Thursday, May 25, 2006

What does burning do to the soil?

This is what the Lao Lao watershed looked like a few weeks ago:

Here is a photo (taken this week) of pretty much the same area after a recent burn:

The first photo was taken just downhill from the second photo, but they both show pretty much the same spot. In the first photo, there are lots of grasses and a few trees covering the hill (but not much swordgrass). In the second photo, there is almost no cover and the trees look like they are dying. This area burned a few weeks ago.

At first glance, you might think that the post-burning photo looks nicer. The ground isn't covered by all those weeds and it looks like a pretty nice lawn (a lawn on an extremely steep slope).

Let's take a closer look

When I inspected the Lao Lao revegetation site a few weeks ago (on April 27, before the burnings), the soil looked like this:

I'm not a soil scientist, but from the color, one can deduce that it is composed of mainly clay. Clay is made up of very fine particles. The soil looks dry, but it looks like it has some moisture (hence, the grass).

After the burning, the soil looks like this:

Those rocky looking things aren't rocks. They are what remains of the soil.

The fires competely dried out the soil and as a result the soil has started cracking. The fires have also caused the clay soil to form into small pellets (sorry for the lack of a better term). The pellets are very dry and hard, but crumble if you rub them between your fingers. They crunch under your feet as you walk through the charred remains of the vegetation. If I had to guess, I would say that there is almost no moisture in the soil (there might not be any).

The fires will have caused a series of chemical changes in the soil, too.

Brush fires can cause something called the hydrophobicity phenomena. This happens when something in burned organic matter decomposes to form a substance that creates an almost waterproof layer around soil.

Jeremy Shaw thinks this has happened at Lao Lao.

When the hydrophobcity phenomena occurs, soil is unable to soak up precipitation. When this happens, erosion rates increase dramatically due to the fact that course soils wash away much more easily than fine soils. This is especially true in locations with steeps slopes, such as the Lao Lao watershed.

Just how steep are the slopes at Lao Lao? Maybe this photo will help illustrate:

Those slopes are very steep. When it starts raining, anything not held in place by vegetation or some type of other mechanism will flow straight into the streams draining the watershed. From there it ends up on the reef flats and causes a whole host of problems.

In the following photo, Jeremy Shaw is kicking up the charred soil:

In addition to charred soil, Jeremy is also kicking up a lot of the ash and other charred organic materials. He thinks that once the Green Season arrives that this additional input of sediment will cause even more problems downstream and out on the coral reefs.

So how did this all happen? Was it an act of God? Was it an accident?

My best guess would have to be arson. Slash and burn has been a technique for clearing land on Saipan (and around the world) for centuries. When clearing with slash and burn techniques, a person clearing some land cuts down a few trees in an area. Then the felled trees are allowed to dry and are set ablaze. The fire burns until it runs out of fuel or until it reaches vegetation that is not dry enough to burn.

It has been a few weeks since the first burn, so it is difficult to see where it started and ended, because most of the ash has already washed away. The most recent burn, however, is still fresh enough to allow one to follow the path of charred vegetation

My best guess is that the most recent burn started just above the big water tank and then burned along the top of the ridge until it ran out of fuel.

I guess it is possible that the fire started at the top of the ridge and moved downhill, but it is much more likely that it started at the bottom of the ridge and moved uphill.

Guess what we found at the bottom of the ridge, where it looks like the fire originated:

Is this proof that it was arson? Maybe. Or maybe not. Maybe someone was just cutting some firewood.

We have always hypothesized that hunters were burning the area to attract deer, which feed on the young swordgrass and tree shoots that pop up after the fire.

Well, guess what else we found:

Is that what I think it is? Maybe. Or maybe not. Maybe someone's pet rabbit got loose.

We have called a special meeting to discuss what our next steps will be.

Nobody is pointing fingers, but if we want to be succesful, we NEED to stop the burnings.

That's all I'm going to say about that for now. I guess if anything, this new development will make our jobs a lot more interesting.


AS said...

That is amazing. Thank you for the in-depth look at this problem.

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