[music throughout]
My name is Sean Healey,
I study forest dynamics.
I study changes in carbon,
changes in forest structure when they are disturbed,
that is burned or cut down.
I also study recovery,
which is what we see a lot of following the eruption of Mount St. Helens.
I visited it for the first time 20 years after, it was like a moonscape.
A lot of it was still like a moonscape:
you know, pieces of pumice on the ground, no vegetation.
It has been a place of extraordinary change over the last forty years.
From really beautiful old growth forest to a moonscape.
And then back again to a lot of unbroken
sort-of twenty year-old forests, pretty high canopy cover.
Uh, a lot of trees there now.
From the fringes of the blast zone,
you definitely see patterns of revegetation.
They start slowly, with a little bit of greening,
but by the time you get to the end of the time series,
it looks pretty dark green. And you see the same thing on the ground.
You see you know maybe not a forty year old forest,
but something that looks thirty years old.
And in this part of the country, trees are pretty tall after thirty years.
What we’re looking at is Mount St. Helens
in southwestern Washington, in 1973.
Snow cover is what you see as white there,
right in the middle of the mountain.
It’s surrounded by some dark, deep reddish forests,
which is basically how this MSS imagery sees older growth forests.
To the east of it we se a lot of very active timber harvests.
All of those patches of blue are recent clear cuts.
Just to the north of the mountain, there,
you see a very large clear cut.
That one happens to be about two square miles,
which is really really large by today’s standards.
And then Spirit Lake, just to the northeast of the volcano.
You will see that the shape of that lake will change,
radically, after the volcano erupts.
Well, in this image, I mean, if you remember that large clear cut
that I pointed out you can see that it is starting to go from that bluish cast
to having more red, which indicates, you know, recovery following harvest.
And six years has passed, so you would definitely expect that
in a very productive forest like this one.
And it has been joined by a lot more clear cuts in that area, too.
I have looked at a lot of Landsat scenes.
This is the biggest change from one image to another that I can think of.
Forests and everything else, many miles to the north, were just incinerated.
The blast and the heat and the ash really levelled a lot.
It looks from this image like Spirit Lake, just to the northeast of the volcano
is not there, but it is there, it’s covered in logs.
And most of the water in that lake got splashed up six hundred feet or more,
and sort of dragged all of the trees that were on that wall down.
Over the forty years since the eruption,
you know, we do see that recovery happens first
in the places that didn’t have that huge blast of heat
and were not covered with many, many meters of ash or mud.
In ’84, the first Thematic Mapper Landsat was launched.
So that really changes our ability to display this kind of image
in something closer to true color so it’s much more intuitive:
green means green.
So from this point forward you can sort of visualize
the encroaching greenness
which indicates the re-vegetating and re-foresting areas after the eruption.
Well, I’ll also point out some of the remaining old growth forest
that you can see in that very dark patch.
In the MSS imagery, that was dark red.
It’s just much more intuitive to spot that kind of dark green
when we can display it in true color.
You can see in the early ‘90s clear cuts starting to pop up.
So basically it looks like it is going from green
to some kind of brownish color.
Even as the outer edge of these clearcuts are growing and filling in,
the inner edges are greening up and turning green.
And by the end of the time series they look pretty green,
even though we just saw them get cut thirty years before that.
So we have basically made a model
that uses the imagery to predict what percent cover there is.
Basically, how thick is the tree canopy here.
And through the time series, you can see much of the blast zone
going from red, which is zero trees,
to pretty green, which is, you know, pretty solid tree cover.
And that is really solid documentation of the recovery of the forest in this area.
You can also see its neighboring places go from green to red
in the opposite direction. Obviously there are harvests happening
and those are taking away forest canopy
at the same time that recovery is adding forest canopy
in the place that was affected by the eruption.
The really unique part about the Landsat record
is the fact that it goes back to the ‘70s.
I can’t imagine what it would be like to describe
what this volcano did without having a time series of Landsat.
We’ve got eight years before the eruption and forty years after the eruption.
There is no other asset in the sky that can show us what Landsat does,
in terms of the effect of the eruption
and also the effect of the recovery following the eruption.
Uh, it’s just amazingly lucky that Landsat was up there
in its spot in the sky to watch this whole thing unfold. 105 00:06:16,060 --> 00:00:00,000 Let’s hear it for Landsat.