Can trees cause pollution?

Having too much of something is often harmful, even trees.

In Japan, cypress and cedar trees are causing massive amounts of nitrogen runoff into local streams. Because algae feed off nitrogen, the amount of algae was significantly increased in these rivers, they produce natural toxins, and deal mechanical damage to other organisms(They sucks oxygen out of the water), therefore harming the environment.

However, it is not because cypress and cedar are naturally harmful to the environment, these are the same type of trees in Amazon forest and keep the planet cool. In fact, these trees are planted artificially for commercial use 50 years ago, at a time when Japan has a large demand for wood. However, later for a number of reasons including labour cost and legislation, Japan began to increasingly import wood.

This leads to an overabundance of trees which causes today's problem. Because older trees use less nitrogen than younger trees, and these forest are so full go older trees that block the sunlight access for younger trees, leaving large amount of nitrogen being washed down to the river and caused algae blooms.

Scientist are proposing that owners of these land should thinner the first and let younger trees grow.

http://www.seeker.com/excess-trees-in-japan-are-harming-the-environment-1769934569.html