Forest Canopy, Understory, N, and Climate

Public release date: 30-May-2008
Swedish University of Agricultural Sciences

Contact: Dr. Thomas H. DeLuca
tom_deluca@tws.org
406-586-1600 x110

Forest canopies help determine natural fertilization rates
Scientists clarify controls on boreal nitrogen
resource that dictates long-term forest
productivity

Nitrogen is the primary nutrient that dictates
productivity (and thus carbon consumption) in
boreal forests. In pristine boreal ecosystems,
most new nitrogen enters the forest through
cyanobacteria living on the shoots of feather
mosses, which grows in dense cushions on the
forest floor. These bacteria convert nitrogen
from the atmosphere to a form that can be used by
other living organisms, a process referred to as
“nitrogen-fixation.” The researchers showed that
this natural fertilization process appears to be
partially controlled by trees and shrubs that sit
above the feather mosses.

In the summer of 2006, the researchers placed
small tubes, called resin lysimeters, in the moss
layer to catch nitrogen deposited on the feather
moss carpets from the above canopy and then
monitored nitrogen fixation rates in the mosses.
The studies revealed that when high levels of
nitrogen were deposited on the moss cushion from
above, a condition typical of young forests,
nitrogen fixation was extremely low. In older,
low-productivity forests, very little nitrogen
was deposited on the moss cushion, resulting in
extremely high nitrogen fixation rates.

Nitrogen fixation is an energy demanding process.
Thus, when mosses are exposed to high
concentrations of bioavailable nitrogen, the
cyanobacteria will consume this resident nitrogen
rather than expending energy on fixing new
nitrogen. Thus the nitrogen content of canopy
throughfall acts as a regulator of newly fixed
nitrogen into these boreal forests. For this same
reason, elevated nitrogen deposition from
pollution likely reduces moss nitrogen fixation
rates. The moss would initially buffer the forest
against the effect of nitrogen added as pollution
or fertilizer; however, chronic elevated nitrogen
inputs would ultimately eliminate this natural
source of forest fertility.

The feather moss-cyanobacterial association
provides a unique model system in which to study
nitrogen feedback mechanisms. The cyanobacteria
reside on the leaves, thus the nitrogen status of
the canopy throughfall directly influences
nitrogen fixation in the feather mosses. This
direct expression of a nutrient feedback
mechanism could not be detected in other nitrogen
fixing plant species, such as legumes, that house
their nitrogen fixing bacteria below ground and
where soils and decomposing litter intercept and
modify the nitrogen from throughfall before it
reaches the bacteria.

These findings are important from a global
standpoint, because feather mosses (and
associated cyanobacteria) are the primary source
of biologically fixed nitrogen in the boreal
forest biome. The dominating feathermoss
Pleurozium schreberi is also found in arctic and
temperate biomes and thus may be the widest
distributed individual nitrogen-fixing plant
species on Earth. Understanding feed back
mechanisms among dominating organisms that
regulate fundamental ecosystem processes are
integral to our ability to predict long term
outcomes of global carbon dynamics.

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Dr. DeLuca is a senior forest ecologist with The
Wilderness Society, 503 West Mendenhall, Bozeman,
MT 59715, and a Guest Professor in the Department
of Forest Ecology and Management, Swedish
University of Agricultural Sciences Umeå 901-83
Sweden.

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