T he key to1 unlocking

the secretshidden inisotope signalsis an instrumentcalled the massspectrometer.The age oforganic materi-als can bedeterminedwith fair accu-racy back40,000 years by

comparing the extent of disintegration of radioactive carbon to stablecarbon atoms. Very small samples of material are burned and puregases are then injected into the instrument. The pure gases contain twotypes of atoms: one of common weight and one slightly heavier. Thegases are forced, under pressure, down a tube and around a corner. Athigh speed the ones with higher mass bang against the wall near thestart of the turn. The lighter ones strike further down the tube.

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TREES ASENVIRONMENTALHISTORIANS

by R.H. Waring

T

he burial was complete asMowa packed the lastbasketful of earth aroundthe lifeless body of her ema-

ciated son. She stood, wiped tearsfrom her eyes, and turned to lookat the river valley where her ances-tors had built the largest earthenburial mounds in North America.New strains of corn provided hertribe great wealth to barter forcopper from the shores of LakeSuperior, and for obsidian from theYellowstone region. Since 1000AD the cultivation of corn hadincreased to provide three-quartersof her people's diet. But now, inthe year 1225, the rains had failedand parched fields stood brown,framed against the ghostly outlineof a leafless forest. For the nextcentury the central part of thecontinent from the MississippiValley to the Allegheny Rangewould be filled with dust andsmoke: her tribe would be scat-tered and only remnants of herpeople's culture preserved.

Archaeologists, working togetherwith other scientists, are now re-vealing many secrets of prehistoriccivilization from chemical cluesfound in the skeletal remains ofprimitive people and from thewood of ancient trees. As our pres-ent world awakens to the realiza-tion that the next generation mayexperience climatic change moreabrupt and more extreme thanthose that destroyed previouscultures, we search for new tech-

niques to interpret fluctuations inpast climates and the environ-mental change that resulted. To ac-complish this, a new technique hasbeen developed to examine theatomic composition of bone,wood, sediment and glacial ice.Then it translates those chemicalclues into weather reports ofancient climates to show how theyaffected past civilizations. Thesame chemical clues are beingdeposited today in these materialsand may be current harbingers ofmajor changes to our lives in thenear future.

The bones of Mowa's ancestors,buried at various layers in themounds, left a record of genera-tions increasingly dependent on

corn. Corn is a subtropical grasswith a unique way of incorporatingcarbon dioxide into plant matterduring photosynthesis that differsfrom other major food plants inMowa's or her tribe's diet. Moreprecisely, the ratio of the stableisotope of carbon with an atomicweight of 13, relative to that with aweight of 12, is more than doublein corn compared to other nativefood plants. The extent to whichthe sample of bone collagenbecomes enriched in the heavierisotope of carbon provides anaccurate record of the tribe's in-creasing dependence on corn as astaple in their diet.

How do we know that theclimate changed around 1200 AD?

FORESTWORLD 10 v^ INTER 19S. V C /. 4

Paleobotanists and archaeologistsobtain some clues by observing thekinds of plant materials buried insediments of known age. The ageof organic materials can he deter-mined with fair accuracy back40.000 years by comparing theextent of disintegration of radioac-ti\ e carbon with an atomic weightof 14, compared to stable carbonatoms with an atomic weight of 12.This is known as carbon 14 dating,clen termed radiocarbon dating.Other sets of stable isotopes ofcarbon, hydrogen and oxygen arefound in organic materials inspecific ratios determined byclimatic conditions the day thematerial was formed, and theseratios remain constant throughtime.

The key to unlocking the secretshidden in isotope signals is an in-trument called a mass spectrome-

ter. Very small samples of materialare burned and pure gases ofcarbon dioxide, nitrogen, oxygen.hydrogen or sulfur dioxide areseparated and then injected intothe instrument.

The pure gases of these elementscontain two types of atoms: one ofcommon weight, and one muchrarer and slightly heavier in mass.The gases are forced, under pres-sure, down a tube and around acorner. At high speed the oneswith higher mass bang against thewall near the start of the turn. Thelighter ones strike further down thetube. By carefully positioningcounters at correct places along thetube's wall, it is possible to deter-mine the relative number of strikesof heavy vs. lighter atoms of eachelement.

To determine climate conditionsduring the famine period, we needa piece of undecomposed woodfrom a prehistoric campfire orburied artifact. On this material.the disintegration of radiocarbon(carbon 14) is measured to deter-mine the approximate age. Afterthat we might analyze for the stableisotope ratios of hydrogen. oxygenand carbon. .

The stable isotope ratio of hydro-gen 2 to hydrogen 1 in precipita-tion reflects the average yearlytemperature because the air tem-perature to a large extent deter-mines the hydrogen isotopiccomposition of precipitation. Ifwarm tropical storms brought rainto the forests and adjacent cornfields in middle America, thetemperature of the water would behigher than if snowmelt irrigatedthe hills and valleys. Indeed, whenwe look across all of North Amer-ica, we see that the average iso-topic composition of precipitationgenerally mirrors the averagetemperature. In northern Canada.we find the lowest fraction ofheavy isotopes of hydrogen wheremost precipitation falls as snow. Asthe climate warms toward the GulfCoast, the fraction of heavy iso-topes of hydrogen progressivelyincreases in precipitation. During adrought there is little rain, but raintemperatures tend to be warmerand this shows up through anenrichment in the heavier isotopeof hydrogen in tree wood cellulose.

The air also holds less moistureduring a drought. This lowers thehumidity of the air and affects theisotopic ratios of oxygen 18 tooxygen 16 in the cellulose of treerings. When the humidity is low,more of the lighter isotopes ofoxygen escape as a gas duringphotosynthesis, leaving behindmore of the heavy form to he incor-porated into the tree's wood.

Of the vast majority of plants thatphotosynthesize differently thancorn, small increases in the stableisotope ratio of carbon 13 tocarbon 12 in tree wood cellulose isindicative of stress. The moreenriched a sample of wood cellu-lose is in the heavier isotope. themore closed were the pores in theplant's leaves when photosynthesisoccurred (Figure 1). During adrought, lack of water causes leafpores to close, reducing water losswhile also restricting photosynthe-sis and growth.

Today the greenhouse" effect of

climatic warming, brought aboutby burning of fossil fuels andrelease of carbon dioxide and othertrace gases, is predicted to be mostdramatic at higher latitudes closerto the poles. The reason for this isthat the atmosphere near the polesis less buffered against change thanelsewhere. It is not as thick nor aswell mixed by winds as the atmos-phere nearer the equator. Ecolo-gists are now examining thepristine cedar forests of southeast-ern Alaska to see whether changes

Researchers, looking for clues todiscover why some trees appear tobe damaged or dying, use stableisotopes to test climatic change inthe history of the forest.I

have already occurred. The forestsof Alaska yellow cedar are very oldand much prized, both culturallyand economically. Yet many treesare now dying. Forest pathologistscannot identify the cause: no singledisease brings on tree death. Manyof the trees started life more thanhalf a millennium ago, but few arereproducing from seed. In manyrespects, the decline of Alaskayellow cedar exhibits symptoms ofstress due to climatic change.

To test the hypothesis thatclimatic change has caused thecedar in southeastern Alaska toreduce its growth and vigor.ecologists are looking first at thehydrogen isotopic composition in

,•V)141 r, 11 %% I NTFR

wood cores representing tree ringsover the last two centuries. Theyhope to find evidence of a 2 to 4degrees Fahrenheit increase in tem-perature. Such an increase shouldresult in a slight reduction inrelative humidity during the grow-ing season. This in turn should benoted by enrichment in the fractionof the heavier isotope of oxygen in

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wind from heavily industrializedparts of North America, manyforests appear to have been understress during the last few decades,despite fairly stable climatic con-ditions at mid-latitudes. Why isthis? Acid rain and other formsof air pollution have washed overthese forests in increasing wavesduring this time span and pollu-

T rees UnderI Stress Hold

Their Breath.Changes in climatecan sometimes beharmful to trees,so scientists areusing stable iso-topes to check forevidence of harm-ful climaticchanges in pastenvironments.These same chemi-cal clues are beingleft in trees todayand may be able totell us of changesin our lives com-ing in the nearfuture. The graphshows the relativescale of poreopenings in leaves

2.0 of various speciesof trees growing indifferent soils.

leave no record of major weatherchanges over the last few decades,but the carbon isotopes indicatethat closure of leaf pores is con-straining photosynthesis, then weshould expect to find some recordof changing sulfur and/or nitrogenisotopic ratios in the wood cellu-lose of more recent tree rings. Thiskind of signal should increasedramatically in trees close to amajor source of pollution, as it isknown to in more sensitive plantslike mosses.

Forests still stand in North Amer-ica that were growing before thecontinent was colonized by Euro-peans. We have attempted tomaintain some ancient forests fortheir beauty and for the protectionthey provide other forms of life. Indoing so we also protect an irre-placeable wealth of information onpast climates, pollution, and otherenvironmental changes that isstored annually by chemical scribeswithin tree wood cellulose.

Isotope techniques offer the op-portunity to tap these wooden databanks. Combined with isotopicanalyses of other materials. archae-ologists and ecologists may tell usmore about how earlier peopleadapted to the Earth's changing en-vironment. From this legacy wecan perhaps also better forecast thefuture ecological consequences instore for us. f

yez-pr,&,..ort,16-,„7.aca-0 1.0Increasing opening of leaf pores

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The more that pores on a leaf open to let in carbon dioxide, the lowerthe enrichment of heavy isotope (carbon 13) in the leaf or wood cellu-lose. The more pores close on the leaves—when the tree "holds itsbreath" because of the stress from a dramatic change in the climate—the

rort, the amount of heavy isotope. (Rising line indicates isotope deple-tion.) Adapted from E.H.DeLuci4 NiX H Schlesinger. W.D Billings. Eco/ogv, Vol 69. p 303 . 311. 19148.

the same tree rings over the lasttwo centuries. Maritime tree spe-cies such as cedar tend to be verysensitive to lowering air humidity.They should respond to lowerhumidity by closing the pores intheir leaves, causing wood cellu-lose to show an increasing enrich-ment in the heavier isotope ofcarbon 13. Reduced photosynthe-sis, combined with higher tempera-tures, would threaten the cedars byforcing them to respire morecarbon while acquiring less.

In Europe, as well as down-

tion is known to induce leafpores to close.

Acid rain is composed of sulfurand nitrogen compounds originat-ing from the combustion of fossilfuels or from fertilizers producedfrom atmospheric nitrogen. Theisotopic composition of sulfur andnitrogen from these extraneoussources can differ substantiallyfrom those trees normally obtainfrom weathering of rocks andnatural, unfertilized soils. If thehydrogen and oxygen isotopicscribes of weather in tree rings

Richard Waring has been a professorin the College of Forestry at OregonState Unimrsit y in Corvallis for 25Years. He recently took a leave-of-absence to the Ecosystem Center atWoods Hole, Mass., to learn moreabout the use of stable isotopes andremote sensing in studies of forestecosystems.

Figure ReferenceEdwards. RT. R.O. Aravena. P. Fritz. and A.V.Morgan. 1985. Interpreting paleochmate from18 and 2„ in plant cellulose: comparison withevidence from fossil insects and relict perma-frost in southwest Ontario. Canadian Journal ofEarth Sciences 22.1-20-1-26.