Tree Avalanche!

The Tree Avalanche in motion is in some ways like an avalanche of mud in reverse.  A mud avalanche starts off slowly – in fact, you often don’t even notice the start. Often the start is that all the trees have been removed. This does not immediately cause an avalanche, because the roots are still present. The avalanche comes later when the roots have rotted and there is nothing more holding the soil together.

There is usually an event that starts the avalanche – often days and days of rain so that the soil is saturated and the water has nowhere to go. Since there is little vegetation on the slope, there is nothing to capture the water and cause it to go far underground. Pretty soon, the entire upper level of soil has become saturated with water and often, a layer of water develops between the upper levels of soil and the subsoil. The subsoil may be a clay mixture that gets very slick when wet.

At this point the weight of the water-logged topsoil is the push that starts the entire topsoil layer sliding downhill – building up speed as it goes and destroying everything in its path until it reaches the bottom of the hill.

The Tree Avalanche starts in reverse. The most important thing on denuded slopes is to plant trees that quickly put down roots in order to prevent the topsoil from coming loose. The trees that nature uses for this are called pioneer species: They grow fast, tend to have fairly shallow roots, and produce some food for wildlife. A pioneer forest tends to be pretty difficult to walk through, with lots of small trees. We plant pioneer tree species, rapidly protecting the soil from further erosion.

One of the keys to the Tree Avalanche is that when you plant 100 trees, a survival rate of 85 to 90 percent is normal. Instead of replanting with pioneer species where there are failed trees, we do what nature would do: introduce variety in the form of the tree species of the climax forest. These are slower growing trees that require shade. So, even within the first two or three years, we are introducing diversity. These trees will always be slower growing than the pioneer species and will remain after all the pioneer species are removed.

At the beginning, we plant fairly close – the goal is to plant in such a way that there is space to grow for 6 to 8 years. In the tropics, the trees we planted can be 8 inches or more in diameter and 50 feet tall by that time, and the canopy will have closed, so we need to remove some trees to let the remaining trees continue growing. Between 30 to 45 percent of the pioneer species will be harvested at this first thinning. More trees will be thinned out every four or so years, and at 25 years, all the remaining pioneer species will be removed, leaving the trees of the climax forest, which by then will be of a considerable size and producing seeds.

What accelerates the Tree Avalanche is that the wood from trees that are removed is used to buy more trees (and the land to plant them on) that will also return wood in the future that in turn will buy even more trees. The cycle continues forever. In fact, this is one of the ways that the Tree Avalanche differs from other avalanches: With other avalanches, when they reach the bottom of the hill, they stop (thankfully!) but there is nothing that will stop the Tree Avalanche once it gets going.