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Bacteria living on old growth trees sustain forest growth

June 8, 2011 - Washington

A study has revealed that bacteria living on large, ancient trees are very important in helping forests grow, as they accumulate more moss which is useful for providing nitrogen.

In the study, three types of trees were taken 1) large, old trees; 2) mosses that grow along their branches; and 3) a group of bacteria called cyanobacteria associated with the mosses.

The cyanobacteria can take nitrogen from the atmosphere and make it available to plants through a process called 'nitrogen fixation' that very few organisms can do.

The growth and development of many forests are thought to be limited by the availability of nitrogen.

By collecting mosses on the forest floor and then at 15 and 30 metres up into the forest canopy, the study was able to show that the cyanobacteria are more abundant in mosses high above the ground, and that they 'fix' twice as much nitrogen as those associated with mosses on the forest floor.

Moss is the crucial element. The amount of nitrogen coming from the canopy depends on trees having mosses.

"You need trees that are large enough and old enough to start accumulating mosses before you can have the cyanobacteria that are associated with the mosses," said Dr. Zoë Lindo, a post-doctoral fellow in the Department of Biology at McGill University.

"Many trees don't start to accumulate mosses until they're more than 100 years old. So it's really the density of very large old trees that are draped in moss that is important at a forest stand level. We surveyed trees that are estimated as being between 500 and 800 years old," Dr. Lindo added.


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