[Nebbe] This eastern cottonwood is Iowa's largest tree, rooted between the edge of a cornfield and a gravel road near Otranto in Mitchell County.

Its trunk is almost 30 feet around, and it stands 102 feet tall.

But how do we know which trees are the biggest?

Who measures and who keeps track of that?

[Mark Rao] 74.3 for the total height.

[Nebbe] Meet Mark Rao, Iowa's unofficial giant tree hunter.

For nearly 50 years, he has traveled extensively throughout the state, carefully measuring and documenting Iowa's largest trees for the DNR's Big Trees of Iowa program.

And he does it all simply for the love of trees.

[Rao] The first thing we need to do is determine the circumference.

Every state has got a big tree program.

And so, in Iowa, I'm trying to find the biggest ones that we have here in our state.

I usually refer to it as big tree hunting.

I've been measuring trees since about 1972.

This extreme flare here.

I started out with just learning to identify trees, and then I started noticing some of the really big ones, which impressed me.

I'm the only one, really, in the state of Iowa that has devoted this much time and energy to looking for and finding big trees.

Okay, so take a look through there.

Keep both eyes.

I'm a volunteer for the DNR, but even if I wasn't, I'd still be doing it.

I love trees, so it's just perfect fit for me.

When I see a big tree, there's that sense of awe.

It's just so impressive.

By measuring them, it's a way to quantify how big a tree is.

So, without those measurements, it's just a big tree.

With the measurements, we know exactly how big it is.

And then it gets interesting because you can compare similar trees that are also big.

[Nebbe] In the 1940s, the American Forestry Association came up with a point system for measuring tree size.

One point for each inch in circumference, one point for each foot in height.

Then you take the average spread of the crown of the tree in feet, divide that by four, add all of those numbers together, and voila.

That is quantifiably how big your tree is compared to other big trees in Iowa.

[Rao] One of the things that's rather exceptional about this site is there's two American elms here, and they have survived all the waves of Dutch elm disease, which is pretty interesting.

We had a circumference today of 17 feet, 5 inches.

When I first measured this tree in 1979, it was 12 feet something.

So, it's grown a lot.

It's always exciting to update measurements and find an increase in size.

I kind of view myself as the guy here in Iowa, and I'm getting older, and I would like to pass some of the knowledge along to somebody that's interested.

That's one nice thing about trees.

We never have any arguments.

We always get along.

And I always miss them when they're gone.

So, yeah, I consider a lot of my trees that I've been monitoring for many years to be close friends.

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