Environmental Engineering Reference
In-Depth Information
management concept called “Nachhaltigkeit” (translated into English as
the neologism, “sustainability”) was developed by the Germans. From about
1,800, this new forestry practice spread over Europe, particularly Northern Europe.
Nachhaltigkeit involved detailed determination of how to best manage forests to
produce wood and other goods in perpetuity. In German, the root Nachhaltigkeit
alone means nothing more than having a lasting or sustained effect.
Nachhaltigkeit involved detailed determination of how to best manage forests to
produce wood and other goods. Essential to this objective was manipulating
density by spacing trees on a given site, either by planting trees or by thinning
a naturally regenerated stand of trees following a timber harvest or a natural
disturbance. Additionally, one needed to determine how long one should wait
before harvesting a stand of trees and then planting a new stand. This spacing/
length-of-rotation problem had long been solved for crop plants through experi-
mentation and observation. To produce trees as long-lived crops, elaborate long-
term data collection started on the height; size in diameter; amount of wood; size of
crowns in forest stands of different densities at sites with different environmental
conditions. Eventually, a forest modeling concept called the “yield table” approach
developed and became the signature of modern forestry. Nowadays, some of the
historical forest data-sets used in yield tables have grown to 200+ years of
continual record [
50
].
On a given kind of site (same soils, same rainfall, etc.), trees growing in even-
aged, single-species stands (such as forest plantations) tend to grow to the same
height at a given age, regardless of density of trees [
51
,
52
]. At low densities, trees
have with large diameters and crowns and on an equivalent high-density location,
the trees would have small diameters and crowns - but the heights of the trees
would be the same in both cases. In a yield table, decades and sometimes centuries
of forest stand data are arranged by the height of the trees at a given location reach
at a given age, usually the typical age of tree harvest. The tree height at this standard
age is called a “site index” and is used to signify the overall quality of a location for
growing trees. A site index is the canopy height one would expect a forest to attain
in a reference length of time. If, for example, the dominant trees in the canopy of
a forest attain a height of 30 m in 50 years (the reference time) then the forest would
have a site index of 30. Sites with larger site indices would reach the 30 m height
earlier than 50 years; sites with lower site indices would reach 30 m later (if at all).
Using yield tables, one can assess the volume of wood in managed stands of
equivalent heights on different sites.
Site index is clearly defined in terms of the basic data that goes into a yield table and
it can be directly determined by measuring the heights of trees on even-aged stands at
the reference age. Associating site index with actual plots of land is a learned skill and
an art at the same time. A capable site surveyor can judge site index by reconnaissance
of land in a particular region and can make a good wage practicing this trade. Along
with such arcane practices as ax-throwing, log-rolling, and tree-felling, forestry
schools have regular intercollegiate competitions of judging site indices among their
students as part of “Forestry Field Day” celebrations. At the edge of most universities
with a college of forestry, one finds plots of trees planted at different densities for field
