Crown Spread    Edward Frank
   Jan 11, 2006 18:15 PST 


Looking over many of our reports I find we have little in the way of
crown spread information. This was made more apparent with work on the
Pa Big Tree website, which is using the American Forests big tree
formula to rank the size of trees. Crown spread is one component of
that formula.

Will Blozan has defined the methodology for measuring crown spread in
his Tree Measuring Guidelines. One involves averaging the greatest
spread and the minimum spread through the central portion of the crown
to obtain an average crown spread. Another is the spoke method, where a
series of ten measurements, from the outer limits of the branches to the
center of the tree, are averaged together and multiplied by two to
obtain an average crown spread.

Is there a better way to do it, an easier way to measure crown spread?
It is sometimes difficult to gauge spread when the lowest limbs are 80
to 100 feet up in tree. I measured the spread on the NE's tallest
hemlock a few days ago - so this is on my mind. I walked around the
tree, tripping on limbs and rock trying to identify the points where the
branch tips were directly overhead using my clinometer, then measured to
the tree... By the way the tree has a crown spread of just 45 feet, and
maximum lateral spread of 31.5 feet.

The average of the greatest and shortest is essentially the same as the
spoke method, using the equivalent of four spokes. Are ten spokes the
right number, too many, or not enough? Can we figure out some
methodology that will make these measurement easier and will lead to
them being taken more often?


Re: Crown Spread
  Jan 12, 2006 05:26 PST 

I have always wondered why spread doesn't get any respect. With the current Big Tree formulas, spread is heavily discounted. If you are looking for "Big" trees, and one has a 135' spread, doesn't that say something about that tree? Compared to another of the same species? I am guessing that the Big tree points originated with those who tallied board feet, and not overall mass (volume?) I would be interested in ideas that included full points for average spread at least.

Crown spread techniques    Robert Leverett
   Jan 12, 2006 11:41 PST 

Ed, Scott,

     I run hot and cold on crown spread measurements, not because I
think crown spread is unimportant, but because accuracy is hard to
achieve even with settled definitions. I definitely think crown spread
on really big trees is as important as either of the other two common
measurements. For narrow-crowned species like red spruce, I don't hold
that view and seldom take crown spread.

     In terms of measuring spread, over the years I tried everything.
Back in the 1995-1997 period when Will Blozan, Jack Sobon, and I were
writing "Stalking the Forest Monarchs", Will and Micheal Davie tried one
method of crown measurement by positioning themselves on opposite sides
of the tree and shooting the intervening distance with their lasers,
rotating and doing it again, until the tree had been circled. The
average of the cross-crown distances was the average crown spread. I
proposed an alternative of shooting a "spoke" to the trunk and adding
the radius, moving and shooting again, until the tree had been circled.
The average of the spokes x 2 was the average crown spread. I chose that
method instead of the one Will and Mike used, not because it was better,
but because in those days, I usually was the only one with a laser. The
spoke method is still my preferred technique, so why don't I use it
routinely. Well, because getting to the opposite side of a tree is often
impossible. On city trees, I'd often wind up in someone's bedroom,
floundering in a raging torrent, dodging cars in a busy road, or staring
hopelessly up into the co-mingled branches of two trees. Increasingly,
my solution has been to back off far enough to see what I think are the
ends of the crown spread, shoot their distances from a fixed point with
my laser, measure the intervening angle with a compass, and apply the
law of cosines. I can then project the linear distance between the end
points onto a level plane and compute that distance as the lateral
spread. The problem is that most compasses are a poor devices for
accurately measuring horizontal angle. I sendom can read mine
confidentaly to within a degree. Tip my new digital compass just a bit
and watch the numbers spin. Woo! I can get the time of day with the
device, the temperature, time myself with the stop watch, but can't get
the darned thing to do what I bought it for. I can see when it is level.
Whoever design the level was either a space alien or had the eyes of an

    Accurately computing either maximum linear spreads (ends are not
necessarily at the same elevations) or lateral spreads from a distance
is held hostage to the lack of a cheap device that accurately returns
horizontal angles. The Impulse Laser series by LTI has an add-on device
that connects to the the laser at one end and a tripod at the other.
Price of the combination? Oh, over $2,500.

    In my earlier days of tree measuring I had access to a transit. It
was a pain to lug around, but horizontal angles were never a problem.
However, we weren't measuring spreads in those days, just heights. The
monocular that Will and Jess are using will allow computation of a
lateral spread, but no more than 7 or 8 feet. So there is no cheap
solution to horizontal angles. For the present, crown spread
measurements will have to remain the province of: (1) direct lateral
measurement from beneath the end points (preferred), (2) some version of
the spoke method, or (3) measurement at a distance using the law of
cosines. One can work out various scenarios for making the computations
most efficiently where access to the end points is limited. Past
diagrams that I've circulated have that have differing designs, have
been motivated by assumptions about the direct accessibility of one or
more end points. My diagarms seldom are accompanied by adequate
explanations of the underlying assumptions I was making. It wasn't clear
why I wasn't doing the measuring in a more direct way. However, I can
dust of the designs, add some explanatory comments and pass them to you
Ed for posting on the website.

    In terms of what to measure, I have settled on the greatest linear
spread, the greatest linear limb length, and the average lateral crown
spread. Although, I cringe at the work involved to compute all three, I
think I would vote for these measures to be incorporated into an overall
measurement protocal that we might adopt to do any of four classes of
measuring. The above would fit into the champion trees class of

Robert T. Leverett
RE: Crown Spread   edward coyle
  Jan 12, 2006 13:49 PST 


The simplest method is to measure maximum spread. One measurement. It might
take several tries to get the maximum, but that is no different than finding
the highest branch. No averaging of anything. Unless we are considering
averaging the three tallest points on a tree to obtain the height!
The maximum spread, as is the greatest height, should be considered the
absolute. It is not necessary to mimic the AF technique.

Ed C
Re: Crown Spread   Jess Riddle
  Jan 12, 2006 18:24 PST 

Ed, Ed and others,

I'm beginning to measure more spreads with one of two motivating
factors: the tree has an impressive spread, or the tree may qualify
as a state champion. In either case, I measure both quantities for
comparisons sake. I'm also interested in seeing the maximum spread
data used in ranking formula based of proportions of the greatest
known dimensions. I prefer maximum spread to maximum lateral spread,
a term which I find confusing.

On the way back from the Forest Summit in October, Will Blozan and I
discussed the obstacles to implementing a ranking system based of
proportions of the greatest known dimensions. The lack of a reference
list of maximum dimensions is a problem...  We've made progress on data 
entry for the list and other related lists has started, but is proceeding slowly.

Jess Riddle
RE: Crown Spread   Robert Leverett
  Jan 13, 2006 05:52 PST 


I read with interest the different points of view on crown spread
measures. If we are trying to capture overall "bigness", the longest
distance between two points in the crown would seem to have merit,
though not as a substitute for maximum horizontal spread whether limited
to one side of the tree or across the crown. I would think that trees
that have huge crowns that stretch a long distance both horizontally and
vertically deserve to be recognized in a way that takes both dimensions
into consideration. It does complicate the measuring process.

Crown spread mania    Robert Leverett
   Jan 13, 2006 09:14 PST 

Jess, Will, Ed, Ed, Darian, Scott, et al:

...  Another measure is maximum
limb length defined as the linear distance from the center of the tree
where the limb originates to the end point of the most distant point of
the limb structure, i.e. some twig. We can extend this idea to be the
maximum linear (as opposed to strictly horizontal) spread from one side
of the crown to the other. This idea of course takes in the vertical
aspect. The longest linear spread of the Sunderland sycamore is 154 feet
as opposed to the 151.5 feet for the longest horizontal spread. As
mentioned in a prior e-mail on the subject, getting accurate
measurements for any of these concepts is the trick where visibility or
accessibility raise their ugly heads.

    A related concept is the longest linear distance one can travel
while remaining inside the crown (more or less). This measurement is
probably virtually impossible to determine accurately, but I can clearly
see that the pin oak on the Mercy Hospital Campus has a longer linear
inside crown spread (LICS) than it does for maximum horizontal spread
across the crown. While I wouldn't want to attempt making this latter
measurement as part of every crown measurement exercise, when fully
documenting a huge tree for historical purposes, as well as others, it
might help to set giant cherrybark oaks such as grow in Congaree apart
from shorter broad-crowned trees. Any thoughts?

Re: Crown Spread   Jess Riddle
  Jan 13, 2006 13:30 PST 


One of the principal reasons I prefer measuring maximum spread over
measuring limb like is that the former is more a measure of the whole
tree rather than just part of the tree. True, maximum spread does not
include every branch, but the crown is a major functional unit of a
tree and the maximum spread tells you something about that whole unit.
Also maximum spread often is not as simple as long limb + trunk
diameter + long limb. The widest part of the crown is frequently not
centered on the trunk, and the path of the maximum spread does not
coincide with any limbs.

Re: Crown Spread   Edward Frank
  Jan 17, 2006 08:06 PST 

ENTS, Don, Will,

I can see arguments in favor of using maximum spread for in the dimension
formula. It is a single number that reflects a maximum for that aspect of
the tree. We are maximum height for the tree, so it can be argued we should
use maximum spread for the crown measure. I agree that this measurement
does not reflect the height or thickness of the crown, the shape of the
crown, nor its density. I agree that a parameter for crown size should be
included in the dimension formula.

I favor average spread for the crown measure over maximum spread. Why do I
favor average spread? The same limitations outlined above still apply. One
of the main reasons is as Don suggests below - maximum crown spread tends
to favor asymmetry over size. A tree with a long narrow asymmetrical shaped
crown would receive more points than a tree with a round shaped crown with a
slightly smaller maximum dimension.   I think the average crown spread gives
a fairer (not perfect, but fairer) approximation of the areal extent of the
crown than does maximum crown spread, and therefore is a better value for
use in the formula.

I would also argue that the girth measurement also reflects an average -
circumference is a stand-in for diameter in this parameter. Tree trunks
tend to be slightly asymmetrical. The measured circumference is less than
the circumference of a perfectly round tree equal in diameter to the largest
diameter ( long axis of asymmetry ) of the tree, and is greater than the
circumference of a perfectly round tree equal in diameter to the shortest
diameter (short axis) of the same tree trunk. So the cbh is essentially an
average of the circumferences that would be obtained by using the long and
short axis of an asymmetrical trunk.

Either measure, maximum spread or average spread, will produce a workable
formula. So the question is simply deciding which one is best and then we
should use the that one in the formula. I hope we can quickly come to a
consensus - if not I would encourage Will to simply choose one or the other
so we can proceed with implementation.

Ed Frank