Environmental Engineering Reference
In-Depth Information
All of the difi culties arising from the choice of NPP as the principal analytical
denominator are greatly magnii ed when the calculation calls (as in all analyses done
by the Austrian authors publishing HANPP estimates) for the NPP of potential
rather than actual vegetation. Although all large-scale estimates of actual NPP have
been subject to substantial errors because of their reliance on theoretical models or
on combinations of models and remote sensing, smaller-scale studies of HANPP
could use rather accurate measures of NEP derived from actual i eld measurements.
Such an option is, obviously, impossible when quantifying potential production,
and the outcome will be only as good as the initial inputs of the key variables
driving the calculation and the overall ability of a chosen vegetation model to
reproduce the real world.
All Austrian-authored assessments calculate the potential NPP using the Lund-
Potsdam-Jena global dynamic vegetation model, one of more than a dozen more
or less complex models used to simulate photosynthesis on large scales (Sitch et al.
2003). An intercomparison of such models used to simulate actual annual produc-
tion showed a substantial range of results, with the highest global NPP value twice
the lowest rate; even after four extreme values were excluded, the extreme values
of the remaining 12 assessment differed by 40% (Cramer et al. 1999). Choosing a
different model or changing some of the input parameters of a preferred model may
thus result in differences easily amounting to 10%-15%.
Sometimes we can safely identify the direction of the most likely error, but when
faced with half a dozen or a dozen uncertainties we cannot be sure, even if each
one of them is relatively small (no greater than 5%), what the aggregate effect will
be. The fortuitous cancellation of opposite errors would make no difference, but
simple arithmetic shows that if both the harvested phytomass total and the chosen
(actual or potential) NPP value have minimum unavoidable errors of
15%, then
the extreme HANPP shares would be about 27% less and 33% more than the mean
rate, bracketing a range of possible outcomes that is simply too large to assess real
concerns and to inform effective policies.
Finally, there has been no uniform approach to calculating the HANPP, and the
published values are often cited without making clear what the numbers represent.
A
sensu stricto
dei nition of human appropriation of photosynthetic products is
rather self-evident as it includes all crop harvests (be they directly for food or for
animal feeding, raw materials, or medicinal or ornamental uses) and all harvests
of woody phytomass (be it for fuel, construction timber, or roundwood to be made
into plywood and furniture or pulped to make cardboard and paper for packaging,
±
