Unique archive provides new opportunities for researchers

One of the most exciting rooms in the SLU building in Umeå is a small concrete bunker, filled with almost a million bore core samples. 

‘I've spent a few hours in here...’ Fredrik Johansson laughs as he shows us around. The shelves almost touch the ceiling, and are crammed with cardboard boxes, neatly labelled in black ink. Year and district.

Fredrik Johansson is an environmental analysis assistant at the Department of Forest Resources Management. It is thanks to him that we’re able to move around and find our way among the boxes, which are all full of bore core samples. 

The samples all come from the National Forest Inventory's sample areas and have been collected since the early 1950s. Every year, around 6,000 sample trees are drilled and measured, so the archive is constantly growing. 

For several years, Fredrik has been going through the material, sorting, cleaning, packaging – and thinking.

‘It's been quite difficult to put it all together. It was especially hard to understand how they recorded and categorised things in the past. But after a while, I got the hang of it,’ says Fredrik Johansson.

The world's biggest game of Mikado

Fredrik estimates that the entire archive contains around 700,000 to 800,000 bore core samples.m When he began his “clean-up operation”, tattered boxes were stacked haphazardly, filled with bundles of samples. The bundles were held together with rubber bands that had long since withered, causing the samples to mix. It may well have been the world's biggest game of Mikado pick-up sticks. 

Today, things are very different. Most of the material has been sorted and reviewed, and we can easily find a 60-year-old sample from an alder tree. The annual rings are clearly visible. For those who have never handled samples like this, it is striking how fragile they are. They are about as thick as a match and will easily break.

Samples were discarded 

The National Forest Inventory has been ongoing since 1923 – over a hundred years. Unfortunately, the bore core samples from the arly years are no longer available. At some point, possibly when the then Forestry College moved from Stockholm to Umeå in the late 1970s, some of the old samples were removed, their fate unknown. 

“The archive list at the National Archives states that the drill cuttings were discarded ‘at the Forestry College, at an unknown time’. I have read that many times and imagined how the samples were burned in a stove at SLU Stockholm”, says Anna-Lena Axelsson, coordinator of SLU's environmental analysis programme for forests, who is working on developing the National Forest Inventory as a research infrastructure.

It may sound crazy that the samples were simply discarded. But being able to study the samples later on was not the original purpose of the NFI. As with many other things, the usefulness of something is not obvious until much later.

“One small consolation is that some of the data from the lost samples is available. Data for sample trees from the first National Forest Inventory in 1923-1929 has recently been digitised,” says Anna-Lena Axelsson.

Available for research

An organized archive is easier to navigate, but the greatest benefit of the recent overhaul is, of course, that the material is much more accessible to researchers. One researcher who has recently used the archive is Kelley Bassett.

Kelley, originally from Minnesota, USA, is a doctoral student in the Department of Forest Ecology and Management in Umeå. She is in her final year of a part-time doctoral project, studying the biogeochemistry of stemwood. More specifically, she is examining how forest resource availability changes over time and what effects this may have on forest growth and carbon uptake.

The past informs the future

Kelley is using the National Forest Inventory's tree-core archive, which spans seven decades, to examine how resource availability has changed in Swedish forests. She is also studying changes in wood density and carbon content,  knowledge that could be very helpful in better estimating carbon stocks in boreal forests overall.

By examining how things have changed throughout history, we can perhaps better understand what future trends might look like, such as nitrogen availability in forests. The study's results could bring about some really positive outcomes," states Kelley.

Without the National Forest Inventory and the work carried out by Fredrik Johansson, Bassett's research would not have been possible. It was not obvious from the outset that the samples would be researched in this way. Rather, it was a classic example of saving material because it ‘might be useful’ and only realising much later that it was, in fact, a gold mine.

About the National Forest Inventory

Right from the start of the National Forest Inventory in 1923, sample trees were drilled and a bore core was extracted. The number of annual rings (the age of the tree at breast height) was registered, and the width of the latest rings was noted on the sample tree form. In the 1950s, the cores were collected and measured in a stereomicroscope once back from the field; from 1953 to the present day, measurements of annual ring widths are available digitally.

The main purpose of the bore cores has always been to produce up-to-date statistics on forest growth. The annual ring data is mainly used to calculate diameter growth, which forms the basis for basal area growth and volume growth. The annual ring data has also been used to calculate annual ring indices and in other growth analyses.

The cores are part of a systematic inventory. Each sample can also be linked to additional information about the characteristics of the tree, forest and soil.

Interested in collaboration?

Are you interested in the National Forest Inventory's data for your own research? Contact Bertil Westerlund (below).