Gathering the Laurels

I grew up enjoying the site of mountain-laurel blooming in profusion each June. It is a common understory shrub in the Hudson Highlands, one of the northeast-southwest-trending chains of mountains that make up the northern Appalachians through downstate New York. Kalmia latifolia is an evergreen member of the Ericaceae (heath family), but it is definitely most noticeable when it flowers. The pink-tinged white blossoms are arranged in corymbs; the petals are attached to form pentangular bowls. The corymbs are several inches across; each flower is about an three-quarters to an inch in diameter. It is a showy plant.

Mountain-laurel in flower in Maryland.
Mountain-laurel in flower in Maryland.

In addition to their large size, the flowers are set against glossy, dark green foliage. K. latifolia has the tendency to form large, continuous thickets, sometimes covering part of an acre on rocky, well drained hillsides. In the southern Appalachians, they can attain the size of a small tree (over 20 feet tall), but in the northeastern states they are usually five or six feet tall.

I haven’t seen any mountain-laurel on Martha’s Vineyard yet, but it does occur here and is likely to be found “up-island” in the towns of West Tisbury, Chilmark, and Aquinnah. The Gay Head and Martha’s Vineyard Moraines create a series of northeast-southwest-trending ridges that form the upland portion of the island. The terrain is dotted with large boulders (especially in the Gay Head Moraine areas) and was apparently originally forested with more beeches and maples (Northern Hardwood assemblage), tempering the dominant Oak-Heath presence that still dominates the down-island portion of the island, which a sandy outwash plain formed by the erosion of the moraine.

Sheep-laurel. (Photo: Bob Cunningham)
Sheep-laurel. (Photo: Bob Cunningham)

The most common member of the heath family in the outwash plain is the sheep-laurel, Kalmia angustifolia. This is a much smaller plant than K. latifolia, getting only about three feet all, but it has the same tendency to grow in extensive thickets. It is a common part of the Oak-Heath assemblage wherever it is found. It has evergreen leaves that hang down at a sharp angle. As its trivial name implies, the leaves of sheep laurel are narrow. It can also be distinguished from other Kalmia species because its branches end in terminal whorls of leaves, while the flowers emerge from the stem beneath the terminal leaves. The flowers resemble those of mountain laurel, but are a deep pink and much smaller.

The third member of the genus that is found in the northeastern U.S. is Kalmia polifolia, the bog-laurel. While the other two species are associated with mesic or even dry habitats, K. polifolia lives up to its name and is strongly associated with hydric conditions, although not necessarily bogs. K. polifolia is distinguished from K. angustifolia by the position of its flowers, which are terminal rather than growing further down the stem like the sheep-laurel’s. K. polifolia also blooms in the spring (April or May) rather than in the early summer.

Bog-laurel
Bog-laurel

All Kalmia prefer acidic soil conditions. But while sheep-laurel grows on a variety of sites ranging from wet sphagnum bogs to dry jack pine forests, bog-laurel is confined to wetlands (it is also called “swamp-laurel”). Mountain-laurel, on the other hand, prefers drier sites, but can tolerate moist soils at the edges of wetlands. While the sheep-laurel is widespread on Martha’s Vineyard, the bog-laurel is not found here at all.

The fourth member of the heath family found in the Northeast is Kalmia procumbens, the alpine-azalea, but as its name suggests, it is confined to the tundra-like areas on the higher peaks of New Hampshire and Maine and grows to be only 4 inches tall.

All parts of all members of the genus are poisonous to many mammals. The sheep-laurel gets its name for its morbid effect on that species, but the plant is poisonous to all livestock, as well as to humans. Some mammals, caribou for example, can tolerate it, and many birds depend on the fruits for winter forage.

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Do Red Maples Hide Bogs?

When I was pursuing a Ph.D. at New York University in the late 1980s I was casting about for a dissertation topic and had settled on a project that came about as a result of my being exposed to ground-penetrating radar and pollen analysis. My advisor Jim Mellett had been a vertebrate paleontologist for most of his career, but had begun using the radar machine to begin a second leg of his professional life as a forensic scientist. Cal Heusser, another member of my committee, had been a Quaternary palynologist for his entire career and had done pioneering work in the higher latitudes of both hemispheres.

 

Bog with deciduous and coniferous trees at the edge.
Bog with deciduous and coniferous trees at the edge.

Bogs are not very common below a particular latitude because the conditions that make them abundant—a flat landscape with a drainage deranged by glacial moraine—is not often encountered. During the 10,000 years since the retreat of the ice sheets from the mid-latitudes most of the morainal “dams” that backed up water in valleys had been broken by erosion, leaving water to enter at the upslope side of a pond and depart by the downslope route. Bogs require—among several criteria—that water leave only by evaporation. Bogs can also go away when they fill with sediment and water simply drains across the area they once covered.

It was the latter that particularly interested me. I thought perhaps the present sites of some “red maple swamps” could once have been bogs. The low oxygen conditions at the bottom of bogs insure the preservation of pollen and the sediment record is often continuous. Perhaps there were “lost bogs” under some maple swamps that would provide local climate records of the early Holocene where none presently were known.

Red maple swamp in Maine.
Red maple swamp in Maine.

The ground-penetrating radar is able to distinguish differences in density in sediment layers. I hoped to be able to find the closely packed predominantly vegetative peat layers buried below the mineral-rich shallower layers that had covered them. A series of transits across a maple swamp would determine whether it was candidate for coring and bringing up pollen-laden peat.

Red maples (Acer rubrum) often grow around the edges of bogs because they are tolerant of a wide range of physical environmental conditions. They are called variously “swamp maple” or “water maple” (among other vernacular names) because they thrive in both saturated conditions, but they are also happy on rocky ridges where many trees cannot find purchase. They also grow along side other maples in more moderate conditions, but in less hospitable places they can form nearly solid stands.

Compared to sugar (A. saccharum) and black (A. nigrum) maples red maple is a smaller, less long-lived tree; it does not dominate the canopy where these trees are present in the forest. It is not an understory tree like striped maple (A. pensylvanicum) and is more common along watercourses where it can be found with silver maple (A. saccarhinum) and box elder (A. negundo).

Red maple flowers.
Red maple flowers.

Red maple lives up to its name: the leaf buds and petioles, the flowers, and the samaras (seeds) are all tinged with red. In the fall the foliage is often a brilliant scarlet, while sugar maple tends to be a reddish orange and several other maples are yellow-leaved in autumn. These characteristics (and its tolerance of difficult conditions) have made it a popular street tree and ornamental in designed landscapes. Several cultivars have been developed. (It should not be confused with the marooned-leaved cultivars of Norway maple.)

It seemed likely to me that red maple swamps would be the next successional step as a bog filled in. They are often already present around the margins of existing bogs, their windblown samaras allow them to distribute themselves relatively rapidly, and they don’t mind “wet feet.”

But I didn’t pursue this research project. Instead I decided to “take a master’s” and leave the biology program to join a geology Ph.D. program in Massachusetts, where I ended up studying Pliocene climate using microfossils in deep-sea sediments. I did hear that Prof. Mellett did have a subsequent student who did independently arrive at the same idea. However, I never learned her name or was able to read anything she published on the topic.

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.

beschel
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.

seriesoflichens_large
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.