Its great that you are providing this type of information to the public, and I’m very intrigued by the regressions and formulae of age vs time.

The charts I found after following the 2nd link in comment 35 have a series of formulae which are typically something like T=5*D^0.8 where T is time (age) and D is diameter in centimetres. That can be re-arranged with a little algebra, into D=0.2*T^1.25

Now what that seems to mean, with an exponent (power term) of more than 1.0, is that trees are actually growing faster and faster as they get older !! That’s very counter-intuitive. However since the plotted data came from different trees, not the same trees over time, I can only conclude that the trees in the past were growing at a much, much faster rate than the trees at present. In other words when those older trees were say 50 years old, they were already much bigger than the current crop of trees of that age.

I note the Bowle & Jones 2008 paper suggests that the forests in the past were much more open-canopied, so this presumably produced the faster tree growth, despite droughts etc. However their ring width data in the Appendix, if I am reading it right, doesn’t really support that conclusion. So what’s going on?

As trees age and increase in size (diameter), their annual growth ring width declines. Even if the area of new sapwood is constant each year, it’s an ever-thinner ring around an ever-larger circle. Hence the trees that are more than say 20-30 years age and still growing slowly, are very unlikely to pick up growth in old age so as to recover what they missed out on in their youth.

So, if the modern trees are being suppressed by excessive canopy, and the diameter vs time charts are correctly drawn, then it looks to me like if you want have any big trees at all in your Chicago area forests in the future, you’d better get out the axe and firestick and start thinning out the patches where the young trees are, and give them a chance to get going with a full head of steam!

I’m not sure if you are referring to the formula used in our article, or one used by John Dwyer.

http://treetalk.mortonarb.org/areas-of-interest/do-you-know/what-are-the-ages-of-trees-in-your-neighborhood/597/

Our data, as well as John’s, are actually not based on average growth rates, but on an equation (a power function in our case) expressing the relationship between tree age vs. tree dbh. The actual data and equations are shown in the link below, scroll down to the graphs:

http://plantconservation.us/resources.html#treerings

These equations have r-squared values that range from about 0.87 to 0.90, which means that 87-90% of the variation in the individual tree ages is explained by their diameters. That’s actually pretty good, and means that the equations have strong predictive value. ne reason for this is that the data are taken from forest grown trees, which partition by habitats. As you note, it’s location, location, location.

I keep reminding people that because our data are from forest-grown tree species, they cannot be applied to different species, nor to open grown urban trees, which are usually much younger at a given diameter. John Dwyer has addressed this (http://treetalk.mortonarb.org/areas-of-interest/do-you-know/what-are-the-ages-of-trees-in-your-neighborhood/597/).

I completely agree with you on your example of variation in tree age vs. dbh for white oak. Even for forest-grown trees, our white oak data show a different growth pattern from other trees that we have examined. See the bottom left graph for white oak. It is clear that the age-dbh relationship does not exactly follow the regression line. It actually reaches a plateau at about 150 years beginning at about 35 centimeters, resembling a logarithmic function. If we had tried to get a significant regression only using trees that were greater than 25 centimeters, there would be no relationship and we could not predict tree age.

Nevertheless, there are two 60-centimeter trees that are about 250 years old, further supporting your comments.

Another important point is that it is dangerous to extend tree age estimates beyond the last point on a regression line. We provide estimates of tree ages up to 100 centimeters, but we have no data for trees that large. There are many examples of larger trees out there for many species, but most have rotten cores and cannot be aged, or it is impossible to hit the tree center and get an accurate age. The growth curves of almost every tree species also eventually start to plateau as they get extremely large because proportionally more biomass is required to build a growth ring for a larger tree than for a smaller tree. For this reason, as well as other factors of tree ageing, tree age-size relationships are almost always unpredictable for old growth trees. Attached is a very interesting article, with photos, on characteristics of old growth trees by Neil Pederson.

Marlin Bowles
The Morton Arboretum

Trunk diameter depends a lot on the species of tree, age of the tree and where it is growing. I have two white oak tree trunk sections from the Arboretum that are both about 32″ in diameter. One is 141 years old; the other is 302 years old with virtually the same diameter.

Based on the formula in the article, a 32″ diameter white oak should be about 249 years old. One of my samples is significantly younger than the average, the other significantly older.

The same saying applies to tree diameters and growth rates as well as to home values – growth rates and diameters are based on location, location, location. The only way to accurately assess the age of the tree is to take an increment core sample and count the rings.

Ed Hedborn
The Morton Arboretum

I’m sorry, but since our chart stops short of the size of this tree, its almost impossible to estimate its age. All i can suggest is that it might be older than the maximum estimate on the chart.

Marlin