posted on Dec, 18 2006 @ 07:06 PM
Originally posted by forestlady
...Dave 54, I realize that you are positing a theory, which I respect...
Not theory. The gas exchange in a forest canopy was modeled and theorized for years. Actual data was hard to obtain, since forests are open to the
atmosphere and gas sampling is difficult. In the mid 1990's, The Forest Service Southern Research Station decided to find out for certain. They
rented several large 'big top' circus tents and erected them over both native mixed age/mixed species forests, and planted even-aged forests.
Sealed the tents, installed lighting and sensors, and measured the actual gas exchanges. The previously held conventional wisdom was determined to be
correct. Mid seral forests absorb the most carbon and old growth does not. In light of the findings, the authors of a widely circulated 1992 article
in Nature magazine (their modeling asserted old growth is a net carbon sink, but their modeling methodology was widely criticized) publicly repudiated
their earlier paper in 2001.
How forests affect microclimate is highly variable. A canopy shades the ground, but raises the temperature of the boundary layer immediately above
the canopy. Relative humidity is slightly higher below due to the lower temperature, and seasonal soil moisture stays longer. Air exchange is
restricted below the canopy, as wind flow is reduced.
Hydrologically, it has long been known that in forested watersheds maximum water yield is obtained with the forest at 80% normal stocking. Tree
density above that level actually decreases water yield, as the canopy intercepts precipitation. Below that 80% level, increasing solar radiation on
the ground reduces soil moisture. This is for temperate forests. Tropical forests may have a different response.
How all these factors interplay is very complex. And since forests provide many other attributes and values than atmosphere mitigation the various
costs and benefits of different forest management philosophies cannot be decided by science alone. All factors, social, economic, and political as
well as scientific, must be considered. As scientists are trained to ignore the other values (and in most cases consider only their narrow field of
expertise), they are singularly unqualified to make the final decisions on forest management or global warming.