While I agree that the trees themselves are taking in carbon, the
ecosystem as a whole may be losing carbon. This could be the case
because following a management event, (whether converting a grassland to
a forest, clear felling and replanting a forest stand as has been
practiced in Europe for many years or dare I say it simply due to
Climate Change) there may be a rapid change in the different pools of
organic carbon in the system. Both grasslands and forest systems can
store significant amounts of carbon in the soil but the carbon in these
systems is of different chemical composition. A change in the system
can lead to a loss of carbon from one pool more rapidly than another
pool can accumulate. What happens to soil carbon during changes in a
system caused by any reason, seems to me, to be one of the biggest
questions regarding terrestrial carbon cycles.
I would also be less critical of the statement "it is argued that
the root loss due to grazing can increase the net C content in the
soil". Carbon lost from the plants due to grazing is not simply lost
from the soil. It is usually returned straight back to the soil in as
the excrement of whatever creature ate the plant. All so called
healthy/unspoiled ecosystems many of which have high soil carbon
contents also support communities of grazers and detritivores all
processing organic material and cycling nutrients. This process is what
leads to the production of the recalcitrant carbon pools that can
persist in the soil thus increasing the total soil carbon content while
at the same time recycling the nutrients required to support new plants
growth.
In both cases it is important to understand that the atmosphere is NOT
the only source of carbon in the system. Tracking carbon in and out of
the system is important and in many cases not that easy.
Phill
Mike Marsh wrote:
With aknowledgement to Wayne Tyson for his comment regarding the
importance of a theoretical foundation and practical research before
acceptance of such statements as those Sr. de Alba Avila questions,
doesn't it defy the laws of chemistry, physics, and common sense to
say that rapidly growing trees are putting more carbon into the
atmosphere than they store? Excuse me, but do the proposers of this
idea think that the rapidly growing trees are CREATING the carbon
atoms that they are using to build tree tissues, and that they are
respiring. Perhaps they have not heard of photosynthesis. It is true
that the trees are being built largely out of thin air, but the carbon
being added to tree trunks is carbon that is being REMOVED from the
atmosphere. Certainly some carbon goes back into the atmosphere as a
result of respiration needed to convert the simple sugars created by
photosynthesis into more complex cellular materials and into the woody
xylem that represents long-term carbon storage, but the sum total of
carbon ending up in those two compartments: carbon stored in wood
and carbon respired back into the atmosphere, must be equal to the
amount of carbon originally withdrawn from the atmosphere. Ecological
studies report that carbon storage is greatest, rather than least, in
rapidly growing young stands of trees.
His other quoted statement: "it is argued that the root loss due to
grazing can increase the net C content in the soil" is equally
difficult to understand. In grassland soils, carbon captured (again,
FROM THE ATMOSPHERE) through photosynthesis in the aboveground grass
blades is stored in roots as starch, xylem and as other plant tissues.
Thus root loss implies loss of carbon from the soil, not a gain, as
grasses use stored carbon (in starches and sugars) to build structures
aboveground which can capture solar energy. Thus stressing a grassland
ecosystem by heavy grazing will increase the rate of withdrawal of
carbon from the soil, as plants seek to replace lost foliage, thereby
causing loss of root mass and loss of carbon from the soil.
In both cases, it is important to understand that the source of all
the carbon in the system is the atmosphere, and subsequent disposition
of the carbon needs to be accounted for, as Odum does so well for
energy in his energy flow diagrams.
mike
