Environmental Engineering Reference
In-Depth Information
printing, and writing). That is the i rst, low-estimate choice by Vitousek et al.
(1986).
A
sensu lato
dei nition is much more elastic: there is no clear natural cutoff for
inclusion, while many impacts that should be obviously included are difi cult to
quantify in a satisfactory manner. Grazing by domestic animals should be included,
as that kind of consumption would not take place without humans—and yet all
grazing that is done in a truly sustainable manner (by keeping the animal densities
well below the available annual phytomass production or by ensuring that they
do not compete with the resident wild grazers) does not diminish the overall pho-
tosynthetic capability of a site: in fact, some grazing may promote growth. And if
the phytomass were not consumed by domesticated herbivores it would not neces-
sarily be “appropriated” by other vertebrates: the ungrazed grass would die during
the winter or arid season and eventually decompose. Moreover, domesticated grazers
also return much of the partially digested phytomass in their wastes, actually pro-
moting patchy grasslands productivity.
This is not the only case in which strict logic would demand an adjustment of
the actually harvested phytomass. Increasingly common conservation tillage and
no-till practices either recycle most of the residual phytomass (straws, stalks) or do
not remove any of it, leaving it to decomposers and other heterotophs. Signii cant
shares of cereal straws removed for the bedding and feeding of ruminant animals
are returned to i elds (made available to heterotrophs) as manures. And most logging
operations do not remove the tapering treetops and any branches from forests. All
of these realities should be considered when calculating the difference between the
NPP of potential vegetation and the NPP remaining in ecosystems after harvests: in
some cases the difference will be marginal, in others it will be substantial. By a
logical extension, the regular burning of grasslands that is done to prevent the
reestablishment of woody phytomass should be also included in the HANPP, as
should all phytomass burned by shifting cultivators and all forest i res caused by
human negligence or arson.
A comprehensive global estimate of phytomass consumed in anthropogenic
i res used the best available published estimates of the share of human-induced
large-scale vegetation i res in different countries (mostly between 80% and 95% in
the tropics, but only 15% in Canada) and a set of assumptions to calculate the
biomass burned in small (shifting cultivation) i res (Lauk and Erb 2009). The exer-
cise resulted in the annual burning of 3.5-2.9 Gt of dry matter, with one-third (1-1.4
Gt) attributed to shifting cultivation and with sub-Saharan Africa's grassland i res
accounting for the largest share of the total (2.2 Gt/year).
