Dating With Lichens

Lichen-encrusted tombstone
Lichen-encrusted tombstone

A visit to an old graveyard, particularly one that has not been cared for, will generally reveal tombstones covered in lichens. Lichens are composite organisms; they are a symbiotic relationship between a fungus and a green alga (or a cyanobacterium). The fungus provides the physical infrastructure and the algae do photosynthetic duty to supply sugars. The lichen takes a form that resembles neither the fungus nor the alga with the symbiotic partners interpenetrating each other to create a life-form that resembles a primitive plant.

Lichens grow on tombstones because they are adapted to colonizing very dry environments that have very little in the way of nutrients available. They are found on barren rock right up into the polar regions, where the extreme cold adds an additional challenge. In better-maintained cemeteries, the lichens are generally scraped off because they soon begin to obscure the engraving on the stone. In addition, the lichens also chemically degrade the surfaces to which they cling, breaking down the rock into its constituent minerals.

http://www.jon-nelson.com/lichens-unusual-partners
Old-man’s beard (Photo: Jon Nelson)

Lichens have many different habits, but most fall into three categories: crustose, foliose, or fructiose. The most well-known fructiose variety is probably the misnamed “beard moss,” which hangs in great bedraggled mats from spruces in boreal forests, especially along the ocean.

Foliose species are characterized by having leaf-like sheets, often arranged in a rosette pattern. Crustose lichens may look almost painted on to the surface where they are growing or resemble a gray-green stubble.

Austrian botanist Roland Beschel developed lichenometry in the 1950s. He was looking for a method of dating glacial moraines in Alpine valleys. These linear piles of boulders, gravel, and sand stretched across the valleys, marking the location where a glacier had advanced and then remained, its rate of advance equaling the rate at which it was melting back. Eventually the rate of melting exceeded the rate of advance, and the glacier retreated up the valley. Some valleys in the Alps have a whole series of these moraines with the older ones (furthest down the valley) having been deposited before the beginning of written records. Crustose lichens are the slowest growing lichens, and they often grow in regular circles and are therefore relatively easy to measure.

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Roland Beschel

Beschel measured the diameters of crustose lichens that were growing on stone surfaces that could be dated independently. This includes tombstones, as well as very old buildings and bridges for which there are either dated cornerstones or written records for their date of construction. In this way, Beschel established a relationship between the increasing diameter of the lichen and the increasing age of the structure on which it grew. He allowed for a period of approximately a decade between the erection of the structure and the commencement of lichen growth. The next step was to find and measure lichens on rocks that were part of the valley moraines.

Many of these moraines were deposited during the so-called “Little Ice Age,” which came and went episodically from the Renaissance (15th century) through the 19th century. Beschel could measure the diameter of lichens on these natural landforms and then back-calculate an age for them through the curves that he had derived using the man-made structures covered with lichens. In the 60 years since Beschel first published the technique, it has been used throughout the world to date Holocene landforms that were too young to date reliably with radiocarbon and too old to date reliably using historical records.

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Circular lichens

Lichens grow most slowly in the most northern or most southern climates, so the utility of lichenometry is greatest in the polar and subpolar regions. Anyone who wishes to date stone walls, foundations, or other stone structures that have been abandoned and have become encrusted with lichens, needs only to create an age-diameter curve using local dated surfaces like tombstones. It is important to create a age-diameter relationship locally because the rate of lichen growth will vary regionally, giving a different slope to the curve in different locations.

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Hypoallergenic Flowers

It has been found that allergenic plants are often favored by landscapers: ‘School after school is landscaped with the most allergenic plants possible. Even at hospitals I see landscaping so explosively allergenic that it makes me shudder.

Horticulturalist Thomas Leo Ogren, author of Allergy-Free Gardening: The Revolutionary Guide to Healthy Landscaping (www.achooallergy.com)

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Male flower is on the right

Some allergy-sufferers, fearing a reaction, decline to have flowers in their homes or to visit gardens (let alone do the gardening themselves). Pollen is the vector that aggravates the immune system. Ogren points out that landscapers tend to plant male plants (which produce the pollen) because they do not produce fruit, which then has to be cleaned up … by the landscaper.

Species that have male and female flowers on separate plants are referred to as “dioecious,” literally “two houses.” So, for the allergenic person, one option is to plant only female plants in his or her own gardens and harvest only female plants to bring indoors as cut flowers. It also be noted that not all pollen is created equal. Pollen grains are disseminated by many different means.

Bat at a flower
Bat at a flower

Many people will have seen the dramatic stop-action films of bats sipping nectar from tropical flowers and emerging with a snout covered in yellow dust, which they then transport to the next flower. Some birds also carry out this task, as do myriad insects in addition to bees.

Pollen grains that are transported this way tend to be larger and cannot be carried far in the air. They are much less likely to make their way to a human nasal passage from the flower, and therefore not an appreciable hazard to the pollen-allergic. This article at achooallergy.com includes a list of plants that may be safely planted and another (longer) list of plants to avoid. It also lists a number of steps that the allergenic can take to minimize exposure to pollen.

Dahloan Hembre, a Florida resident, was diagnosed with pollen allergies, particularly to oaks and her progressive-minded allergist suggested that she alter the landscaping around her house to reduce the number of allergens present. This meant taking down a beautiful old oak and a pecan, and removing several shrubs. The oak was replaced by a dogwood and the shrubs by azaleas. Her flower beds filled up with snapdragons and daisies. In other words, hypoallergenic does not mean boring or ugly.

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Spray roses

When it comes to sending cut flowers to the allergenic there is also no need to panic. Many quite standard-issue choices are not aggravating to the immune system. For example, roses, especially those that are still tightly budded, are a safe choice. Spray roses are better than long-stemmed roses. In the aster family, dahlias and chrysanthemums are generally not an irritant. In lilies the pollen-bearing anthers are so large and singular that they can be removed, so that the allergen never makes it out of the florist shop. Many florists will fill out an arrangement with baby’s breath. Apparently the double-flowered variety is preferable to the single-flowered. If you are sending potted plants, hydrangeas, begonias, cactuses, and orchids come in many varieties and, as they are insect-pollinated, do not have pollen grains that stay air-borne for long.

Modern molecular genetics is presently engaged in an effort to develop plants of normally allergenic species that are hypoallergenic. Through a process called “gene silencing” the sequence that produces the allergens in pollen is turned off, resulting in a plant that is otherwise normal, but hypoallergenic.

Existing techniques are “post-transcriptional,” which means that the messenger RNA (mRNA) is prevented from producing a protein from the portion of the genome that includes the allergen. All silencing is “epigenetic,” which means that the DNA sequence that builds structures is not altered. Rather, portions of it are prevented from expression for subsequent generations. This has been observed as a natural process, but can also be induced. Current work is focused on crop and weed species, which are widespread and wind-pollinated.

Why Plants Are Where They Are

Why are there different plants in different places? If you revisit a place decades later, will the same plants still be there, only having grown larger? How do plants get where they are? All of these questions (and more) are asked by plant biogeographers. Many students of biology who pursue the subject in school out of a childhood fascination with natural history will be stunned to find out that they have sit through lecture after lecture about photosynthesis and chloroplasts, the parts of the flower, plant genetics and other topics before the course may (or may not) eventually get around to talking about plant distribution.

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Eugenius Warming

The descriptive stage of plant biogeography was largely accomplished in the 19th century by explorers like Alexander von Humboldt and Charles Darwin (on the voyage of the HMS Beagle). Interpretive plant biogeography took the raw data of the explorers and tried to find patterns in it.

Danish botanist Eugenius Warming taught the first university course on ecology at the University of København in the 1880s and presented a summary of the world’s biomes in his landmark work Plantesamfund. Biomes are large-scale collections of ecosystems that have similar environmental conditions (e.g., rainfall), but generally different plant (and animal) species on different continents. The dynamic stage of plant biogeography involves the study of the development and interaction among plants, including the idea of plant succession, the change in the composition of plant communities over time.

Through the late 19th and into the early 20th century, an entire vocabulary was invented to describe newly understood phenomena. Physical environments were described as xeric, mesic, or hydric, depending on the amount of rainfall they received and the amount of water they retained. The plant communities varied accordingly. Series of communities that developed in these environments were called xeroseres, mesoseres and hydroseres, respectively.

If the plants themselves were seen as the driving force in succession, then the evolution of communities was said to be autogenic. That is, the plants were modifying the inorganic environment to make it more suitable for them. For example, pioneer species of plants may secrete chemicals that physical break down rock to begin the process of soil formation. When the physical environment is in control of plant distribution it is referred to as an allogenic plant succession. Locations at environmental extremes (polar, desert) are likely to be examples of this phenomenon.

Pioneer community
Pioneer community

The landscape may be denuded of plants in several ways precipitating the recovery of vegetative cover through the process of succession. The surface of the land may be rendered by barren by erosion (by water or ice), or on steep slopes by mass wasting (i.e. landslides). Geological causes include faulting, mountain-building and volcanism.

Climatic causes include droughts, flooding, snowiness, protracted windiness (which removes soil), and freezing and thawing. Lightning and the ensuing fires can reduce thousands of acres to ash. Other biota may remove some or all plants from the landscape. Grazing by livestock eliminates species that cannot withstand the constant clipping. Insects (locusts, gypsy moths) may defoliate entire plant communities. Beavers may drown acres of plants. Pathogens like chestnut blight may selectively remove dominant species from a community and cause reshuffling of dominance in the remaining species. Plants move into an area in the form of seeds, or as partially pieces of plant that can establish itself vegetatively.

Ecesis is the process of germination, growth and reproduction. Once individual plants are established in this manner, many pioneer species spread by aggregation, spreading like carpets over the landscape. The first plants will compete aggressively for light, water, nutrients and pollinators. Eventually the community reaches a state where it shares the available resources and can perpetuate itself indefinitely. Frederic Clements referred to this as the climax stage.

Beech-maple climax forest
Beech-maple climax forest

Clements originally conceived of each region has having a single (mono-) climax community. Subclimax stages were recognized as being stalled on the way to climax by some limiting factor (wind, water or the lack of an important nutrient). A disclimax stage was produced when an invading factor (e.g. chestnut blight) reversed the direction of seral progression. Clements and his followers eventually recognized polyclimaxes, in which the nature of the climax community was affected by various factors. The concept of an inevitable climax community has been largely abandoned since the 1950s as the ideas of Henry Gleason became accepted. See separate entry.

The Plant Succession Debate

The ecologist community of the 20th century was divided over the nature of plant succession on both an ecological and a geological timescale. During the last glacial advance of the Pleistocene Epoch ice sheets covered nearly all of Canada and the northern third (more or less) of the United States. Beginning approximately 18,000 years ago, these continental glaciers began to melt back, uncovering a landscape that had been entirely denuded of vegetation (not to mention soil). The pattern of re-vegetation of these post-glacial landscapes was the focus of the debate.

Seral stages
Seral stages

Frederic Clements, a botanist with the Carnegie Institution, developed a theory of plant succession in early 20th century that introduced the idea of a “climax community” to the professional lexicon. Clements described a series of intermediate communities (seres) that occupied a given location between the reintroduction of vegetation (pioneer species) and the climax (final) stage. In addition to series beginning with the absence of vegetation, the progression of communities could also start after recovering from a disturbance. The most well-known example of this is “old field succession,” which describes the recovery of a natural landscape on abandoned agricultural land.

Frederic Clements
Frederic Clements

Clements descriptive work was underlain with the belief that the species within communities were inter-adapted, and that all members of a particular seral stage would be reintroduced to a given locality at essentially the same time and come to dominate the place in a unison manner. Clements thought of plant communities as analogous to organisms. In an organism all the organs work together to make the body function; in a plant community all the species work together (associations) to make energy flow the community properly.

Henry Gleason, a younger contemporary of Clements, initially endorsed and employed Clementsian concepts in his early research in the Midwest, but after moving to the New York Botanical Garden, Gleason began to develop a new theory of succession. Gleason first questioned the analogy between an organism and a plant association. His own fieldwork suggested that plants were not found together in a statistically significant manner. Rather, a given plant species’ own ecology determined its distribution, regardless of what other species were present.

Henry Gleason
Henry Gleason

This individualist” model of plant distribution did not gain much ground in the ecology community for several decades. In the 1950s further work by Robert Whittaker began the overturning of the Clementsian model in the professional community, but the Clementsian terminology was used in ecology classes right through the rest of the 20th century. While the plant ecology community works on a timescale of hundreds of years at most, the paleoecology community, employing the sedimentary record, works on a timescale of thousands (to millions) of years.

In addition to the actual organic remains of plant parts (leaves, bark, flowers etc.), plant communities of the past can be reconstructed from fossil pollen. Bogs are a primary source of fossil pollen because they are isolated from other water bodies, and the pollen that collects in them represents only the immediate vicinity and is therefore more likely to represent a single plant community rather an amalgamation of several. However, pollen recovered from small ponds and larger lakes is also used in the absence of bogs and fens.

Thompson Webb of Brown University led a large-scale effort to use pollen records to document the recovery of North American plant communities in the wake of the retreat of Pleistocene ice sheets. Beginning in the 1970s and culminating the 1980s with the COHMAP (Cooperative Holocene Mapping Project), Thompson’s group collected pollen records (dated with 14C with an accelerator mass spectrometer) and mapped the northward migration of plant species over several thousand years.

Pollen map for spruce
Pollen map for spruce

The primary conclusion germane to this discussion is that Gleason’s ideas were borne out by the pollen data. In the aftermath of continental glaciation, Clements climax communities did not march northward together as an association. Instead between the windswept barrenness of the glacial front and the “climax forest community” of the present day, all kinds of non-analogous plant communities occupied the landscape at any given location.