Friday, February 15, 2008

Cajanus tube demystified

One of the questions I keep on hearing is: what is the Cajanus tube and how it is used? Actually it is all very simple, but in scientific articles there is never enough space to describe everything comprehensively. So hopefully this will make things clearer.

At least in this case one image tells more than tens of words, so this is how the Cajanus tube looks like:

As you see, it is little like a periscope, but it is used for looking upwards. The balancing system at the top keeps the tube leveled, and the user looks vertically through the tube that has a small mirror (placed at 45 degrees angle) at the bottom and a crosshair at the top. The whole system is usually placed on a supportive staff. This allows the measurer to define exactly whether some point is covered by the canopy. It can be used for measuring crown radiuses or canopy cover. In canopy cover estimation the standard method is to establish some transects in the study plot, and then measure the percentage of the transect covered by the crowns.

The Cajanus tube has already a long history. It was designed by Werner Cajanus, who was the first professor in forest inventory in Finland, already in the 1910’s for use in crown radius measurements. Risto Sarvas showed how it can be used for canopy cover measurements in the 50’s, but after that I have not found any Finnish publications until LAI detectives. Actually nobody of our forest inventory staff here in Joensuu had even seen a Cajanus tube before I borrowed the one in the picture from Metla’s Suonenjoki research station.

The final question of an interested reader should now be “where to get one?” As far as I know, nobody makes these anymore. There is one fairly similar commercial version called the GRS densitometer, but it is nowhere as useful as the Cajanus tube, because it has no self-leveling system (the user must use the in-built leveling bubbles to keep it vertical). So the remaining option is to build one with your own hands – for example plywood and mirror can be used for the tube, and the leveling system can be made of some screws and iron bars. Or maybe an average researcher should just find someone handier to do the job…