The idea that a forest can cause pollution might seem counter-intuitive. After all, deforestation is a huge problem, and planting more trees is usually part of the solution to various problems (habitat destruction, global warming, air pollution, soil erosion, etc). But there are always exceptions, as some scientists in Japan have discovered. The forests in question are not just any kind of forest, though, but abandoned tree plantations.
"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."
But since, many were abandoned, possibly because of Japan's harsh demographic reality. Because these plantations were designed to allow as many trees to reach full maturity as possible, they are now overcrowded and dominated by these tall trees that block most of the light from ever reaching the ground, so there is little undergrowth, unlike in a natural ecosystem where everything is in balance.
The mature, slow-growing trees require less nutrients than younger trees, so more nitrogen collects on the forest floor from all the decaying organic matter (needles, etc) that accumulates there. With no smaller vegetation to make use of it, it washes away with each new rainstorm, and these runoffs can create massive algal blooms once they reach the see, potentially contributing to so-called "dead zones" where little can survive (see the map above for the locations of the biggest ones).
That's how these trees cause as much pollution as some industrial farms, which similarly release fertilizer nitrogen runoffs.