Dendrochronology – Stories Told By trees
Alexandra Kessler, University of Zurich
Trees are everywhere, in our backyards, framing streets, and in forests all over the world. Most of them outlive us by many years and then some, remembering environmental events much more accurately then we could remember a meal from last week. Through the science of dendrochronology, we can access the huge archive of information stored in each and every tree. We start with an introduction into a tree’s life:
The life of a tree starts by fighting against huge odds to be able to survive the first couple of years. They get eaten by animals, attacked by fungi and have to fight against the environment on top of that: a roasting sun, the cold in the winter or the wind (with razor sharp snow in cold climate) pressing them to the ground.
The year of a seasonally growing tree is very busy: Trees grow every spring as fast as they can to repair seasonal winter damage, regenerate, grow taller and, if they still have some energy left, they grow in width. All of their energy for this must be made from photosynthesis (using sunlight to produce sugar). Just below the bark they grow long cells with thin walls, creating a light brown ring. Trees try to make the most of summer months when they get the most energy from the sun, hoping not to run out of water or get too hot. During autumn, darker and shorter cells with more lignum (the material making wood) grow denser than the spring cells. Then the tree shuts down, preparing for the cold by reducing the amount of water in the cells and adding sugar to lower the freezing temperature of their cells.
One of these pairs of light and dark cells is called a tree ring, representing one year of growth. If the tree has a good year, it may use the increased energy to produce broader rings. If the tree has a bad year, it may produce either narrow rings or no rings at all, as most or all its energy goes into repair and regeneration. The amount of energy a tree has to use depends on regional effects, like changes in temperature and precipitation, and local effects that only impact a few trees, such as water or mineral shortage, longer snow cover in concave slopes and competition between nearby trees. Counting these rings tells us how old the tree is, and measuring their width tells us how it has fared over the years. The tree ring succession acts like a barcode and can be extended by looking at older trees or trees which have been preserved in swamps or in lateral moraines of glaciers (see figure 1). Using tree rings like this, we can reconstruct a very long record of tree rings going back 11,000 years. If you find a tree in a swamp for example you could find out its age by going into the international database of tree ring records and compare its barcode to the record.
Dendrochronology puts these barcodes of life into context. During bad years, trees can be limited by water, temperature, snow cover, or other variables. Thus, the ups and downs can be correlated with events like temperature fluctuations. The trees can therefore give us valuable information about the climate of the past. By comparing trees from all over the world, we are able to find a common climate signal, telling the story of global climate fluctuations through time. We can observe large-scale events like atmospheric warming, as the trees have this information imprinted in their cell tissue. Even the tree in your backyard can tell you how the weather has been of every year it has survived, its memory written into the tree rings.
On the way up to Camp 17, Jeremy Littell (USGS Research Scientist and JIRP Faculty) and other JIRPers collected samples from trees near tree line. Jeremy would drill a tube into the tree, which was about half a centimeter in diameter (quarter an inch). These core samples from living and dead trees were then treated with sand paper to make the tree rings more visible. Under the microscope we could then count the number of tree rings. One tree with the thickness of a lower arm was 100 years old, that made it quite hard to tell the rings apart because they were that near to each other. The oldest one of the collected samples was about 200 years old.
So why do people working on a glacier care about trees? Trees and glaciers do actually respond similarly to the climate. When there is a year with a lot of snow and is cold, a glacier is happy. A tree however will be freezing and sad, creating only a small ring. Therefore, a glacier’s mass balance and the tree’s growth patterns, represented in the thickness of the tree ring, correspond. This means, that we can reconstruct a glacier’s mass balance in a time when there was no JIRP around to measure it.
-Kessler Alexandra with the help of Jeremy Littell, thanks for Molly this is now in actual English, ☺ and thanks to Annie... it did not get too long!