Environmental Engineering Reference
In-Depth Information
sents the true dynamics of primary productivity in grasslands. In contrast, harvests
of aboveground tissues of annual crops leave behind dead roots, and Imhoff et al.
(2004a) included them among human appropriation—but the Austrian group
excludes them from HANPP calculations because that dead phytomass is fully avail-
able to decomposers and soil heterotrophs.
The second problem with limiting the NPP account to only the aboveground
phytomass concerns the harvesting of belowground tubers, roots, and seeds. The
i rst category represents a massive harvest of white and sweet potatoes, yams,
cassava, and taro, totaling some 700 Mt worldwide; the second category includes
sugar beets (now more than 200 Mt/year) and many vegetable root crops, ranging
from carrots, onions, and garlic to celeriac and Japanese
gobo
; the most prominent
component of the third category is peanuts, whose harvest is now close to 40 Mt/
year. The grand total of these underground harvests is now thus on the order of 1
Gt of fresh phytomass, but their “appropriation” is charged against the aboveground
NPP!
More important, even before humans begin any harvesting, the NPP of crops and
forests is reduced, often substantially, by incessant heterotrophic attacks. Here the
fundamental realities of phytomass harvests clash with both the choice of the ana-
lytical denominator (NPP) and the correct understanding of the key operative term
(“appropriation”). On the most general semantic level we should ask what is meant
by saying that humans “appropriate” (or “co-opt”) a certain share of the Earth's
annual photosynthetic production. “Appropriate” may be a felicitous choice of a
verb intended to capture the entirety of human intervention: it is obviously superior
to “consumed,” as the connection of the latter verb to phytomass evokes i rst food,
and on second thought also wood for construction and pulp.
The HANPP may be dei ned in ways that subsume not only direct harvests of
phytomass for food, feed, and fuel but also many indirect claims humans make on
the biosphere's photosynthetic production: the annual burning of grasslands to
maintain open pastures for large herds of domesticated animals is spatially the most
extensive example of such interventions. But the very i rst dei nition of that inclusive
verb in
Merriam-Webster's Dictionary,
“to take exclusive possession of,” also shows
that the way it was used by Vitousek et al. (1986) is not accurate: the biosphere
operates in ways that make it impossible for humans to take exclusive possession
of any phytomass.
Viral, bacterial, and fungal infections affect every staple grain: infestations by
fungi, including wheat rust (
Pucinia triticum
), rice blast (
Pyricularia oryzae
), and
corn ear rot (
Aspergillus l avus
), are particularly common. Insect depredations can
