Ever wonder
what the relative capture of CO2 is from an acre of mature forest vs regrowth
vs an acre of pasture or corn? For forests it goes like this:
After
harvest, for a few decades, forests are net sources of Carbon (C) to the
atmosphere. That is because the C
capture from the atmosphere is low (no trees, low net primary productivity,
NPP) and the C loss from the ecosystem to the atmosphere stays the same or increases
after harvest because soil microorganisms keep eating soil organic matter and
respiring it as CO2.
Then, at
some point a few decades after harvest, things switch because tree C capture
from the atmosphere increases to the point where NPP is greater than C losses
from soil microbial decomposition. After
this switch, for hundreds of years, forests can be net sinks for atmospheric C.
In theory,
at some point, an old growth forest should have net C capture of zero as inputs
from NPP balance outputs from microbial respiration. In reality, there haven’t
been many observations of this theoretical steady state – most forests have net
C capture for centuries, but it becomes low in the oldest forests.
Rain fed
agriculture, row crop or pasture, almost always has lower annual NPP than the
adjacent mature forest vegetation.
However, whether an agricultural system has negative or positive net
atmospheric C capture (the balance of NPP inputs and microbial respiration losses)
depends a lot on how management practices have changed soil practices
recently.
Under some
circumstances, reducing tillage can briefly turn ag systems into a C sink that
accrues atmospheric C for a decade or so.
Conversion from annual to perennial vegetation, row crops to pasture, is
even better – creating a large sink for atmospheric C for several decades by
increasing storage of soil C.
Like
forests, row crops and pastures that have been under the same management regime
for long times capture zero atmospheric C because NPP inputs to soil are
balanced by microbial outputs. (This assumes all of the harvested product
returns to the atmosphere as CO2 quickly when humans eat them or burn them for
fuel). The time scales are shorter for these ag systems. While forests might take centuries to reach
this steady state, ag systems might take less than a century. (Jason Kaye,
Professor of Soil Biogeochemistry, Penn State University)
For a
comparison of carbon storage and accumulation in US Forests based on region and
forest type check out the following paper by the US Forest Service: CarbonStorage and Accumulation in United States Forest Ecosystems
Fun Facts:
A 40 acre
woodlot of relatively mature (50-year old) deciduous trees absorbs, or
sequesters, approximately 30,000 pounds of carbon dioxide per acre in a typical
year. And would be emitting about 22,000 pounds of oxygen per acre. The
contribution varies with the age of the forest and species involved. (Timothy
J. Fahey, Ecology Professor, Department of Natural Resources, Cornell
University)
The EPA has
calculated the amount of carbon dioxide emitted by the average car at about
five metric tons, more than 11,000 pounds, so a single acre of woods would be
countering the emissions of about 2.7 cars. For 40 acres, that would be about
109 cars.
