Climatic Change Over the Last Millennium in Tasmania Reconstructed from Ttee-Rings E
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TheHolocene 2,3 (19921pp.205-277 Climatic change over the last millennium in Tasmania reconstructed from ttee-rings E. Cook*,T.Birdt, M. Petersono,M. Barbettif,B. Buckleyn, R. D'Arrigo*and R. Francey** (*Lamont-DohertyGeological Observatory, Palisades, New York 10964,USA; fTasmania Tradesand Labor Council, New Town, Tasmania7008, Australia; "TasmaniaForestry Commission,Hobart, Tasmania7001, Australia; iUniversity of Sydney,Sydney, New South Wales2006, Australia; x*CSlRO Division of AtmosphericResearch, Mordialloc, Victoria 3195,Australia) ReceivedL4 January1.992; revised manuscript accepted 26 May 1992 Abstract Tree-ring widths from millennium-old Huon pine (Lagarostrobosfranklinii) trees have been used to reconstructwann season(November-April) temperaturesback to AD 900 for Tasmania.The reconstructionindicates that the most recent 100 years of climate have been highly unusual,with the coldest and warmest 25-year periods occurring from 1890 to l9L4 and 1965 to 1989, respectively. Although the most recent 25-year period is warmer than any comparable period over the past 1090 years,it is not yet statisticallyunprecedented. Some evidencefor the Medieval Warm Period and the 'Little Ice Age' can be found in the reconstruction, especiallyduring the twelfth and seventeenth centuries respectively.However, the latter is weakly expressedcompared to many records from the northern hemisphere,suggesting that the southernoceans may havesignificantly moderated its effect on Tasmania.Regular oscillationson the order of 30, 56, 80 and 180years in length have been identified in A the reconstructionusing spectralanalysis. These oscillationscannot be easilylinked to climatic forcing HOLOCENE functions related to solar or volcanic activity. Thus, they are best regardedas arisingfrom the internal RESEARCH PAPER dynamicsof the ocean-atmosphere-cryospheresystem. Key words: climatic variations,tree-rings, Lagarostrobos franklinii,Tasmania, Medieval Warm Period, Little Ice Age, spectral analysis. IntrodUCtion One of the few regions of the Southern Hemisphere that has the potential for millennia-long, annual tree-ring chron- Comparedto the availableinstrumental and historical climate ologies is Tasmania,with its temperate rainforests of long- records of the Northern Hemisphere (e.g., Lamb, 1977),the lived conifers.This palaeoclimaticresource was evaluatedin Southern Hemisphere has relatively few high-resolution cli- the 1970s(Ogden, 1978a,btDunwiddie and LaMarche, 1980), matic time series that extend back more than 100 years but never fully exploited. Recently, Cook et al. (199I) re- (Barry, 1-978).Yet, to properly evaluaterecent climatic trends ported on the development of a climatically-sensitiveHuon and fluctuations (e.g., Hansen and Lebedeff, 1987), much pine (LagarostrobosfrankliniiC.J. Quinn) tree-ring chronol- longer recordsare needed(Bryson, 1974).To fulfill this need, ogy from Tasmania that extends back to AD 900. They long paleoclimaticrecords with annual to decadaltime scale demonstratedthat the yearly variations in ring width corre- resolution must be developedfor the Southern Hemisphere. lated well with changes in warm-season(i.e., November- Tree rings (e.g.,LaMarche,1975; Norton et a1.,1989;Villalba, April) temperature. They then went on to draw inferences 1990) and ice cores from high snow accumulationrate areas about the signiflcance of the present-day warming trend (e.g., Morgan, 1985;Thompson el al.,1985; Aristarain et al., apparentin the seriesand speculatedabout the occurrenceof 1990) have demonstratedthe capacity to produce such re- the 'Little Ice Age' (LIA) and Medieval Warm Period cords.However, exceptfor Antarctica and the high Andes bf (MWP) in Tasmania.However, they did not derive quantita- South America, the availability of suitable ice cores is ex- tive estimatesof past temperature from that record. In this tremely low. This essentiallyleaves tree-rings as the primary paper,we describemore fully the developmentand character- source of high-resolution paleoclimatic information in the istics of this tree-ring chronology and extend the analysesof temperatesouthern latitudes. Cook et al, (199t) to the actual reconstructionof Novernber- @ Edward Amold 7992 206 The Holocene 2 (1992) April averagetemperatures for Tasmaniaback to AD 900.In especiallyin subalpine environments.However, it is reason- so doing, we will provide the first well-verified estimatesof able in this climatically-temperateregion to expect radial past temperature changein the Australia-New Zealand sec- growth to commence sometime in late spring (i.e., No- tor of the SouthernHemisphere that extend back through the vember-December) and end in mid- to late-summer (i.e., LIA and MWP as broadly defined by Lamb (1965), Lamb February-March), with final cell wall thickening and lignifica- (1977), and Grove (1988).This researchbuilds upon earlier tion probably occurring in the early autumn months. tree-ring reconstructionsof climate for Tasmania(Campbell, A total of 56 radii from 23 Huon pine trees were used in 1982;LaMarche and Pittock, 1982) and New Zealand (Nor- developing the tree-ring chronology. These included incre- ton, 1987; Norton et al., 7989), but none of these records ment cores from 17 living trees and samplesfrom six dead extend back before 1730. trees killed by a fire in 196L.The oldest tree dated back to AD779. However, for purposes of dendroclimatic analysis and reconstruction,the final chronologywas truncatedbefore The Huon pine tree-ring data AD 900where the samplesize dropped below three trees and sevenradii. The Huon pine ring widths were rigorously cross- The tree-ring data used here were collected from a disjunct dated and measured to a precision of 10.01 mm following stand of subalpineHuon pine growing at an elevation of 950 established methods (Fritts, 1976; Pilcher, 1989). By the metres near Lake Johnston on Mount Read in western convention used to date Southern Hemisphere tree-rings Tasmania (Figure L). Other tree speciesassociated with this (Schulman,1956), the calendaryear assignedto each annual stand are deciduous beech (Nothofagusgunni), pencil pine ring was the year in which radial growth began,e.9., the ring (Athrotaxus cuppressoides),King Billy pine (A. selaginoides), formed in the 1988/89growing seasonwas dated as L988. the putative hybrid A. laxifulia and Diselma archeri, a Among the key yearsused for cross-datingwere a number of 'light shrubby conifer that achievessmall tree status on Mount significantlynarrow rings, numerous rings' (Filion er al., Read. AII of these associatedspecies are natural endemicsin 1986)characterized by poorly lignified, low-densitylatewood 'frost the subalpine forests of western Tasmania. However, until bands,and several rings' (LaMarche and Hirsghboeck, recently, Huon pine was thought tq grow only in more L984) characterized by cell wall damage caused by the temperate rainforestsbelow 700 m elevation (Gibson, 1986). formation of ice crystalsin the newly'formed tracheids.These This belief was shatteredby the discoveryof high-elevation, key yearsare listed in Table L. They do not indicate any clear subalpine standsnear FrenchmansCap (Hickey and Felton, associationwith known explosive volcanic eruptions as has 1988) and Lake Johnston (Peterson, 1990). Although in a been suggestedin some Northern Hemisphere tree-ring re- subalpinelocation, the Lake Johnstonstand has a fully closed cords (LaMarche and Hirschboeck, 1984;Filion et aL.,t986). canopy and extremely dense understory vegetation. Huon Nor is there much evidence for the joint occurrence of pine is extremely shade-tolerantand can exist as a suppressed narrow, light, and frost rings exceptby chancealone. understory tree for decades,if not centuries.Its reproduction Figure 2 showsa plot of the average,smoothed ring widths is principally vegetative although seedlingscan occasionally from AD 900 to 1988 with nonparametric 95% confidence be found. Little is known about the phenologyof Huon pine, intervals estimated using the bootstrap (Cook, 1990). Be- neath it is a plot showingthe changein samplesize per year. important propertiesof the ring- I 45' t46' t47" 148' Theseplots illustrate several width data used in developing the standardized tree-ring chronologyfor climatic reconstruction.With regardsto non- 100km climatic growth trends related to tree size, age,and canopy status, there is little evidence for the classical negative (Fritts 41" exponential trend in ring width et a1.,1969), except perhaps after 1400. The overall growth trend is broadly convex with a peak in growth occurring around 1300.This pattern of growth justifies the use of more data-adaptive methods of tree-ring standardizationas advocatedby Cook (1987), specifically the use of the cubic smoothing spline (Cook and Peters, 1981). In this case,it is apparent that a smoothingspline with a fixed 50% frequencyresponse cutoff 410 of 500 years will remove little else than the trend in growth attributable to changing tree size, age, and canopy status. With regard to sample size,the maximum replication occurs in the middle of the data and is fairly constantfrom 1300to 1900.The decreasein sample size after 1900 is due to two factors: the inclusion of dead trees in the compositethat all end no later than 1958,and severegrowth suppressionin the 1898-1913period in some living trees that made their ring widths, in that interval, undatableand unmeasurable.Prior to 1300,the decline in samplesize is simply due to the decreas- ing number of very old trees in the sample. For the purpose of later climatic