New research suggests not all trees are 'green.' Some trees cause pollution. A new study out of Japan suggests some forests are polluters on par with industrial farms and urban expanses.

Deforestation is one the largest drivers of climate change, second only the proliferation of greenhouse gases. Trees, of course, are much revered for their ability to suck planet-warming CO2 from the atmosphere. Forests also help clean water, cool the environment and host the biodiversity and ecological balance that bolsters our natural resources.

For all these reasons, trees are regularly celebrated for their environmental benefits. But a group of scientists say several abandoned tree plantations in Japan are causing more harm than good.

The problem is nitrogen -- too much of it. The forests are leaching it, and it's ending up in local waterways, causing algae blooms that damage local ecosystems.

These tree plantations, made of up of aging timber, aren't forests. They're abandoned farms.

"Many Japanese cedar and Japanese cypress plantations were established in the 1950s and 1960s -- 60 percent of those on private land," lead study author Masaaki Chiwa, an assistant professor of agriculture at Kyushu University in Japan, explained in a press release. "These are not natural forests; they were meant for commercial purposes."

Because these forests were planted in a way that allowed for natural maturation, they are dominated by mature timber. The abandoned tree farms are overcrowded, meaning little sunlight can reach the forest floor and encourage undergrowth. As a result, there is a glut of excess nitrogen in the form of decaying needles lining the forest floors.

With the aging, slow-growing trees requiring less nutrients, more nitrogen collects on the forest floor. And with no smaller trees, plants and shrubs to suck it up, it quickly washes away with each new rainstorm -- much as runoff may carry excess fertilizers, spawning massive algal blooms.

The problem is not entirely ignored. Farmers and conservations have recently been working to thin the aging forests, and Chiwa and his colleagues are conducting research to determine whether the strategy is improving the situation.

"We have been measuring water quality to evaluate the effect of forest thinning on water quality including nitrogen loss," Chiwa said.

The work of Chiwa and his colleagues was recently detailed in the Journal of Environmental Quality.