Determining if a Tree is a Witness Tree
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Just because a tree standing majestically in front of you on the battlefield, perhaps in the Rose Woods or along West Confederate Avenue, can also be seen (in younger form) in a photograph taken by William Tipton in 1905, this does not mean, by itself, that the tree is a Witness Tree. After all, many of the large trees which stand very alive today on the hallowed grounds might very well have first sprouted in 1865 or 1870. All of the enormous trees that line the walkways of the Soldiers’ National Cemetery in Gettysburg, for example, were planted as part of the landscaping plan of the cemetery in 1865 or so.
And though a tree “looks mature” in a turn of the century picture, it still does not guarantee it had been born before the Battle of Gettysburg. Different trees – even of the same species – have been shown to grow at very different rates.
So how do we determine which trees are Witness Trees? A two-step process is involved, in which we determine
- if the tree in an old photograph is actually the same one present in a given location; and if so, then, after taking a modern photograph which replicates exactly the older image,
- we calculate the growth rate of the tree by comparing the relative sizes of the diameters of the tree in the old and modern photograph.
Allowing for some variation in the growth rate over time, if the calculation shows that the tree was at least 20-30 years old at the time of the battle, then we can claim with 99% certainty that, “Yes, this is a Witness Tree.”
Step 1: Is this the same tree?
(1) We must first determine if a potential Witness Tree appears in an old photograph. To do this, it is critical to take a modern photo of the tree that replicates as precisely as possible the older image.
This means taking a picture that matches the early photograph in its distance from the subject tree, the angle, and the height. Most of the time, there are other identifiable objects in the picture which make this task much easier: a monument, a rock, a turn in a roadway. Only in this way can the subtle and often distinctive twists and turns of a given tree be seen in both the older and newer images.
We present here two examples of this task.
Example A. The white oak behind the 147th PA monument on Geary Avenue.
Here is an easy tree to pick out from a crowd (Photo 1). The north-facing U-shaped meander is highly distinctive. We may note that the tree, a white oak, would never be suspected of being a Witness Tree, it is so small in diameter. A much larger chestnut oak tree grows on Geary Avenue in front of and justly slightly north of the star shaped monument of the 147th PA, but this tree so far has escaped identification in older photographs (Photo 2).
Example B. Black Walnut tree on Geary Avenue.
This tree is not as obvious a candidate for Witness Tree status. Luckily, this picture from William Tipton includes several great landmarks which can be used to line up an exact replica photograph: the road sign on the right, the rock in the distance on the right, and of course the location just to the right center of the road.
Having taken the picture, we can see that the walnut trees subtle S-curves in the old and new photographs match up very nicely. The modern tree is clearly larger in diameter than the younger version, and since the location of the tree matches up so nicely, we can conclude that this is indeed the same tree.
Step 2: Estimating the Age of the Tree
a. The key to estimating the age of our tree is to compare the diameter of the tree as it appears in the two photos. Obviously, in order to do this, you would place the two photographs – then and now – and measure the width of the tree with a ruler at the same location on the tree, and create a ratio.
For the ratio to be accurate, it is helpful, perhaps necessary, to have some other object in the picture whose size has remained fixed over time, typically a rock or a monument. This way, once you set the two photographs next to each other on your computer screen, if you can get the two rocks or monuments to appear the same size, then you can get a fairly accurate reading on the diameters of the tree.
b. Next, we can take the diameter of the tree as it is today (divide the circumference, which we determine by wrapping a tape measure around the trunk of the tree at a breast-high level), and, by multiplying it by the ratio of the relative sizes of the diameter found in step “a”, to estimate the diameter of the tree as the time it was photographed in the older picture.
c. by dividing the number of years passed between the then and now photographs by the difference in the diameters, we calculate the tree’s average growth rate since the earlier photograph was taken:
The growth rate of the tree tells us how many years it takes for the tree to grow one inch in diameter.
d. finally, we can approximate the diameter of the tree at the time of the Battle of Gettysburg by performing the following calculation:
e. next we can multiply the present diameter by the growth rate to determine the age of the tree. We can then subtract the year it was found to be born from 1863 to determine its estimated age during the battle.
Step 3. Making the final determination: is it a Witness Tree?
The natural qualification to all of this is, what if the tree grew faster in its early years, only to slow down once it turned 20 or 30 years old? In other words, suppose the growth rate of the tree between the years 1900 and 2022 is large enough to allow us to predict the tree was born in 1855, thus making it old enough to be a Witness Tree; isn’t it possible that this tree might have actually first sprouted in 1865, and then grew quickly for three decades, before slowing down to the rate found in our calculation?
Because the answer to this question is “yes, of course!”, we must be conservative in making the final decision to call a given tree a Witness Tree.
Thus, in order to award a tree Witness Tree status, our calculation must allow for some variation in the tree’s growth rate in its earliest years. We will only designate a tree to be a WT Witness Tree if there is a sufficient cushion, or space, between the predicted birth year and 1863.
The closer the older photograph was to having been taken to 1863, the better. A tree whose photograph was taken in 1895 or 1905 will be designated a Witness Tree if we estimate its birth date to be at least 20 years before the battle. If the photograph was taken in the 1920s or 1930s, we will want to calculate the birth date to be at least 30 to 40 years before 1863.
Happily, what we will find is that the vast majority of designated Witness Trees on this site are estimated to be at least 200 years old. This is a very conservative margin of error to allow for in our designations.
More than 9 out of 10 Witness Trees on this site are white oaks and hickories. We provide numerous examples of photographs of stumps of these species in various places on this site, and the stumps show that these two species our generally quite consistent in their growth rates, and that in their earliest years, do not in fact grow any more quickly (in width) than in their mature years.
Example A. The white oak behind the 147th PA monument on Geary Avenue.
a. the early photograph of the 147th PA monument was taken, according to the NPS, in the “1900s”. Let us be conservative and assume the photo was taken in 1910. Measuring the diameter of the tree the same height in both photos (approximately breast high), we find the ratio of the tree’s diameter to be 0.57. This means that the tree is a very slow grower: its diameter has not even doubled in the past 112 years!
To be conservative, I take my measurement at a spot in the older photograph where the trunk is narrower, and in the modern photo where it is thicker.
b. the white oak’s circumference today is 71.5”, making its diameter 22.8”. We multiply 22.8 by 0.57 to find that the diameter of this tree in 1910 was 13”.
c. now we apply the formula above to find the tree’s average growth rate over the past 112 years:
d. the estimated diameter of the tree in 1863 is found as follows:
e. finally, to estimate the age of this tree, we multiply its present diameter by the calculated growth rate:
Step 3: Is this a Witness Tree.
Since the calculation shows the tree to be well over 200 years old, we can safely designate this white oak tree to be a Witness Tree.
Note that it is not important to believe that this tree is actually 260 years old. It is simply and easily old enough to be called a Witness Tree, and a mature one at that, with 99% confidence, and that is all that matters.
Example B. Black Walnut tree on Geary Avenue.
a. this early photograph of Geary Avenue was taken, according to the NPS, in the “c. 1900”. Measuring the diameter of the tree the same height in both photos (approximately breast high), we find the ratio of the tree’s diameter (early tree to modern tree) to be 0.66. This means that this tree is also a very slow grower: its diameter has not even doubled in the past 122 years!
b. the white oak’s circumference today is 96”, making its diameter 30.5”. We multiply 30.5 by 0.66 to find that the diameter of this tree in 1900 was 16.8”.
c. now we apply the formula above to find the tree’s average growth rate over the past 112 years:
d. the estimated diameter of the tree in 1863 is found as follows:
e. finally, to estimate the age of this tree, we multiply its present diameter by the calculated growth rate:
Step 3: Is this a Witness Tree?
Since the calculation shows the tree to be well over 200 years old, we can safely designate this black walnut tree to be a Witness Tree.
Once again, we note that it is not important to believe that this tree is actually 260 years old. It is simply and easily old enough to be called a Witness Tree, and a mature one at that, with 99% confidence, and that is all that matters.