Gregg Müller | Sources: Gregg Müller On Muller on Lichenometry and Environmental History | Environmental History, 11.3

sources

GREGG MÜLLER ON MULLER ON LICHENOMETRY AND ENVIRONMENTAL HISTORY

FEW PEOPLE WOULD connect the splashes of color of lichen crusts on rock with unraveling the history of landscapes, yet the meek but intriguing lichen can help researchers tell the stories of places.

Perhaps the most well-known application of lichens in environmental study has been their use as indicator species for air quality, particularly in northern Europe.¹ Less well known is lichenometric dating, which essentially uses the size of lichens to date surfaces. If the relationship between the age of a lichen and its size is known, then the minimum age of the surface on which a lichen is growing can be inferred.

Lichens can live for a remarkably long time (perhaps as long as 3,ooo years) and can grow in environments that are too harsh for most other organisms.² The primary application of lichenometry has been in the dating of relatively recent (less than 1,000 years) glacial and alpine landscape features where the time spans involved—and the lack of other suitable dating techniques such as dendrochronology (tree rings), radiometric carbon (carbon dating), or human artifacts—make dating problematic.³

The most common application of the technique has been in dating naturally occurring surfaces—rockfalls, glacial moraines, mud flows, flood regimes, periglacial surfaces, and other geomorphic features. There has been curiously little application in the fields of archaeology, anthropology, and history even though anthropogenic surfaces and historical objects are often indispensable in establishing lichen growth rates.⁴

My own work has focused on dating mining remains and earthworks associated with gold mining in the Box-Ironbark forests around Bendigo in central Victoria, Australia, considered to be a historical landscape of national and in-ternational significance.⁵ The mining sites are located in relatively dry, open forest, with low relief, irregular rainfall and hot summers, and are about as distant from arctic glaciers as one can imagine, but they share a number of char-acteristics with alpine environments in respect to dating. The relatively short time frames involved (about 150 years) and the lack of organic material make radiocarbon dating problematic. The variability of the climate means many of the native trees in central Victoria do not exhibit annual rings, so dendrochronology cannot be used. The sequence of mining exploitation further complicates the issue.

In the mid 1800s, hundreds of thousands of miners from all over the world descended on these gold fields, clearing the forests, turning over the soil, washing, panning, sluicing, digging deep mines, throwing up hills of "mullock" and in general disturbing the ground surface over huge areas. The introduction of industrial scale mining and the working and reworking of the fields up to the present day has resulted in a palimpsest of remains, where even small areas may have overlapping layers of early small-scale alluvial workings, deep mining, hydraulic sluicing, and recent fossicking (small-scale searching for gold, often reworking old mining sites, now usually undertaken with metal detectors or by panning).

In spite of the authorities' bent for record keeping, much of the early mining activity went unrecorded in any detail, and subsequent reworking of the diggings, much of it during times of economic downturn among poor rural workers who turned their hands to the search for a bit of "color" to supplement other income, has resulted in a patchwork of activity and little or no documentation.⁶ This lack of documentation has been exacerbated by the disregard in which these forests have been held, and it is only relatively recently that the history and significance of "the forgotten forests" has attracted serious attention.⁷

Artifacts, water races, walls, shacks, and other traces of activity are scattered beneath the regenerated woodlands and forest. Archaeological and historical research in these settings often relies upon relative dating techniques based upon the presence or absence of artifacts, and evidence of mining styles, and can only give broadly indicative dates.⁸ Often building materials and mine machinery were scavenged and recycled to new mining sites, increasing confusion for the historian. But as the forests have regrown, so lichens have colonized the bare rock surfaces left by miners, providing the material for lichenometric dating.

A LICHENOMETRIC APPROACH

AS NOTED, LICHENOMETRIC dating relies on establishing a relationship between the size of a lichen and its age. The growth rate of lichens is rarely measured directly because of the long time frames required, so researchers usually rely on using control surfaces of known age—gravestones, buildings, embankments, rock walls, and the like—or well-dated geomorphic events such as ava-lanches or landslides—to establish growth rates. Researchers then construct a growth curve for that partic-ular lichen and locality.

A number of meth-odologies exist for data collection and analysis, but they essentially fall into two groups. The first approach seeks to find the largest single lichen on the surface, which is considered to be the oldest lichen growing in the most advantageous location, representing the earliest lichen to colonize the surface. The age of the surface is estimated by referring to a maximum growth curve calculated from control surfaces for that particular lichen taxon which yields a minimum age for the surface (see Table 1).

Table 1. Growth of Buellia Albula, Cement Grave Monuments, Bendigo Cemetery, n=122.

Table compiled by author.

The second tech-nique, which relies on large surfaces ( greater than 100m2), takes a population approach. A large single age surface is divided into many small sub-plots, usually individual boulders on a moraine or talus slope (or mine spoil heap). The biggest lichen within each of the sub-plots is recorded and a fre-quency histogram of largest lichen sizes constructed. This will usually have a normal distribution. The peak of the distribution yields the "average" (strictly the modal) size of this group of largest lichens (see Table 2). This data can be used to construct growth curves, and in calculating the age of unknown surfaces in a similar manner to the first method. Recent researchers prefer this approach since it is amenable to stricter statistical testing. The choice depends on the needs of the researcher and on the area of the surface to be dated—small surfaces are unlikely to have sufficient lichens to allow a population approach.

Table 2. Size-Class Frequency, Rhizocarpon sp. Lichens.

Table compiled by author.

Rocky Valley Dam wall, Bogong High Plains. The largest lichen was measured on 190 boulders on the dam wall (35 years of growth). The most likely size of a lichen of this taxon to be found on a surface 35 years old is therefore 10.5 mm.

The beauty of the frequency analysis approach is that it can be used to establish the dates of multi-aged surfaces. If two events separated in time have occurred, and the more recent activity has not totally obscured the older surface, the distribution of largest lichens will have two peaks (usually a larger, younger peak, and a subsidiary, older peak). While this technique has not been tested in historical contexts, geomorphological studies indicate that it shows promise. The technique also may have application in the dating of wildfires and other lichen extinction processes, such as floods and changes in snow cover.⁹

Figure 1. Lichen Encrusted Drystone Wall.

Photo courtesy Gregg Müller.

Lichenometric dating technique focuses on saxicolous (rock growing) lichens and requires that the species be relatively slow-growing and generally circular in outline, have sharp borders, be sufficiently abundant, and have a geographical distribution and ecological niche suitable to the study. In spite of these limitations over fifty taxa of lichen have been used in lichenometric dating.¹⁰

Each species of lichen grows at a different rate depending on the local environment, substrate, aspect, and climate. In the case of alpine or arctic environments, growth rates and lichen taxonomy are fairly well established, but in other areas well-dated control surfaces are required. The classic source for control surfaces is a cemetery, since it offers a range of rock types often of local provenance, well-dated surfaces (headstones), regular orientation of growing surfaces, and environmental uniformity.

In spite of their beauty, diversity, adaptability, and intriguing biology (they are not plants, nor even individual organisms, but a symbiosis between a fungus and alga) lichen do not fit into the "charismatic flora" class, like giant trees, rare orchids, or colorful wildflowers. In fact, the complexity of their biology makes identification and taxonomy difficult. In Australia this is compounded since they are relatively little-studied, and much of the reference material is held in overseas collections, a legacy of Australia's colonial past (but that's another story).

The central Victorian project has started from a low knowledge base, initially demonstrating that the technique works on known artifacts (gravestones in Bendigo cemetery).¹¹ A considerable number of undated grave monuments occur in the three cemeteries in Bendigo, and the technique has been successfully applied to ascribe indicative dates to the monuments. Field work is continuing on a number of lichen taxa growing on a range of surfaces including cement, quartz, and the mudstone country rock.

While the dating of mining sites has considerable interest for those intrigued by mining landscapes, there are wider implications for environ-mental historians. There is considerable debate among foresters, ecologists, and historians about the original composition of the Box-Ironbark forests. Many eucalypt species will resprout from the stump when felled, and the "forgotten forests" are now primarily composed of multi-stemmed coppice regrowth as a result of the substantial clearing that took place for firewood, building, mine construction, and charcoal burning during the gold rushes. (One Tree Hill, part of the Greater Bendigo National Park, now clothed in forest, bears testament to the efficiency with which this clearing was carried out.) Dating of individual mining sites will give valuable insights into the timing of local clearing events, informing modern forest and conservation management practices.

Figure 2. Lichen Encrusted Granite Gravestone, Bendigo Cemetery.

Photo courtesy Gregg Müller.

Geomorphological studies have used a broad range of artifacts and anthropogenic surfaces as control surfaces, including Native American game fences in Colorado, dry stone walls in Wales, zeppelin anchors in Spitsbergen, and cemeteries across four continents. The application of the technique in dating movements in standing stone circles in Britain and fish net racks in Sweden suggests a wider possible application for the technique in historical and archaeological studies.¹² Recent work in the Sierra Nevada indicates that the technique may have application in arid environments if suitable taxa and control surfaces can be found.¹³

Greg Müller is associate lecturer in the School of Outdoor Education and Environment, La Trobe University, Bendigo, Victoria, Australia. His major areas of research are environmental history and management, and the ecological effects of recreation.

NOTES

^1. Margalith Galun and Reuven Ronen, "Interaction of Lichens and Pollutants", in CRC Handbook of Lichenology, ed. Margalith Galun (Boca Raton, Fla.: CRC Press, Inc. 1988), 55–72.

^2. J. B. Benedict, "Recent Glacial History of an Alpine Area in the Colorado Front Range, U.S.A.: Establishing a Lichen-Growth Curve," Journal of Glaciology 6 (1967): 817–32.

^3. John A. Matthews, "Lichenometric Dating: A Review with Particular Reference to 'Little Ice Age' Moraines in Southern Norway," in Dating in Exposed and Surface Contexts, ed. Charlotte Beck (Albuquerque: University of New Mexico Press, 1994), 185–212; J. L. Innes, "Lichenometry," Progress in Physical Geography 9 (1985): 187–254.

^4. Matthews, "Lichenometric Dating"; Innes, "Lichenometry"; J. L. Innes, "The Use Of Lichens In Dating," in CRC Handbook Of Lichenology, ed. Galun , 75–91; G. S. Maas and M. G. Macklin, "The Impact of Recent Climate Change on Flooding and Sediment Supply within a Mediterranean Mountain Catchment, Southwestern Crete, Greece," Earth Surface Processes and Landforms 27 (2002): 1087–1105; K. P. Timoney and J. Marsh, "Lichen Trimlines in Northern Alberta: Establishment, Growth Rates, and Historic Water Levels," The Bryologist 107 (2004): 429−40; V. Winchester, "An Assessment of Lichenometry as a Method for Dating Recent Stone Movements in Two Stone Circles in Cumbria and Oxfordshire," Botanical Journal of the Linnean Society 96 (1988): 57–68; N. D. Broadbent and K. I. Bergqvist, "Lichenometric Chronology and Archaeological Features on Raised Beaches: Preliminary Results from the Swedish North Bothnian Coastal Region," Arctic and Alpine Research 18 (1986): 297–306.

^5. M. Pearson, J. Lennon, and D. Marshall, Heritage Action Plan Castlemaine Diggings National Heritage Park (Melbourne: Parks Victoria, 2002).

^6. D. Bannear, Historic Mining Sites in The Castlemaine/Fryers Creek Mining Divisions, in North Central Goldfields Project (Melbourne: Department of Conservation and Natural Resources North West Area, 1993).

^7. J. Lennon, Case Study of the Cultural Landscapes of the Central Victorian Goldfields, in Australia: State of the Environment Technical Paper Series (Natural and Cultural Heritage) (Canberra: Department of the Environment, 1994); J. Calder, and I. McCann, The Forgotten Forests : A Field Guide to Victoria's Box and Ironbark Country (Melbourne: Victorian National Parks Association, 1994).

^8. Bannear, Historic Mining Sites in the Castlemaine, 68.

^9. W. B. Bull and M. T. Brandon, "Lichen Dating of Earthquake-Generated Regional Rockfall Events, Southern Alps, New Zealand," Geological Society of America Bulletin 110 (1998): 60–84; W. D. Erskine, N. Terrazzolo, and R. F. Warner, "River Rehabilitation from Hydrogeomorphic Impacts of a Large Hydro-Electric Power Project: Snowy River, Australia," Regulated Rivers: Research and Management 110 (1999): 3–24; P. E. Carrara and J. T. Andrews, "Problems and Application of Lichenometry to Geomorphic Studies, San Juan Mountains, Colorado," Arctic and Alpine Research 5 (1973): 373–84.

^10. Innes, "The Use Of Lichens In Dating," 75–91.

^11. Greg Müller, "Growth of the Lichen Buellia Albula and its Potential for Lichenometric Dating in South-Eastern Australia," Geographical Research 43 (2005): 267–73.

^12. On Native American game fences, see Benedict, "Recent Glacial History," 817–32; on zeppelin anchors in Spitsbergen, see A. Werner, "Lichen Growth Rates for the Northwest Coast of Spitsbergen, Svalbard," Arctic and Alpine Research 22 (1990): 129–40; on standing stone circles in Britain, see Winchester, "An Assessment of Lichenometry," 57–68; on fish net racks in Sweden, see Broadbent and Bergqvist, "Lichenometric Chronology," 297–306.

^13. Pearson, Lennon, and Marshall, Heritage Action Plan.

Content in the History Cooperative database is intended for personal, noncommercial use only. You may not reproduce, publish, distribute, transmit, participate in the transfer or sale of, modify, create derivative works from, display, or in any way exploit the History Cooperative database in whole or in part without the written permission of the copyright holder.