Environmental Engineering Reference
In-Depth Information
3
Biomass Productivities
The biosphere's productivity can be quantii ed as a cascading series of variables: the
most inclusive ones, the two rates at the cascade's top, gross and net primary pro-
ductivity, cannot be measured directly and can be quantii ed (far from accurately)
only thanks to our improved understanding of photosynthetic processes, the envi-
ronmental responses of autotrophs, and the properties of plant metabolism. In
contrast, the rates at the cascade's end can be readily measured in mass or energy
terms (although in practice they are often estimated on the basis of limited i eld
sampling): crop yields (expressed mostly in t/ha) and timber harvests (commonly
quantii ed as m
3
/ha) are the most common values of interest. But data for crop and
wood yields do not capture the complete biomass productivity and must be adjusted
in order to i nd the totals of initially produced phytomass or zoomass and hence to
express more accurately the actual intensity of harvests and to assess the overall
impact of human actions on the affected natural ecosystems and on various agro-
ecosystems or aquacultures.
Primary Productivity
Gross primary productivity
(GPP) is the most inclusive measure, subsuming the total
amount of new phytomass that is photosynthesized during a given period of time
by all autotrophs. The rate excludes all photorespiratory losses that accompany the
conversion of solar radiation to the chemical energy of phytomass, and its global
tally can neglect marginal contributions by chemotrophic prokaryotes that form
new biomass in the absence of sunlight. A large part of this new photosynthate is
not deposited as new plant tissues but is rapidly reoxidized during
autotrophic
respiration
(R
A
). Plant respiration reduces the l ux of i xed carbon, but (unlike
the entirely wasteful photorespiration) it cannot be seen as a loss because the
