Environmental Engineering Reference
In-Depth Information
photochemical and thermal oxidation of organic matter, and carbon supplied to
root symbionts. Over longer periods of time and on larger scales, the accounts must
also include phytomass losses resulting from such natural disturbances as i res and
destructive l oods, which can cause substantial episodic destruction of plant growth
(the effects of droughts should be rel ected in a reduced GPP). This means that the
commonly used NEP values are only best approximations and that an all-
encompassing dei nition of NEP should explicitly incorporate all of the carbon l ows
leaving an ecosystem (Randerson et al. 2002). If all of the primary productivity
components that cannot be captured by standard aboveground sampling were to
add just 5% to the total, there would be no need for concern, but the truncated
assessments of NEP result in much larger errors.
For comparative purposes, both the NPP and the NEP are normally expressed in
annual terms, although the ecological literature contains references to maximum
daily, monthly, and seasonal productivities. Here are a few examples of annual forest
NEP (with all values expressed as shares of GPP) calculated from data in Luyssaret
et al. (2007): about 13% for a boreal humid evergreen forest, around 22% for
temperate evergreen as well as deciduous forests, and just 11% for a tropical humid
evergreen forest. Daily means of NPP that are sustainable during the weeks of the
most rapid growth are about 20 g/m
2
(200 kg/ha) for the photosynthetically more
efi cient C
4
plants and 5-15 g/m
2
(average of about 13 g/m
2
) for C
3
species. As a
general rule, short-term rates should not be extrapolated to obtain yearly l uxes,
and there should be explicit references to what is included in any particular rates.
Calculating and Monitoring Global Primary Productivity
The global estimates of NPP made prior to the 1970s are either too low or too high
when compared with the most likely range of phytomass production that emerged
in the last three decades of the twentieth century with more i eld studies and better
models. Although Liebig (1862) was credited by Lieth (2005) with the i rst estimate
of global NPP, a careful reading of Liebig's original text shows that his was only a
conditional example aimed at explaining the cyclical nature of photosynthesis and
respiration, not a deliberate estimate of actual phytomass productivity.
As a result, the oldest published NPP estimate was made by Ebermayer (1882).
It was based on typical yields of Bavarian forests and crops applied to woodland,
grassland, cropland, and barren areas, and it amounted to only 24.5 Gt C/year.
Schroeder (1919) and Noddack (1937) estimated even lower totals at, respectively,
16 and 15 Gt C/year, while Deevey (1960) suggested as much as 82 Gt C/year as
