EOLOGICAL ANDclimatic historyshare commonthemes with history,

politics, and many otherdisciplines, that of the need toestablish a timescale ofevents. In pre-history,timescales are critical tounderstanding evolution ofbiological, tectonic, andclimatic systems that haveinteracted to produce the

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contemporary surface of theEarth. Many chronologicaltools have emerged to allowelapsed time to be accuratelyestimated for timescalesranging from decades tomillions of years.

One of the most widelyused techniques is‘Lichenometry’, the use ofsymbiotic fungi in the form oflichens, growing on rocksurfaces or other suitable

substrates, to obtain anapproximate date of thedeposition of the surface. Thisarticle describes themethodology of lichenometry,the usefulness of the methodin dating surfaces over the last500 years, a period in whichradiocarbon dating isrelatively inefficient, anddiscusses how the techniquehas contributed to the debateover climatic change and

global warming.

Lichens

Lichens are very commonorganisms on Earth and arefound in a range ofenvironments including thesurfaces of rocks, trees, andman-made structures. A lichenis an intimate associationbetween two quite differentmicroorganisms, viz., an algaand a fungus resulting in a

G

LICHENS,LICHENOMETRYAND GLOBALWARMING

Richard Armstrong describes an alternative method of dating

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Features

‘macroorganism’, the lichenthallus, the morphology ofwhich is quite unlike that ofthe two original constituents.The two organisms are sointimately associated that theterm mutualism or symbiosishas been applied to it. Lichenswere the first examples ofsymbiosis in whichtranslocation between theconstituent symbionts wasdemonstrated especially themovement of carbohydratefrom the algae to the fungus.It is possible that the fungussupplies the algae with lowconcentrations of nutrients orvitamins but this translocationhas not been convincinglydemonstrated. A fullerdescription of lichen growthand physiology is given in aprevious article in theMicrobiologist (Armstrong,2003, Vol 4, No.4).

There are three major typesof lichen, viz., the fruticosetype in which the lichenthallus is attached to thesubstratum at a single pointand forms a complex

branched structure, the foliosetype that comprises a series ofradially arranged leaf-likelobes, and the crustose typethat is tightly attached to thesubstratum. The foliose andcrustose types of lichen growradially over the substratumrather like a fungus on an agarplate but growth rates arevery slow. Many foliosespecies have rates of radialextension between 2 and 5mm per year but manycrustose lichens grow muchmore slowly with rates of lessthan 0.5 mm per year. Somespecies grow so slowly thatlarger thalli growing in theArctic may live to be over5000 years old, thus makingthem candidates for oldestorganism on Earth. It is thisslow growth and the longevityof crustose lichens that havemade them especially useful inlichenometry (Innes, 1985).

Lichenometry

Lichenometry depends onthe assumption that if the lagtime before colonisation of a

Fig. 1: A population of yellow-green thalli of the lichenRhizocarpon geographicum on scree boulders in the PacificNorthwest, USA. The yellow-green islands of areolae can beclearly seen as well as the black fungal hypothallus at the margin.

substratum by a lichen isknown and the growth of thelichen can be measured, thena minimum date can beobtained by measuring thediameter (or another propertyrelated to size) of the largestlichen at the site. The methodis particularly useful inregions above and beyond thetree-line and especially inArctic-Alpine environmentswhere lichens grow veryslowly and have greatlongevity. In suchenvironments, it is possible todate deposits up to thousandsof years old but, in themajority of cases, the methodis most useful for dating overthe past 500 years.

The ‘Rhizocarpon’ groupof lichens

The majority oflichenometric studies havebeen conducted using aspecific group of crustoselichens, viz., the yellow-greenspecies of the genusRhizocarpon (Fig. 1). Thistype of lichen is abundant inmany Arctic-Alpineenvironments, grows veryslowly (0.02 – 2 mm yr-1), andlives to a considerable age.Morphologically, the lichencomprises discreet areolaethat contain the cells of thealga Trebouxia, located on afungal medulla which isattached to the substratumand which extends into ablack algal-free marginal zonearound the thallus called thehypothallus (Armstrong &Smith 1987). Within eachareola there is a cortical layer15-80 mm thick, an algallayer, and medullary tissue.The fungal hypothallusextends radially and to grow,relies on carbohydratesupplied from the areolae(Armstrong & Smith, 1987).Primary areolae near the edgeof the hypothallus maydevelop from free-living algalcells on the substratum thatare trapped by the hypothalluswhile secondary areolae maydevelop from zoospores

produced within the thallus. Different species of

Rhizocarpon may colonise asurface at different rates, e.g.,species of the sectionRhizocarpon may coloniseearlier than the speciesRhizocarpon alpicola.However, R. alpicola growsfaster than most members ofthe section Rhizocarpon andeventually becomes the largestlichen on the substratum(Innes, 1985).

Measurements

Several measurements oflichen thallus size have beensuggested as the most usefulin lichenometric studies. Theuse of the ‘largest inscribedcircle’, i.e., the largest circlethat can be drawn within anindividual thallus wassuggested by Locke et. al.,(1979) and has been adoptedby several workers and isequivalent to using the‘shortest diameter’ of thethallus. By contrast, manyworkers have suggested that itis the largest diameter of thethallus is the most appropriatemeasure. A major drawback ofthis method, however, is thatindividual lichen thalli canfuse together and therefore,be recorded as a single thallus(Armstrong, 1984). Otherworkers have averaged thelongest and shortest axes orused both measurements toderive estimates of the surfacearea of the thallus (Griffey,1977). In addition, varioussampling methods have beenemployed. Either the singlelargest thallus at the site ismeasured and regarded asrepresentative of age (Webber& Andrews 1973) or several ofthe largest deposits presentare averaged (e.g., Mathews,1974). Some workers haveused the frequencydistribution of thallus size asan indicator of the age of thesubstratum but the problem ofusing such distributions fordating is that they can beinterpreted in several differentways (Innes, 1985).

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Fig. 2: The growth curve of the lichen Rhizocarpon geographicumdetermined by direct measurement. A plot of radial growth rate(RGR) versus thallus diameter on rock surfaces in north Wales.

Establishing a lichengrowth curve

A lichen growth curve foran area can be established bytwo methods. First, directly bymeasuring the radial growthrate of the lichen over aperiod of time or indirectly, bymeasuring the diameter oflichen thalli on surfaces ofknown age. The indirectmethod is regarded as themost useful since it integrates’the effects of climatic changeon lichen growth over longperiods. By contrast, directmeasurements relate only tothe rates of growth over aparticular, usually short,period of time. An example ofthe growth curve of the lichenRhizocarpon geographicumobtained by directmeasurement is shown in Fig2. The growth curve appearsto be non-linear, an earlyphase of increasing growthreaching a maximum in thalliabout 2.5 – 4.5 cm diameterand then a phase of declininggrowth.

A variety of sources canprovide information for datinga substratum and establishinga growth curve includinggravestones, mine spoil heaps,abandoned farms or houses,and stone-walls or cairns. Inaddition, natural deposits canbe used that have been datedaccurately from historicalevents, radiocarbon dating, orthe use of tree growth rings(dendrochronology). Anexample of a lichen growthcurve relating lichen size andage derived in southeastIceland is shown in Fig. 3(Bradwell, 2001). This growthcurve is also non-linear inshape. The relationship isoften described best by athird-order polynomialfunction and which describesthe declining growth rateapparent in larger andtherefore, older thalli, also afeature of the growth curvederived by directmeasurement. This is a featureof many Rhizocarpon growth

curves and could be due tothallus senescence(Armstrong, 1983). Anotherpossibility is that the curvereflects a tendency for morerapid growth to have occurredin the last 100 years, i.e.,environmental change overthe last 300 years could haveaccounted for variations inlichen growth over time.

There are manyuncertainties in lichenometricstudies resulting from a lackof knowledge of fundamentallichen biology (Worsley,1981). For example, there arelimited data on the nature oflichen colonisation and on theinfluence of environmentalfactors on lichen growth rates.Moreover, there is a lack ofreproducibility of manysampling designs, unresolvedquestions relating to theadequacy of using thallusdiameter as an index of age,and many other difficultiesassociated with themethodology (Innes, 1985).Despite these problems,lichenometry, in combination

with other methods, hasproved to be an extremelyvaluable tool in dating thesurfaces of deposits.

Applications oflichenometry

Lichenometry has beenused in many differentcontexts to date surfaces. Thedating of the sequences ofrocks forming glacialmoraines has been the mostwidely used application. Porter(1981) working on themoraines of Mount Rainier inWashington State, USA, (Fig.4) used historical evidence forthe past marginal positions ofthe glaciers and buildingstogether with other structuresin the National Park to providesurfaces of known age from1857 to the present. Thesingle largest circularRhizocarpon geographicumthallus was used as a measureof lichen growth. The studyindicated that the moraineprobably stabilised four yearsearlier than had beensuggested by other studies.

Other applications oflichenometry including thedating of the stone images onEaster Island, stone-walls inEngland, river flooding, sea-level changes, and theoccurrence of landslides(Innes, 1985).

Climatic change

During the last 25 years,there has been increasingconcern as to the impact ofpossible climatic change andespecially global warming dueto the ‘greenhouse effect’.Lichenometric studies haveplayed an important role inthis debate in providing datathat support global warming.McCarroll (1993) used thesize frequency distribution oflichen populations todetermine the ages ofboulders resulting fromavalanches in south Norway.Snow avalanches reflectperiods of high winter snowand rapid spring meltingrather than low temperatures.It was concluded thatincreasing avalanche activityin south Norway wasattributable to the greenhouseeffect. Harrison andWinchester (2000) using acombination of lichenometryand dendrochronology studiedthe 19th and 20th centuryfluctuations of glaciers insouth Chile. There wasevidence for retreat of theglaciers following the ‘little iceage’ maximum between 1850and 1880. Glacier retreatincreased during the 1940sand the degree of synchronyexhibited by different glaciershas suggested a commonclimatic influence. Thispattern has been repeatedaround the world. In centralAsia, there were some smalladvances of the glaciersduring 1908-11, 1911-34 and1960-77 (Narama, 2002).However, significant recessionalso occurred in the 1900sespecially in 1911-34 and1977-78. Recession thenaccelerated after 1990reflecting recent climatic

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Features

Richard A ArmstrongVision SciencesAston University

References

■ Armstrong, R A. (1983) New Phytol 94, 619-622.

■ Armstrong, R A. (1984) New Phytol 98, 497-502.

■ Armstrong, R A. (2003) Microbiologist Vol 4:4, 30-33.

■ Armstrong, R A. and Smith, S.N. (1987) Symbiosis 3,287-300.

■ Bradwell, T. (2001) Geografiska Ann 83, 91-101.

■ Griffey, N J. (1977) Norsk geografisk Tidsskrift 31, 163-172.

■ Harrison, S and Winchester, V. (2000) Arc Antarc Alp Res 32: 55-63.

■ Innes, J L. (1985) Prog Phys Geog 9, 187-254.

■ Locke, W W., Andrews, J T. and Webber, P J. (1979) Brit. Geomorph.Res. Group Bull 26.

■ Mathews, J A. (1974) Norsk geografisk Tidsskrift 28, 215-235.

■ McCarroll, D (1993) Earth Surf Proc Land 18, 527-539.

■ McCarthy, D P. (1999) J Biogeog 26: 379-386.

■ Narama, C. (2002) Catena 48, 21-37.

■ Oerlemans, J. (1994) Science 264, 243-245.

■ Porter, S C. (1981) Arct Alp Res 13, 11-23.

■ Worsley, P. (1981) In: Goudie A (Ed.) Geomorphological techniques,London, 302-305.

Fig 4: The terminal moraine of Emmons glacier, Mount Rainier,Washington State, USA. The boulders deposited by the retreatingglacier are clearly visible and after a certain lag time becomecolonised by crustose lichens.

warming in inland Asia.Records of glacier fluctuationare now compiled by theWorld Glacier Monitoringservice and have derivedestimates of global warmingduring the last 100 years(Oerlemans, 1994).Examination of data from allover the world has confirmedthat the retreat of glaciers hasoccurred over the globe andcan be explained by a linearwarming trend of 0.66 Kelvinper century (Oerlemans,1994).

Conclusions

Lichenometry is one ofmany techniques nowavailable for estimating theelapsed time since theexposure of a substratum. Itsadvantages include an abilityto date surfaces during thelast 500 years, a time intervalin which radiocarbon dating isleast efficient, and provides aquick, cheap, and relativelyaccurate date for asubstratum. Nevertheless,there are many problems

Fig 3: A growth curve of the lichen Rhizocarpon geographicumfrom southeast Iceland determined by lichenometry. The non-linear form of the curve is a notable feature of lichen growthcurves from many sites around the globe.

associated with themethodology (Innes, 1985)with some authors (e.g.McCarthy, 1999) concludingthat because of suspectmethodology, mostlichenometric ages are not infact verifiably accurate.Improvements in methodology,the adoption of carefulsampling regimes, andincreased knowledge of theecology of lichens are likely toimprove accuracy andreliability in future (Innes,1985). Lichenometry, togetherwith complimentarytechniques, is likely tocontinue to play an importantrole in dating a variety ofsurfaces and therefore inproviding data that contributeto the debate regardingclimatic change and globalwarming.