The birch avenue
In 1940, Göte Turesson planted an avenue of birches here. He was a professor interested in the connection between ecology and genetics, and the avenue is an elegant illustration of his research. The birches originate from different places in Sweden, but they all belong to the same species and have grown up together here in Uppsala. Despite that, there are clear differences in when they shred their leaves in the fall and how large they are – differences that must be genetic since they have had identical ecological conditions growing up.
From north to south
The birches planted by Professor Turesson are now more than 80 years old. Most of them are from the northern parts of the country, for example Åre, Luleå and Abisko. You can find them along the northern border of the new residential area in northern Bäcklösa, where they are positioned in a line from east to west.
In the 1980s, an additional avenue of 217 birches from 27 locations were planted. They are connected to the original avenue, but positioned in a line from north to south, on the western side of Dag Hammarskjölds road. They are planted in order according to latitude where they were collected, from Ystad in the south to Kiruna in the north.
When the leaves fall
In the fall there is a noticeable difference in when the leaves of the birches turn to yellow and defoliation occurs. That is because they are genetically coded to go into winter rest at different time points. Studies have shown that darkness plays a key role in the timing of defoliation; longer nights mean it is time to let the leaves go. The further north we go, the earlier the winter starts, so the dormancy signals are triggered earlier in trees from the north than in trees from the south. Since it is the darkness rather than the cold temperatures that triggers the signals, the northern trees go into dormancy earlier than the southern ones even though they are all growing in Uppsala. This is likely also why the trees differ in size – the northern birches get a shorter growth period and therefore cannot grow as large as the southern ones.
There are three different birch species in Sweden, but usually when we say birch we either mean silver birch, Betula pendula, or downy birch, Betula pubescens. The third species is called dwarf birch, Betula nana, and is a small shrub rather than a tree.
The two avenues consist of downy birch. The species name pubescens means hairy, which is suitable because the young shoots of this tree are usually covered with thin hairs. That is a way of distinguishing it from the silver birch, which is otherwise very similar looking. The silver birch has bare shoots, oftentimes covered with small warts. Additionally, the branches of the downy birch are more upright, and its leaves have a slightly rounder shape.
The downy birch is found all over northern Europe and almost all the way to the east coast of Russia. Here in Sweden it is, just like the silver birch, common in the whole country, but especially in the north and on humid soils. It is well-suited for cold climates and form the tree line in the Scandes.
For how long have downy birches grown here?
Downy birch was one of the very first trees to colonize Sweden. As the ice sheet that covered northern Europe melted about 10.000 years ago, downy birches established and spread. It probably came to Uppsala when the ground here became free from ice and rose from the surrounding sea. That happened about 3.000 years ago here in Bäcklösa.
Folding rules, roofs, and medicine
Plants have different strategies to survive and reproduce. While, for instance, the English oak tree goes for stability and resilience, the downy birch is more about fast growth and producing lots of seeds. In contrast to the English oak, the wood of the birch is not very resistant to rot or insects, which is why it is rarely used to build things that will be outside for long periods of time. Instead, birch wood is often used for indoor furniture, plywood, and wooden tools. In fact, downy birch is almost the only type of wood used to make folding rules.
The outdoorsy type knows that birch-bark is hard to beat when it comes to making fire, but it has also had other areas of use throughout history. For example, birch-bark was used to seal the roofs of buildings in the past. Perhaps not very fireproof, but effective for keeping out rain and snow.
Another part of the tree that has been important historically is the birch sap. That is, the fluid used to transport sugars and other compounds between different parts of the tree. During winter, the sugar is stored in the roots, but in the spring, it is needed to provide the buds with energy. The sap rises from the roots to the crown during early spring, making it possible to tap the fluid. Tapping birch sap is not included in the Swedish right of public access, and the sap is not usually found in the grocery store, but during the 18th and 19th century people commonly used it both as a drink and as a medicine to treat several different conditions.