled by a hurricane's prevailing winds.

Most trees came right over, Foster said, but some broke and were left to regrow as they were. Only one tree—a large old oak—resisted the skidder's tow.

"We decided that, godt, in a hurricane that tree wouldn't have fallen, " Foster said. "We went and found one of equal size downslope and pulled it over."

Then began the lengthy task of monitoring. An early revelation was the stability of key indicators like soil temperature, overall productivity, and carbon dioxide and nitrogen gas cycling among the trees, the soil, and the atmosphere.

Another was that the trees didn't die right away. Ninety percent of trees damaged by the winch leafed out regardless, photosynthesizing, drawing water from the earth, and contributing to the forest ecosystem even though they were flat on the ground. As they slowly died, the understory took over. Saplings that had been awaiting their chance shot upward, sprouts grew from the fallen trees' roots, and newly seeded trees got started. Lost production—measured in the amount of leaf litter each fall—recovered in just six years.

"Despite the fact that this looked like a destroyed forest, because it was physically altered in such a major way, it was functioning as an absolutely intact ecosystem, " Foster said.

In trying to understand the forest's unexpected stability, researchers realized that most experience with disturbed sites was at places subjected to the common practice of salvage logging, where fallen trees are cut and dragged out using soil-churning heavy equipment. I nogle tilfælde, as after the 1938 hurricane, the piled debris left behind is burned.

"We're used to looking at sites that were subsequently disturbed after a major wind storm or ice storm by people going in and logging, " Foster said. "The 1938 hurricane was the biggest salvage logging exercise in U.S. history. And it pretty comprehensively turned the 1938 hurricane into one great big cutting operation.

"In almost every case you can think of, if your intent is to encourage the recovery of the forest and ecosystem function with minimal change … doing nothing becomes a viable alternative."

When Audrey Barker-Plotkin arrived at the site eight years after the pulldown, just walking around was a challenge. Today a senior researcher and the author of several studies on the site, she recalled having to weave through tangled branches and wrestle with wiry new growth that all seemed to be at "face level."

"It was like walking through a jungle gym. The plot seems a lot smaller now that you can see through it, " Barker-Plotkin told the visiting scientists. "Just the changes I've seen in 20 years have been really remarkable. … [The site] was different every single year."

Another thing scientists didn't expect, Barker-Plotkin said, was the stability of the tract's species makeup. Researchers thought that more pioneer species like cherry and paper birch—usually fast-growing colonizers of disturbed sites—would take hold. But the stability of even the damaged ecosystem didn't provide much of an opening. While those species did appear in disturbed soil around upturned roots, that was less than 10 percent of the forest floor. Invasive species, another threat at disturbed sites, were also absent, hun sagde.

I dag, hun sagde, the experimental plot has largely recovered structurally, but is still struggling to catch up with the surrounding forest's growth. Tree volume has reached about 80 percent of what it was before the pulldown, but measurements of the nearby control plot show that the surrounding forest has grown 25 percent over the intervening decades as part of New England forests' continued recovery from Colonial-era clear-cutting.

New leaders and a landscape full of questions

Like the long-term processes they measure, the hurricane pulldown and other experiments continue to produce data even as their original investigators' careers come to a close. A smooth transition to new leadership will be essential in maintaining both research continuity and excellence, Foster said. At several sites the group visited, experimental founders handed off presentations to younger researchers, as Foster did to Barker-Plotkin at the hurricane site and Melillo did to University of New Hampshire Professor Serita Frey, a soil microbe expert, at the soil warming experiments.

LTER's new principal investigator, Thompson, spoke of the importance of ensuring the continuity of key long-term experiments even as researchers move on from work that has run its scientific course.

"In some ways, the experiments they set up in the '80s just look so prescient now, " han sagde.

An important question still to be explored is how long recovering forests will keep absorbing carbon, Thompson added. That answer has potentially crucial implications for climate change, since global forests absorb roughly 20 percent of the excess carbon humans emit.

Part of the problem, Thompson said, is that though remnant old-growth patches exist, they may not be good models for understanding forests regrowing on former farmland, since they're typically in poor growing locations, which is why they weren't cut in the first place.

"We know how much [carbon] is in the forests, " Thompson said, "but we don't know how much carbon can be in these forests."

This story is published courtesy of the Harvard Gazette, Harvard University's official newspaper. For additional university news, visit Harvard.edu.