Environmental Engineering Reference
In-Depth Information
successive stages of plant growth (it generally increases with plant age). Its rates are
mostly between 0.3 and 0.65 in grasslands and between 0.55 and 0.75 for boreal
and temperate forests; the highest rates (at least 0.75-0.85) have been attributed to
tropical rain forests. A mean of 0.5 has often been used as the i rst-order approxi-
mation for estimates on larger spatial scales.
Subtracting R
A
from the GPP yields the
net primary productivity
(NPP; NPP =
GPP - R
A
), the amount of phytomass that becomes available to heterotrophic organ-
isms, be they bacteria, insects, or humans. The NPP of major biomes ranges from
negligible amounts in extreme environments (hot or cold deserts) to nearly 1 kg C/m
2
(20 t/ha) in the richest tropical rain forests. Forest NPP is higher than the productiv-
ity of grasslands growing in the same environment because trees have a much higher
leaf area index (LAI), the upper area of foliage per unit area of ground that captures
solar radiation. While grasslands typically have an LAI no higher than 3, and often
less than 2, mean values for forest are above 3 even for boreal growth, and are
commonly more than 5 for multistory temperate and tropical trees with heavy cano-
pies (Myneni et al. 1997; Scurlock, Asner, and Gower 2001).
Large-scale averages of the terrestrial NPP/GPP ratio have been commonly
assumed to be around 0.5, and this mean was coni rmed by calculations based on
four years of satellite observations: between 2000 and 2003, global terrestrial eco-
systems had an NPP/GPP ratio of 0.52 (Zhang et al. 2009). The ratio is generally
lower in densely vegetated ecosystems than in sparsely overgrown regions, forests
have a lower ratio than shrub and herbaceous ecosystems, and the ratio increases
with altitude. Climate exerts a key inl uence: the ratio has a decreasing trend with
a higher precipitation total of up to 2.3 m/year but is static for annual precipitation
above that threshold; as for average temperature, the ratio declines with tempera-
tures between -20°C and 10°C and rises with temperatures increasing between
-10°C and 20°C.
The standard approach is to consider only the R
A
of the phytomass that was
photosynthesized during the period in question, but the quantii cation should also
take into account all respiratory losses from preexisting phytomass, a l ux that, as
Roxburgh et al. (2005) concede, appears to be impossibly difi cult to measure. They
also offer a systematic and revealing decomposition of the measure. Their dei nition
of the instantaneous rate of change of the total phytomass carbon stock—GPP - R
A
- L, where L is the total of nonrespiratory losses, including not only litterfall, root
death, and exudation but also herbivory and natural physical disturbances—
corresponds to what others call the
net ecosystem productivity
(NEP). That l ux,
representing all carbon accumulated by ecosystems, has been usually dei ned as NPP
