Field Sampling:
Sampling for tree-ring studies involves removing
increment cores from living trees
or
cross sections from dead
trees. Dead trees - snags (standing dead), logs, or stumps -
are often referred to as remnant trees. Remnant trees can be cored,
but be careful; it is more difficult to start the
borer in heartwood and decaying sapwood often will jam the borer.
Increment cores are sampled with increment borers. Borers
should be
clean and sharp to obtain
straight, unbroken cores. If a core comes out either broken or
twisted it is time to
clean or sharpen your borer.
It is very important to take good care of the cutting tip of the
borer; keep it out of the dirt and protect it from rocks or other
hard objects. Even when walking between trees, put the borer
back into the handle to protect it. Treat it always as a fine
cutting tool and protect it from anything that will chip or dull it.
Also be careful about getting dirt or small rocks in the borer handle
which can create nicks on the cutting tip when the borer is slipped back in.
Another item to be careful with is the extractor (or spoon).
Don't set it down next to the tree while coring; it is very easy to
lose and then step on and bend. Put it in your back pocket or
jam it in the tree bark.
Also
tie on a piece of brightly colored flagging or ribbon for
better visibility if you loose it.
To start the borer, use the palm of one hand to both push and turn
from the center while holding the borer shaft with
the other. Try to keep the shaft as straight as possible when
starting to avoid cutting a crooked core. On thick-barked
trees, start the borer in a bark furrow rather than in the thicker
areas of the bark; bark often can jam in the shaft and cause the core
to twist. Once the threads have caught, the handle can be
turned using both hands. If coring all the way into a tree,
typically the borer handle will run into the tree (or your knuckles
will) before the full length of the shaft can be used. One
trick for maximizing the length of the borer is to
use a Quad-B handle
(note also the flagging on the end of the spoon in this
photo); directions for making one can be found in
this
article from the journal Tree-Ring Research.
Once you're ready to extract the core, slip the spoon down the shaft
until it jams against the core at the end of the borer. Push
the spoon in firmly with the palm of your hand but avoid bending it,
the spoon is fairly fragile (it shouldn't go all
the way in, usually there will be a half-centimeter or so left
sticking out from the end of the borer).
The teeth at the tip of the spoon will lock the core against the
borer shaft, and backing the borer out 360° will break the core off from the
inside of the tree. If you're trying to get as close to pith
as possible (for example, for age structure studies; see below), backing out
360° will tell you whether pith is to the left or right relative to
the curvature of the rings. Pull the spoon out carefully in
case the core is broken, but ideally your cores should come out
straight, solid, and in one piece if you have a
clean, sharp borer. If the
core is broken, you can slip it into a straw as it you pull out the
spoon.
If the spoon won't go in all the
way because of a jam, twist it around to see if you can get past
it. A jammed borer can occur because of a broken core, bark, a
branch or scar, or rot in the center of the tree (or rotten sapwood if coring a
remnant tree). If you feel any change in tension while coring,
stop immediately. It depends a lot on the site and species but
you will eventually run into heartrot in a tree (or even a whole
stand). If you run into heartrot, the borer will start to turn
easily while coring. On the other hand, if you run into a
branch or scar the borer will be harder to turn. In either case the core can twist inside the
shaft and jam the
borer, so stop before you go too far. One trick for checking to make sure the core is not
twisting is
to check the depth of the
core every few turns. Slip the spoon in just to the point where you feel the
end of the core. Mark that point on the spoon with your thumb, then pull the spoon out and check it on the outside of
the borer shaft; it should equal to the length of the borer sticking
out of the tree.
If you can't get the spoon in all
the way because of a jam, you may be able to clear it in the field
or you may have to it drill out the from the handle end using a long
drill bit. Never try to drill out a jam from the cutting tip!
Nothing metal should ever be used on the cutting tip. Your
field
kit should include a 50-60 cm long drill bit for clearing jams from
the back end of the borer. To make one, take a regular 3/16"
drill bit to a welder to weld on the right length of round stock.
An alternative is to use a long piece of threaded round stock that
you can find in a hardware store; buy a 50-60 cm long piece and then
make a point and flatten two sides of the tip on a grinder to make
an ideal drill for a jammed borer. If the plug hasn't jammed too much of the shaft (only the last cm or
so), often it can be knocked out using either a chop stick or golf
tee. Either of these can be used without damaging the cutting
tip. Again, never use anything metal on the cutting tip!
Put the handle end on your shoe or a branch to avoid damaging the
borer and use a rock to very gently hammer the golf tee or chop
stick to clear the jam. Even a stout, straight branch of the right
size often can be used to clear a small jam. However, if the
jam is too big to gently punch out, you'll have to drill it out with your long drill
bit.
Once you have your core, the next question is how to get it back to
the lab for mounting on
wooden core mounts. Cores are
removed from the field using either plastic or paper straws.
They should be allowed to dry for at least a few days before
mounting; do not mount cores while wet, they shrink after drying and
will break apart on the core mount. I highly recommend paper
straws over plastic; cores can dry
in paper straws while they tend to mold in the plastic ones.
If using plastic straws, make some slices in the straws to allow
some air circulation. Paper straws can be harder
to find than plastic, but I recommend the Chenille Kraft Company's Artstraws
#CK-9031, 900 straws to a box. The straws are 42 cm (16.5")
long, 6mm (1/4") diameter, wide enough to hold both 4.2 and 5.1 mm
cores. Search on the web, lots of places carry them.
Remnant trees can be cored although it is often difficult to get the
borer started on a dry wood surface compared to bark and green wood.
The borer has to be very sharp, and use lots of lubricant such as
WD-40 or beeswax. A better method to sample remnant trees is
to cut a cross section, especially for fire-scarred studies in which
multiple fire scars are recorded on the tree. The easiest way
to cut cross sections is, of course, with a chainsaw.
Chainsaws should not be used without prior instruction from an
experienced sawyer and without safety gear such as saw chaps and ear and eye
protection! For sampling fire-scarred sections, we often
use "plunge" or boring cuts, in which the tip of the saw is used to
bore into the tree; these especially are extremely dangerous without prior
instruction and lots of saw experience! Handsaws also are often
used for taking sections from smaller remnant trees. One trick
for getting cross sections out of the woods and into the shop,
especially old remnant sections in several pieces, is to
wrap them with plastic stretch wrap,
available from several distributors on the web (I usually buy from
, search for
Goodwrappers 3" x 1000 ft stretch wrap).
For age structure studies (examples
here,
here, and
here) we have been using a
powered increment borer setup run by
a Stihl 044 chainsaw motor. This allows for fast,
efficient coring of large numbers of trees, and is an easier way to
core near ground level to obtain better estimates of tree
establishment dates. The power borer we use features
(although note that no one imports these to the
US and they have to be ordered from Canadian distributors)
coupled with a custom chuck
specially made for rapid mounting and removal of regular increment
borers. The chuck is made by in
Montana. The drill attachment has both forward and reverse
gears for both putting in and taking out borers. For our age
structure studies, we have found that a three-person crew is the
most efficient: one person handling the power borer motor with 3-4
borers going at a time (as soon as one is in the tree, s/he moves on
to the next), the second person to back out the core and see if it
is close enough to pith (our protocols call for the core to be no
more than ~5 rings from pith), and the third to straw the core and
write the sample id on it. If the first core is not close
enough to pith, the person with the power borer backs the borer out and
then moves over the estimated distance needed and cores parallel to
the first core (which
you can see in this example, the first core is the one to
the right, slipped back into its hole to give a good angle for the
second). Do not try to angle the second core relative to the
first, but simply move over the estimated distance and core parallel
to the first. If the second is still not close enough (within
an estimated 5 rings), we will take a third, but usually no more
than three total to avoid too much damage to the tree. If the third
core is still not close enough, we take the best of the three.
Sample
Preparation:
Once you have your samples in from the field, there are three practical
considerations for sample analysis that all have to do with being able to see cell
structure within your tree rings. Being able to see cell structures is critical to
your ability to distinguish
ring structures such as
"micro" rings (very narrow rings, sometimes
only a few cells wide; see also
this image) and
"false rings" (or inter-annual latewood
bands; see also
this image). The bottom line is that if you cannot see cell structure in your wood then you cannot do
tree-ring research.
First: Cell alignment. On increment cores, one should be looking directly down on top of the cell
structure to be able to see the rings most clearly.
Cores must be aligned
exactly as they came out of the tree, with the
wood fibers oriented vertical
to the field of view. Sometimes cores will come out of the tree either
twisted or broken; in this case your increment borer either
needs cleaning or
sharpening. A clean and sharp increment borer will cut straight, unbroken
cores every time.
Second: Good surface! I
cannot emphasize enough how important a good, fine surface
is. We use a series of steps to put a surface on both cores and
cross sections (see below). You should be able to
see the cell structure of the rings when finished; if not, the surface is not
good enough.
Third: A good dissecting
microscope. It doesn't matter how well your rings are
aligned or how good
the surface is if you do not have good enough optics to be able
to see the cells within your rings! Put a good chunk of your
lab money into a good microscope. A brand that I highly recommend is
Meiji; they have optics
that I think are comparable to Nikon or Wild scopes that cost
twice as much (or more). Be sure to get 10x oculars (not
15x or 20x!) and a zoom range somewhere around 0.7x to 4x, for
total magnification of 7x to 40x which covers the range you'll
need for tree-ring work. Also a boom arm is important if
you will be working with cross sections. As for a light, I highly recommend
,
which provide a white light that is just perfect for tree-ring research needs.
Mounting Increment Cores:
Cores should be dried for
at least several days before mounting in
wooden core mounts. The mounts provide
stability for the cores and a place to write sample ids, dates, and other notes
about the core. Simple core mounts can be
made by cutting a groove in an ~1 cm wide stick using a table saw or rounded router
bit. It is best to mount cores on separate sticks (rather than
all on a single board) since you'll often want to compare two cores
side-by-side, especially those that come from the same tree.
The most critical component is to glue cores in the core mount with the wood fibers perpendicular to the
field of view.
These photos are of ponderosa pine, and the tracheids can be seen in both the
end view of the core and as a
roughened area on the side of the core.
Glue them with
water soluble white glue in case you mis-mount a core and need to redo it. In this case,
simply soak the core in a pan of water overnight (wrap some string around the
core and mount so it doesn't float away, and you have to weight it down with
something to keep it underwater), let it dry, and then re-mount it correctly.
I like to use
scotch tapeto keep the cores from popping out of the core
mounts while drying, but wrapping them with string also works. Scotch
tape has the advantage of being much faster than wrapping with string and you
don't have to take it off before sanding (simply sand the tape along with
the wood).
Occasionally cores will twist down the length
of the core. If cores are taken with a
sharp and clean increment borer,
there shouldn't be any problem with twisting; cores should come out of the
tree straight and unbroken every time. However, if you do have a twisted core, you
can use a "low-pressure steam-jet generator" ( a tea-kettle or similar with a
small opening for the steam to escape, we use an Erlenmeyer flask with a
small opening in the stopper) to straighten the core. Place the core in
the steam jet and gently untwist it down its length, keeping your fingers on
either side of the steam jet.
Surfacing cores and cross sections
I use a combination of electric hand planers, belt sanders, and hand sanding to
prepare tree-ring samples. For cores, the easiest means to surface them is
to use a series of sanding belts on a
belt sander turned upside down. Start with 220
grit, then move to 320 or even 400 grit sandpapers for a final finish. I
highly recommend Bosch 3x21 sanders (belts 3" wide by 21" circumference).
I have had several that have lasted through years of hard use, it is relatively
easy to find finer grits on the web in the 3x21 size, they are much lighter and
easier to handle than larger sanders, and they have a flat top for stability
when using them for sanding cores. Another invaluable item for sanding,
especially when working with resinous pines and other conifers, are
rubber belt cleaners. The sandpaper
quickly fills with sawdust and resin and these are used to clean the belts.
Also get yourself some sandpaper sheets in 320 and 400 grit for hand sanding of
cores right at the microscope.
For cross sections, the first thing
they often need are gluing. Wood or white glues can be used,
but these tend to run all over the place and are hard to use on old
chunks of wood. I recommend
construction adhesives, especially
PL200, that come in tubes used in a caulking gun.
These glues come out much more solid from the tube and
fill up voids in the wood much
better than the more liquid wood glues. One issue with these
is they do give off a lot of fumes (VOCs), and be sure to use these
only in a well ventilated area. Also it takes more time for these to
set up, so allow for 2-3 days before sanding. On very fragile
sections you can glue them to a backing board, but I do not
recommend that especially for fire-scarred sections; occasionally
you may want to surface the other side of the section to see a fire
scar more clearly.
Once
sections are glued and dry, I like to
first use an electric hand planer
to put a flat surface on the wood. It takes some practice to
use the planer well. Two tricks are do not take much wood off
with any one pass and move the planer across the sample with each
pass. You'll want to take down the high points on the sample
first before trying to run the planer across the entire section.
Also be careful not to take the planer all the way to far edge of
the sample, it will flick off pieces of wood.
After you have a flat surface with the planer,
then go through a series of belts with the belt sander: 150, 200,
320, and 400. If you don't want to have to buy one more piece
of equipment, you can also just use the belt sander and a very
coarse-grit sandpaper (60 or 80) instead of the planer.
However, I highly recommend the planer; with a little bit of
practice it is a much faster way to remove chainsaw cuts on your
cross sections.
Care of Increment Borers
It pays to take good care of your increment borers since these are
the basic tools for dendrochronology (and they are expensive!).
Taking care of them means making sure they stay clean and
sharp. Borers should be cleaned both after a day of use and,
in many situations, often several times during the day when coring
resinous or rotten wood. Anytime the cores are coming out
broken or twisted, first try cleaning the borer, but it also may be
a sign the borer is dull or has a knick in the tip.
To clean borers, most of us use WD-40 which both loosens the resin
and other buildup inside and outside the shaft and reduces the
possibility of rust. To clean the inside of the borer, the
easiest method involves twisting a small amount of fine steel wool
on the threads near the tip of the spoon (extractor) and using that
as a ram to clean the shaft. Spray the inside liberally with
WD-40 to loosen any buildup before cleaning with the steel wool.
Sometimes the steel wool will fall off the tip but can usually be
worked out with the spoon through the tip of the borer. After cleaning the inside with the steel wool on the
spoon, pull it off and then scrub it on the outside of the shaft.
Be careful with the steel wool around the cutting tip; you do not
want to ever put metal on the cutting tip, even the steel wool!
Another option some use is a gun cleaning kit, but that is one more
item to have to take with you when the spoon/steel wool combination
works just fine.
Sharpening your borers takes some skill and practice, but all
dendrochronologists should know how to do this. All field kits
should have a sharpening kit (available from forestry supply
companies) that includes a rough stone for taking the steel down
fast, a flat fine stone for finish work, a conical fine stone for
the inside of the borer, and a bottle of lubricating oil for the
stones (always use the lubricating oil to keep the stones from
becoming clogged). Borers can be touched up at any time in the
field, but if they have a nick in the tip I usually put those away
until I can sharpen them underneath my microscope back in the lab
(be sure to take several borers with you on a collecting trip, that
way you will have backups if one gets too large a nick). If
the nick is too large, more than about 1/4 cm or so, the borer is
useless; to remove it will require that the constricted opening at
the tip would have to be pulled back too far and any core would be
too large for the spoon to be inserted.
I have a series of photos for sharpening a small nick (visible
in this photo as the glint in the upper right on the cutting tip;
yes, even that small of a nick can cause the core to twist or
break). The first step is the nick has to be removed by
flattening the tip to the point it is no
longer a break in the rim; this can be the hardest part
to do since the rest of the borer is still sharp except for this one
place. But it must be done! The best way to do this is
by scrubbing the tip with the rough stone, constantly turning the
borer such that the tip is flattened evenly. The next step is
then to
put the edge back on the outside of the tip;
this can be done first with the rough stone and then with the fine
stone. Do this under your microscope and you can see exactly
when everything is back to a single sharp point around the rim. The
angle should be about 40-50°, not too steep but still sharp enough
to cut the wood. The borer should again be turned constantly to
avoid removing too much steel from any one spot around the rim more
than the rest (try to keep the tip a constant flat plane when viewed
from the side; a depression along the rim will result in uneven
cutting of the wood and catching of the wood fibers, which can also
twist your core as much as a nick). The final step is then
to take the "burr" off the inside of the shaft
with the round stone; do not use the round stone to take
any of the steel off from the inside! The cutting tip is very
much "uni-facial", with only the outside angled in and the inside
straight. And just to be complete, I like to also
sharpen the threads near the tip,
this is very important when trying to core remnant trees but also
just makes it easier to start the borer in any tree.
