Dendrochronology and Lichenometry: Colonization, Growth Rates and Dating of Geomorphological Events on the East Side of the North Patagonian Icefield, Chile
Total Page：16
File Type：pdf, Size：1020Kb
Geomorphology 34Ž. 2000 181±194 www.elsevier.nlrlocatergeomorph Dendrochronology and lichenometry: colonization, growth rates and dating of geomorphological events on the east side of the North Patagonian Icefield, Chile Vanessa Winchester a,), Stephan Harrison b a School of Geography, UniÕersity of Oxford, Oxford OX1 3TB, UK b Centre for Quaternary Science, CoÕentry UniÕersity, CoÕentry CV1 5FB, UK Received 26 May 1999; received in revised form 13 December 1999; accepted 16 December 1999 Abstract This paper highlights the importance for dating accuracy of initial studies of delay before colonization for both trees and lichens and tree age below core height, particularly in recently deglaciated terrain where colonization and growth rates may vary widely due to differences in micro-environment. It demonstrates, for the first time, how dendrochronology and lichenometry can be used together in an assessment of each other's colonization and growth rates, and then cross-correlated to provide a supportive dating framework. The method described for estimating tree age below core height is also new. The results show that on the east side of the North Patagonian Icefield in the Arco and Colonia valleys, Nothofagus age below a core height of 112 cm can vary from 5 to 41 years and delay before colonization may range from a maximum of 22 years near water to a minimum of 93 years on the exposed flanks of the Arenales and Colonia Glaciers. Tree age plus colonization delay supplied a maximum growth rate of 4.7 mmryear for the lichen Placopsis perrugosa and lichen colonization is estimated to take from 2.5 to approximately 13 years. A minimum lichenometric date of 1883 was estimated for an ice-formed trimline at the junction of the Arenales and Colonia glaciers and a maximum dendrochronological date of 1881 for a water-formed trimline in the Arco valley. Tree and lichen ages around the valley suggest that a glacial outburst drained the 1881 high level lake releasing approximately 265 million cubic metres of water. Repeated flooding, with a minimum of 38 high lake levels, is suggested by horizontal sediment lines on the Arco valley walls and moraine flanks. Dating confirmed diminishing flood levels with a last minor flood in 1963. The wider significance of the work is that it should produce more accurate dating of recent glacier fluctuations around the North Patagonia Icefield, an area where dated reference surfaces are extremely scarce. q 2000 Elsevier Science B.V. All rights reserved. Keywords: dendrochronology; lichenometry; colonization; growth rate; trimline; dating ) Corresponding author. Fax: q44-1865-559560. E-mail address: [email protected]Ž. V. Winchester . 0169-555Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0169-555XŽ. 00 00006-4 182 V. Winchester, S. HarrisonrGeomorphology 34() 2000 181±194 1. Introduction the shrinking margin of the same glacier and Villalba et al.Ž. 1990 found a similar optimum rate for the This paper describes the little-explored valleys of species below the Frias Glacier on the northeastern the Arenales, Colonia, and Arco Glaciers on the flank of Mt. Tronador, 600 km north of San Rafael, eastern side on the North Patagonian Icefield, south- with colonization varying from 5±10 years in shel- XX ern Chile, 47816 S, 73813 WŽ. Fig. 1 . It shows how tered spots to 67 years on the exposed valley bottom. dendrochronology and lichenometry can be used to- MercerŽ. 1970 estimated a 70-year delay for gether to date environmental changes and illuminate Nothofagus when working on the western flank of the scale of events following deglaciation. The work the Southern Icefield, while SwedaŽ. 1987 , studying initially focuses on an investigation of the effects of tree age around the Soler Glacier on the eastern side environmental variations on tree age below core of the Northern Icefield, found colonization delays of height, colonization delays, and lichen growth rates. 24 to 30 years on the northern valley side, but 50 This type of investigation, often neglected in previ- years on the valley bottom where there had been a ous geomorphic studies, is mandatory; without it, fire. The longest colonization delay was observed by dating with dendrochronology and lichenometry Nichols and MillerŽ. 1951 who found no trees on the could be seriously misleading. 80-year-old `Little Ice Age' moraines at the foot of Dendrochronological surface exposure dates are the Ameghino Glacier on the eastern side of the derived from tree age, with age calculated by adding Southern Icefield. the annular ring counts taken from cores to an The wide divergence in tree colonization rates estimate of age below core height and years before means that any generalized application of den- colonization. Accurate calculation of all these param- drochronology in this region is inappropriate and, as eters is particularly necessary in areas of recently Warren and SugdenŽ. 1993 emphasized, there is a deglaciated terrain where growth can be slowed by need for detailed case studies. The aim of the present cold katabatic winds, shallow soil on moraine crests, study is to address this need with respect to both extremes of heat and cold, drought and flooding, or dendrochronology and lichenometry which latter late-lying snow on valley bottoms. Following technique relies on accurate estimates of lichen deglaciation, soils are initially sterile and unstable, growth rates. This paper shows how tree age below and potential seedbeds may be distantŽ Matthews, core height and colonization delay and lichen growth 1992. rate and colonization delay can be investigated using Around the North and South Patagonian Icefields, a minimal dating framework to supply an outline there have been no previous studies of the number of control for the local variables affecting growth. It years of tree growth below coring height and tree concludes by using tree and lichen dating to show colonization rates have only been examined in a the scale and sequence of events in the valleys. single study beside the San Rafael Glacier on the northwestern side of the Northern IcefieldŽ Winches- ter and Harrison, 1994. There have been, however, 1.1. PreÕious exploration several studies of colonization showing wide vari- ability. HeusserŽ. 1964 believed that Nothofagus Three previous expeditions in 1958, 1963 and seedlings take about 25 years to become established 1979, have explored the Colonia and Arco valleys. near the San Rafael Glacier. Alternatively, Winch- The first and only one to visit the Arco valley was ester and HarrisonŽ. 1994 used Warren's Ž. 1993 the ExpedicionÂÂ Chileno Japonesa Andes Patagonicos observations to estimate a minimum 6-year delay lead by TanakaŽ. 1980 . Tanaka reported local settlers before Nothofagus colonized bedrock cracks above as saying that between 1920 and 1958, there were Fig. 1. Map of the field area with surface exposure dates provided, in this study, by lichenometry and dendrochronology and glacier ice front positionsŽ. numbered and dashed . V. Winchester, S. HarrisonrGeomorphology 34() 2000 181±194 183 184 V. Winchester, S. HarrisonrGeomorphology 34() 2000 181±194 floods in the Rio Colonia valley every yearŽ except 2. Study area in 1954. between the end of December and January. These lasted around three days, with water rising to a In the Arco valley, the most important features for maximum height of 7 m above the normal summer this study are the height and position of the various level. Icebergs were, apparently, a common summer moraines and a horizontal trimline marking a change feature before 1928, but after this date, they were in vegetation type and densityŽ. Fig. 1 . There are only seen during floods. An outburst flood had oc- three main moraines in the valley: a 25 m high curred just before Tanaka arrived in 1958 and he recessional moraine lies across the glacier foreland, a photographed stranded icebergs halfway up the distal 90 m high terminal moraine defines the eastern end flank of the terminal moraine at the foot of the Arco of Lago Arco, and on the lake's southern shore is a valley. The 1963 expedition, led by Shipton, de- 40 m high lateral moraine bordering a fluvioglacial scended the Colonia Glacier having crossed the ice- terrace. The trimline, lying approximately 120 m field from the northwestŽ. Shipton, 1964 . The pur- above the lake's surface and 30 m above the crest of pose of the 1979 expedition was to study the geology the terminal moraine, runs along both sides of the of the region, with the party reaching the valley from lower valley as far as two rock spurs projecting from Rio Nef and traveling down the east side of the the valley wallsŽ. Fig. 2 . Immediately east of the Colonia valleyŽ Yoshida, 1981, and personal com- northern spur, a truncated debris cone is cut by the munication. line, with secondary forest growing below and ma- Fig. 2. View looking north across Lago Arco showing:Ž. 1 trimline, Ž. 2 sediment lines marking high lake levels on the terminal moraine, Ž. 3 debris flow,Ž. 4 rock spur, Ž. 5 Arco Glacier. V. Winchester, S. HarrisonrGeomorphology 34() 2000 181±194 185 Fig. 3. The east face of Cerro Colonia bluffŽ. 1 showing rapid southward decline in the height of deposits left by the Colonia Glacier Ice during the `Little Ice Age' and a residual moraine fragmentŽ. 2 on the outwash plain. ture forest growing above it. The mature forest is Cerro NegroŽ. also named by us river outlet. Recent characterized by large trees with an extremely dense, ablation and push moraines containing large masses shrubby understorey growing up between fallen of dead ice lie between the latter two calving fronts trunks; the secondary forest contains even-sized, filling the centre of the outwash plain. Lateral closely spaced trees with almost no under storey or moraines line the Colonia valley walls and residual fallen trunks.