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Georg von Arx

Forschungsanstalt WSL

Swiss Federal Research Institute WSL
Zürcherstrasse 111
CH - 8903 Birmensdorf

Tel: +41 44 739 23 16

Michèle Kaennel Dobbertin

Forschungsanstalt WSL

Swiss Federal Research Institute WSL
Communication WSL
Zürcherstrasse 111
CH - 8903 Birmensdorf

Tel: +41 44 739 25 96


Author(s): Michèle Kaennel Dobbertin, Georg von Arx, Daniel Nievergelt et al.
Editorial office: WSL, Switzerland
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A 700-year-old larch tells its story

For many human generations, a larch stood in the Goms valley, in the Swiss canton of Valais, until it had to be felled in 1987 for safety reasons. All that this tree lived through in 700 years is written down in its tree rings. Tree-ring scientists from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL have the tools and the know how to decipher the archived information and be the larch's interperters. A chronology.

Stammscheibe der Lärche

The new exhibition "Tree Stories" in the reception area of the Swiss Federal Research Institute WSL in Birmensdorf ZH provides an interactive and vivid insight into the world of tree rings. Parts of the exhibition are virtually accessible.

A larch germinates above the village of Blitzingen in the Valais at 1720 m above sea level.
1291 Foundation of the Swiss Confederation
1300 A giant in its infancy – even a big tree starts small

In den Kinderschuhen

When the larch above the village of Blitzingen was felled, the lower part of its trunk was rotten. Therefore the forester could only take a complete cross section (or "disc") at a height of 6 meters above ground. As a result, the tree rings formed in the early years of the tree's life are missing on this disc.

Based on site-specific growth curves, scientists can still estimate the number of missing annual rings that have fallen victim to rot and thus also the germination year of a tree. The researchers have estimated the number of missing annual rings at 40 on this trunk disc, the centre of which grew in the year 1300. Hence, our tree germinated about 1260.

Six years of catastrophic weather events in Central Europe
The same yet different – proportions of earlywood and latewood vary
Gleich und doch anders

Tree rings can have different colour gradients. The so-called earlywood is formed at the beginning of the year. It is lighter than the latewood because the cells are larger and thinner at the beginning of a growing period than at the end.

The width of a tree ring – and also its early and late wood parts – are determined by many current and previous year factors. Weather conditions are particularly important. In this example, the latewood of 1324 is clearly wider than that of 1325. The higher proportion of earlywood in 1325 could indicate an early start of growth due to mild temperatures in spring.

1337 First weather observations in Oxford
1342 Medieval deluge in Central and Eastern Europe
1347 The Plague in Europe
1356 The Basel earthquake
Massive growth reduction – a fascinating puzzle
Massiver Wachstumseinbruch

A major change is taking place in the immediate surroundings of the larch which will limit its long-term growth. Faced with the growth reduction of single trees, as is the case with this larch, researchers can only make best guesses.

One possibility is that a loss of part of the crown occurred, for example due to an avalanche, a lightning strike or rockfall. This would have caused fewer needles to be available for photosynthesis, less carbon dioxide (CO2) absorbed from the atmosphere and less carbon processed into wood, resulting in narrower tree rings all around the trunk. This, however, is challenged by the fact that no traumatic resin channels – typical reactions to injuries – are visible in the years following growth reduction. In addition, the tree-ring widths would have gradually returned to their previous levels after the crown recovered.

Another possibility is that a sudden change in the underground water supply occurred, which would have permanently restricted water and nutrient availability. Such a change could have been caused by a landslide or the construction of a suone. In Valais, suonen are historical, mostly-open water pipes with a slight slope, leading the waters of torrents and mountain rivers to irrigate fields, meadows and crops. Some suonen were built in the Middle Ages.

Knocked off balance – locally wide tree ringsEinseitig breite Jahrringe

These five wide tree rings on one side of the stem indicate a tilted position, having been slightly knocked over because of a landslide or a storm.

A tilted conifer compensates for the weight shift with the formation of compression wood on the underside of the trunk. Compression wood is darker than normal wood because it consists of cells with thickened walls. It also characteristically consists of clearly rounded cells.

A very hungry caterpillar – ups and downsDer Nadelfrass des Lärchenwicklers

Every nine years, on average, the tiny larch budmoth (Zeiraphera griseana) reproduces explosively. Its caterpillars feed on needle clusters, causing them to wither and turn yellow-brown.

Around every nine years, the number of caterpillars increases by a factor of up to 30,000. Infested trees flush new needles in summer, but grow less. This can be seen in the much narrower tree rings.

Soon, however, the tide turns. In the following few years, trees flush their needles later, and produce shorter and less nutritious needles. Many caterpillars starve to death. In addition, more and more caterpillars are the victims of parasitic wasps. Within a few years, starvation and natural enemies thus regulate the number of larch budmoths, and tree growth returns to normal.

The Gutenberg Bible
A branch breaks off – how the larch closes its wounds
Ein Ast bricht ab

After an injury, the surrounding wood grows over the damaged area. This seals it off from healthy tissue, forming callous tissue. Injuries caused by, for instance, snow breakage, rockfall, fire, lightning or browsing by game, are usually overgrown within a few years.

Through the microscopic analysis of wafer-thin wood slices, researchers usually date these effects to the exact season. However, they cannot always reliably identify the cause of an injury.

Discovery of America

1517 The Reformation
Copernicus: Turning point in astronomy
Mercator draws a large map of the world
Pope Gregory XIII institutes a new calendar
 1620  The Mayflower sets sail
 1625 Hazel growth – wavelets in tree rings

The wavy pattern in tree rings is called "hazel growth". The causes of this particular growth formation are not yet completely understood. Most likely they are genetically determined. Local growth conditions can also favour hazel growth. 

The name "hazel growth" comes from its similarity to hazelnut wood. Spruce wood with hazel growth is particularly coveted because of its balanced and rich sound properties. Musical instrument makers use it for string instruments in particular. In the 17th century, Antonio Stradivari selected spruces with hazel growth from the Foresta dei violini (violin forest) in the Val di Fiemme in the Dolomites, to make his famous violins.

Harvard University built in Cambridge
A good year – high in the mountains, warmer is better
Ein günstiges Jahr

A tree ring that is consistently wide all around the stem indicates a warm summer. In high altitudes, warmth and the number of sunny days are indeed more relevant for growth than precipitation.

When the temperature at the roots reaches at least 5 to 7°C, growth can resume. The larch uses the nutrients it has stored before the winter and produces “earlywood”, the first layer of wood formed in spring.  Earlywood consists of large, thin-walled wood cells so it can ensure the efficient transport of water and nutrients to the tree crown to supply the needles. The later in the year, the flatter and more thick-walled the cells become. This dark and dense latewood mainly ensures the stability of the tree.

1751 Diderot's Encyclopedia
1776 American Independence
1789 Storming of the Bastille
Whimsical – some tree rings only grow partially
Manche Jahrringe wachsen unvollständig

Some tree rings don’t form around the entire trunk. These "wedging rings" are usually caused by poor growth conditions – here, probably by a cold summer or external stress, such as tilting, injury, insect infestation. It can happen that tree rings are formed only in certain areas of the trunk and become increasingly thinner until they eventually disappear.

Wedging rings complicate the exact dating of a sample. Researchers will end up with different values depending on which radius is chosen when counting the annual rings from pith to bark. If researchers only take cores – as is usual when investigating living trees – they risk "missing out" on annual rings. However, since wedging rings don’t occur simultaneously in all trees of a given site, researchers sample cores from as many trees as possible from the same site. They then overlay the growth patterns of each tree to locate missing rings.


A sticky weapon – resin doesn’t give pests a chance
Klebrige Waffe

Strong bending caused by avalanches or wind can cause small cavities under the bark. These occur when the soft, active layer of divisible cells (cambium) is loosened from the wood cells by bending. Cavities then fill with resin to protect the tree against intruders such as insects, fungi and bacteria.

In a healthy larch, resin is transported through channels called resin ducts. These resin ducts are formed by specialized cells - called epithelial cells - which release resin into the duct. Resin has an antiseptic effect against fungi and bacterial diseases.

Napoleonic Code
The year without a summer – consequences of a volcanic eruption
Das Jahr ohne Sommer

After the eruption of the Indonesian volcano Tambora in April 1815, dust and ash spread through the atmosphere across the globe. Low temperatures and prolonged rainfall caused catastrophic crop failures in North America and parts of Europe in 1816 – the year without a summer – and in the following years.

Several other volcanoes erupted in tropical regions between 1808 and 1815. In addition, cooler temperatures prevailed due to low solar activity. Consequently, the summers from 1813 to 1816 were exceptionally cold in the Alps. This cooling of the climate also restricted tree growth in many places for several years.

In this larch, growth was reduced, but only partially and after a lag. This example illustrates that local environmental factors sometimes overlap with the impact of a more global factor, here a volcanic eruption. However, it also shows that tree-ring researchers need many trees from the same location to draw reliable conclusions.

The Swiss Federal Constitution enters into force
Founding of ETH Zurich
International Committee of the Red Cross founded in Geneva
 1865 The USA abolishes slavery
Breakthrough in the Gotthard tunnel
1885 Founding of the Swiss Federal Research Institute WSL

Dormant buds – larch life insurance
Schlafende Knospen

In old age, the trunk suddenly forms buds. Instead of producing tree rings, the growth layer (the cambium) develops dormant buds. In this larch, these dormant buds did not sprout, but if needed, they would have developed into branches.

Dormant buds, also called dormant eyes, are located under the bark and are hardly visible, if at all. There, they can remain viable for years or even decades. Their sole task is to restore lost organs (branches, twigs or even the entire trunk). In this way they protect the tree against the consequences of branch loss.

1905 E=mc2 of Albert Einstein
1917 October Revolution in Russia
1945 End of the Second World War
Ring of life – transport of water and nutrients

The light outer area, known as the sapwood, is the trunk’s active zone. Its function is to store water and nutrients and transport them to the crown, up to the needles. In larch, after a certain time, sapwood turns into the inactive heartwood. This darker trunk area is more resistant to insects and fungi because of various substances, but it no longer contains living cells.

Young branches consist only of sapwood, while the amount of heartwood increases in older branches and towards the trunk base. In some tree species, such as black locust or oak, the trunk contains only a few sapwood rings, while other species such as larch, spruce, birch or maple have dozens of sapwood rings. Due to the substances stored in it, heartwood is more resistant to weathering and is therefore appropriate for outdoor uses in carpentry.

1966 The first weather satellites
 1971 Women's suffrage in Switzerland
Stained wood – Blue stain fungi rob the tree of nutrients
Bläuepilze rauben wichtige Nährstoffe

Blue stain fungi only infest sapwood, the youngest and active wood layer in the trunk. In this layer, the tree transports water and important nutrients. Blue stain fungi interfere with this transport and also feed on sugar, proteins and starch in the wood. The inner part of the trunk – the harder heartwood – is not affected.

An abrupt end – making room for offspring
Jähes Ende

The now ancient larch grew more and more slanted. At its location, just 50 meters below a pasture hut, it posed a risk to humans. In addition, it stood in the protection forest above the village of Blitzingen. In Switzerland, protection forests are intended to sustainably protect settlements, roads and railway lines against avalanches, rockfalls and debris flows. They are therefore taken care of especially, in particular, to promote regeneration. Old trees are selectively removed to create space and light for a new generation of trees.

The larch formed its last complete ring in 1987 and was felled in September of that year. In May 1988, it was hauled out of the forest using a cable crane. As is often the case with old larches, the lower part of its trunk was rotten. The district forester removed this disc at a height of approx. 6 metres and donated it to the Swiss Federal Research Institute WSL.

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