Environmental Engineering Reference
In-Depth Information
Yet another illustration of a comparatively large extent of premodern i res comes
from comparing wildi res and set i res in North America. Kay (2007) argued that
the potential for i res started by North American natives was most likely many
hundreds to many thousands of times greater than the known lightning ignition
rates; this led him to conclude that, compared to aboriginal burning, i res caused
by lightning may have been largely irrelevant for at least 10,000 years. And Ste-
phens, Martin, and Clinton (2007) estimated that i res ignited by lightning and
Native Americans in California consumed annually about 1.8 Mha. That amounts
to nearly 90% of the total area affected annually by wildi res in the entire United
States during the years 1994-2004, a decade that was characterized as “extreme”
as far as wildi re activity was concerned. Such a description is a perfect example of
a common ahistorical approach to many natural and anthropogenic phenomena.
The l ooding of vegetation by large reservoirs created by dams is obviously an
anthropogenic act that curtails potential phytomass production and should be
included in an appropriation account—but so should an often considerable (but
also unwelcome, owing to its effects on evapotranspiration, the oxygen content of
water, and the survival of i shes) production of phytoplankton and aquatic macro-
phyta created by water storage, particularly in the tropics. And while clear-cutting
of a forest will remove a signii cant amount of phytomass and reduce a site's pro-
ductive potential, such changes will be relatively short-lived in regions with efi cient
forest management, where a replanted forest may eventually reach a productivity
almost as high as the original growth's.
Higher productivities of both i eld crops and well-managed forests may result in
smaller areas devoted to these managed harvests, and as natural vegetation i lls the
vacated space, the national HANPP will decline. As noted in the previous section,
this has indeed been the case in three of a few countries for which HANPP trends
are available, Austria, Spain, and Britain. At the same time, the reduced areas
devoted to intensive cropping and high-yield tree plantations experience greater
environmental burdens (greater fertilizer, pesticide, and herbicide applications and
increased nitrogen losses, including through greater leaching and the resulting eutro-
phication of waters) and may be subject to less desirable agronomic practices
(increased monocropping, reduced crop rotation, soil compaction by heavier machin-
ery). Obviously, such a decline in the HANPP cannot be seen as a purely desirable
development.
In sum, human appropriation of the global NPP is not just a poorly dei ned
measure whose quantii cation depends on an abstract modeled value and on a con-
catenation of variables that cannot be known without considerable error margins.
Search WWH ::




Custom Search