Environmental Engineering Reference
In-Depth Information
a single cut of nonirrigated alfalfa can add more than 5 t/ha, and as many as i ve
cuts are possible (Ludwick 2000). Bradford, Lauenroth, and Burke (2005) quantii ed
such a productivity gain for the Great Plains of the United States, the country's
premiere agricultural region, and concluded that compared with native vegetation,
cultivation is increasing the region's NPP by about 10%, or nearly 100 Mt/year. But
while the primary productivity (and even a temporary phytomass storage) have been
enhanced, the specii c composition has been simplii ed, and the biodiversity of the
entire ecosystem (as well as some of its natural services: in the case of row crops
such as corn, the key concern would be the compromised protection against soil
erosion) has been reduced.
Matters are no easier when looking at the transformations of forests. Shifting
agriculture may remove all natural vegetation within a cultivated patch, but it lets
the original plant communities reassert themselves after a period of cropping, and
if the regeneration period is relatively long and the temporary plantings are sur-
rounded by still intact growth, the regenerated forest may regain most of its original
biodiversity. A great deal of historical evidence shows that even some large-scale
deforestation followed by decades or even centuries of permanent cropping is sub-
stantially reversible.
One of the best-documented examples of this large-scale recovery is the case of
Massachusetts's forests. In 1700, some 85% of the state's area was covered by
forests, but 150 years later Henry David Thoreau noted in his
Journals
that “our
woods are now so reduced that the chopping this winter has been a cutting to the
quick.” By 1870, when the clearing reaching its greatest extent, only about 30% of
the state was covered by trees, but by the end of the twentieth century the cover
was back up to about 70% (Foster and Aber 2004). The history of Massachusetts
forests also illustrates how the interplay of natural and human factors determines
long-term outcomes. The 1938 hurricane, which damaged more than 70% of all
wood volume in the Harvard Forest, was a perfect reminder of the fact that only a
minority of trees in the U.S. Northeast can live out their maximum natural life span.
as the region is (infrequently but assuredly) subject to major hurricanes. And the
massive death of hemlocks, originally one of three dominant species in the state's
natural forests, illustrates the impact of pests against whose attack there is no known
defense.
Contrasts of single measures, even if they are such critical variables as the total
standing phytomass or its annual productivity, cannot capture the true impact of
human transformations of natural ecosystems—but neither can a list of species that
have been endangered or eliminated by such changes, or a calculation of complex
