If you need to take some quick tree core samples to determine the rate of growth last year, one tool you could use is this Swedish-made increment hammer from Haglof. To take a sample, all you do is strike the tree perpendicular to the trunk and and pull the hammer out. You then eject the core with the steel plunger.
Made from high-quality hardened Swedish steel, the 11-1/2″ hammer takes a 0.157″ (4mm) diameter by 3/4″ long sample from either hard or soft wood trees. A 2″ graduated scale on the shaft of the hammer is graduated in increments in 1/20″ or in millimeters for taking a quick reading in the field.
With an investment of $80 you can start taking some core samples. The ejector and the hollowed tip are replaceable if you damage them and cost $18 and $20 respectively.