ARBORETUM
Dendrochronology
Display and text by Associate Professor Brian Atwell
The display can be found in the ground floor foyer of Building E8C (Biological Sciences)
Trees - a window to the past
Counting Tree Rings
What do Tree Rings Tell Us?
Trees in Modern Australia
Tree rings of a very old King Billy Pine
Trees – a window to the past
The secondary stems and roots of land plants lay down wood continuously, producing reinforced roots, powerful trunks and resilient limbs that can survive for millennia. A strong central cylinder of wood is characterised by growth rings. Genetics and environment control wood growth and thereby confer the physical and aesthetic qualities of each individual timber. But wood is slow to degrade so growth rings also tell us much about the past.
Dendrochronology [dendron (tree); khronos (time)] is the art of interpreting tree rings to date wood tissues and document past events (see What do tree rings tell us?). This is possible because trees increase their girth by continually adding new cells to their outer circumference. The oldest wood is thus deep within the centre of each axis, sometimes rotten and weak.
What tissues are found in tree stems - Wood is comprised of several cell types, most notably secondary xylem vessels and tracheids that conduct water and minerals from roots to leaves. The tough polymer lignin is a key strengthening compound in all wood. The outer layers, collectively termed the bark, are even more complex, containing phloem elements responsible for conducting organic molecules and minerals to growing tissues such as the roots.
Where do these tissues arise from? Specialised cambial cells divide to generate many cell types - we have little idea how these ‘stem cells’ are programmed. The vascular cambium lies between the bark and the wood. New daughter cells arise radially outwards to produce bark, while cells on the inner side of the vascular cambium produce new wood. Within the bark, some species produce cork arising from a cork cambium. Cork is common in deciduous species from high latitudes.
Seasonal growth patterns - In spring, xylem vessels and tracheids are relatively large and thin walled, surrounded by paler wood termed earlywood. Later in the summer, cells become narrower with thick dark walls that give the distinctive wood rings you see in all these samples – we term this latewood. In cool temperate climates, wood rings are distinctive and one is laid down each year whereas in the hot savanna and arid zones, rainfall can be erratic, leading to new rings mainly in the wet years. Because seasonal conditions influence tree growth rates, a permanent record is captured in the appearance and chemical composition of growth rings.
Counting trees rings
Transects of many individual trees from one location must be analysed to match their rings if we are to have confidence in our interpretation (see What do tree rings tell us?). In Australia, Snow Gum (Eucalytpus pauciflora) from alpine areas, as well as native conifers such as King Billy Pine (Athrotaxis selaginoides), Huon Pine (Lagarostrobos franklinii) and Celery Top Pine (Phyllocladus aspleniifolius) from Tasmania, have distinct annual growth rings. Look at the moments in history when the King Billy Pine (below) was growing. Contrast this with the Australian Cypress Pine (Callitris spp.) where new wood rings would have been laid down in wet seasons but no new rings would have been left behind after drought years. Timber from the northern Hemisphere generally has very distinctive rings as seen in the huge Oregon pine above. How old is this tree trunk?
How long can trees live? Until recently, a 4,850-year-old Bristle Cone Pine (Pinus longaeva) in the south-western arid zone of North America was thought to be the oldest tree alive. Very recently, a Norway Spruce (Picea abies) in Dalarna, Sweden, was found to be close to 10,000 years old!
While the oldest trees in the world are largely softwoods, there are some notable hardwood species that are very old. Olives (Olea europaea) has been dated to 3,000 years old in Sardinia, Italy. Biblical allusions to the Garden of Gethsemane refer to the olives trees that grow there to this day, many surviving for centuries in this harsh Mediterranean climate. The sacred Jaya Sri Maha Bodhi tree (Ficus religiosa) in Sri Lanka is said be 2,300 years old. Legend tells us that this tree was grown from a cutting of the original Bodhi tree under which Buddha received enlightenment.
The Australian mainland also has long-lived trees, although the tendency of fire to devastate eucalypt forests makes Australia’s hardwoods less ancient than those in other parts of the world. Eucalypts are likely to live for 100 to 400 years. In Tasmania, individuals of the softwood Huon Pine are at least 3,000 years old and some clones (small colonies of naturally occurring identical trees) might be 10,500 years old.
What do tree rings tell us?
Tree rings are full of valuable information. They are especially informative about past climatic events and have great value when combined with other techniques in understanding climate change. Several climatic factors influence wood ring width and composition, particularly rainfall, temperature and carbon dioxide concentration.
We see changes in the past few centuries that indicate rising post-Industrial Revolution CO2 concentrations (‘heavy’ carbon isotopes), the great droughts of the past century and the cool periods that we call mini-Ice Ages, such as 1600-1900.
Other techniques that can be used to identify past climates rely upon stable isotopes (e.g. 18oxygen), pollen, charcoal and fossilised organisms such as phytoplankton and diatoms. ‘Heavy oxygen’ is extracted from ice cores to determine past temperature regimes while other proxies for past climates are variously found in ocean sediments (e.g. Baltic Sea), lake beds (e.g. Lake George near Canberra) and peat beds (e.g. Lynch’s Crater in Queensland). Coral reefs are also priceless records of past climatic events, with floods and temperature fluctuations recorded in the chemistry and banding of the corals. Even lichens and cactus spines can be used together with wood rings to infer past climates.
The timber of each tree species has its own distinctive pattern, rather like finger printing, allowing accurate identification of the tree from which the timber was cut. All trees from a known location have a similar pattern of growth rings but because of variations in climate; trees from other locations will have quite different patterns. Thus dendrochronology can provide a valuable tool for archaeologists and historians. The technique has been used to determine the place of origin of ships, timber in houses and many other wooden items, such as panels for paintings. Differences in tree ring and ray cell patterning can be used to identify the place of origin of timber, such as in Egyptian sarcophagi. Many 15th and 16th century paintings from the Netherlands were painted on panels of Baltic Oak (Quercus petraea) shipped from the Vistula River in Poland.
Trees in modern Australia
Woody plants, like all organisms, evolve continuously under selective pressure imposed by their environment.
The superficial bark layers are very diverse, as you can see from the living trees of the Arboretum. Australian trees are particularly divergent. Some have smooth bark, some rough. The first Europeans were agog that some bark of some angophoras sloughed off while other species kept their bark firmly attached allowing it to be harvested in single sheets. Bark is used by some indigenous peoples for example for shields, cord, twine and canoes. Cork, aspirin, taxol, cinnamon and quinine are all produced from tree bark.
Fire – in Australia, bark has evolved in part to insulate the cambial cells from fires. Some tree species such as ironbarks are highly resistant to fire. Black Ash from the Blue Mountains is fire resistant while the Blue Mountains Ash is mostly killed by fire and re-seeds vigorously. The specimen (image below) shows damage from a ‘cool’ fire after which the vascular cambium has generated some new tissues.
Extremely cold temperatures often lead to deciduous habits (e.g. the Red Cedar, Toona ciliata). When leaves are lost, cork layers in the bark help protect the trunk from water loss and pathogen attack during winter months.
Invertebrates - the trunk of the Tea Tree in this display (image below) is highly irregular and forms an ideal habitat for many invertebrates. Many Paperbarks (Melaleuca spp.) in Australia also have multi-layered barks suited to insect habitat.
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Tree rings of a very old King Billy Pine
Look at the image of the King Billy Pine above. [Or better yet, visit the display in the E8C grpund floor foyer.] You can see wood rings extending just under 1,000 years! Each one tells us about past climate in Tasmania and helps piece together whether climate fluctuations are local or global phenomena. Check the following events in history that occurred when each numbered ring was laid down by this tree.
1. 1215 King John signs the Magna Carta
2. 1347-1351 Black Death kills around a third of the population of Europe
3. 1368 Ming Dynasty Great Wall of China
4. 1450 Johannes Gutenberg designs and builds the first printing press
5. 1492 Christopher Columbus’ first voyage of discovery to the Americas
6. 1519-1522 Magellan begins, Elcano completes, first circumnavigation of the world
7. 1543 Copernicus proposes that the earth is not the centre of the universe
8. 1616 Dutch explorer Dirk Hartog lands on the coast of Western Australia
9. 1632 Shah Jahan orders construction of the Taj Mahal
10. 1665-66 The Great Plague of London
11. 1672 Discovery of bacteria by Leeuwenhoek
12. 1710 Anders Celsius proposed the Celsius temperature scale
13. 1769-70 Lieutenant James Cook explores and maps New Zealand and Australia
14. 1775 Steam engine invented by James Watt, precursor to the industrial revolution
15. 1803-1815 Napoleonic wars
16. 1831 Charles Darwin’s voyage of The Beagle
17. 1901 Federation of six colonies becomes the Commonwealth of Australia
18. 1925 Television invented by John Logie Baird
19. 1940-1945 Invention of the first electronic computer
20. 1947 India achieves independence from Britain
21. 1953 Franklin, Watson and Crick determine the double-helix structure of DNA.
22. 1974 “Lucy”, Australopithecus afarensis discovered in Ethiopia