Your primary field tool will be the increment borer. They are manufactured by several companies (e.g., Haglof, Suunto, Timberline, Mattson), but all contain the same three components (from top to bottom): a handle, a bit, and a core retriever (extractor). They are sold in various bit lengths ranging from 4" to 28" ($100 to $500 US). The components nest together for easy packing. When in use, the bit is inserted into the handle and twisted into the tree (always keep the extractor out of the bit and in a safe place while coring).
The tip of the increment borer is threaded to help pull the hollow bit into the tree. The sharpened surface of the leading edge MUST be protected from damage at all times. The consequences of a dull bit are varied and can range from mild to extreme. A severely nicked or damaged bit can not be re-sharpened and must be replaced. At $75 to $400 per bit, it is best to be careful!
To begin coring, hold the bit just behind the threads and lean into the borer to provide as much body pressure as possible. Slowly turn the bit until the threads have become fully engaged.
After the threads have engaged, you may step back from the increment borer and in a clockwise motion, turn the handle. The bit will proceed into the tree. Hardwoods (e.g., oak) will require a considerable use of strength and energy. If you hit a rot pocket (you will know immediately because of the ease of turning), back out immediately or else your bit may be incredibly difficult to remove (the threads do not engage in reverse unless they already have a bite in the wood).
Your goal is to core the tree slightly past the pith (center) of the stem. To gauge your depth at any given time, you can hold the extractor up to the side of the tree (it is the same length as the bit and will inform you of your progress).
When the proper depth has been achieved, back the bit out one full turn, then insert the extractor into the hollow increment corer bit. Note: place a SLIGHT up-pressure on the back of the extractor to ensure the leading tip stays under your sample. Insert the extractor to its full length (depending upon the species, this may require that you apply some pressure with the heel of your hand near the end). NEVER drive the extractor in with a hammer or other implement.
SLOWLY withdraw the extractor from the the increment handle and you should retrieve an intact core. Remove your borer from the tree as soon as possible to prevent it from being "frozen" in the tree. Current wisdom suggests that the hole you left will scar over quickly and no attempt should be made to treat or plug the hole with any type of substance or object. Just leave it be. If you experience twisting or breaking of the cores, try cleaning and oiling the borer bit (inside and out). Some follow this practice every few cores to reduce core problems.
Immediately place the core into a paper drinking straw, seal the ends, and mark the straw with the sample ID number (this should code to your field data sheets that contain additional information about the tree and site). Be sure not to loose any pieces. If the core breaks, maintain the order of the pieces in the straw. Store the straws in a protective container while in the field (such as a map tube).
Shown here is a collection of basic field tools you will need. They include the INCREMENT BORER, PAPER STRAWS (for core storage), INDELIBLE MARKER PEN, MASKING TAPE (for joining two or more straws together for long cores), BEESWAX (to rub on borer tip to facilitate boring), 2 WOODEN DOWEL RODS and a RUBBER MALLET (to free pieces of wood that may become entrapped in borer bit--this is a pretty common occurrence). A wooden golf tee works well for dislodging material at the tip of the borer bit.
You should always keep a cleaning kit within easy reach (field or lab). We use a cleaning rod designed for .22 caliber rifles with a plastic tip insert that holds either some steel wool or a small cotton pad (both of which should be soaked in WD-40 or equivalent spray lubricant and rust preventative). All bits should be sprayed with WD-40 inside and out prior to storage and minimally after every day of use. Some clean the bits after every few trees to ensure untwisted & unbroken increment cores.