Environmental Engineering Reference
In-Depth Information
is
the standard in commercial evaluations of
forest
year-old counterpart, but its stem storage will be only
40% of the earlier rate.
Savannas and other tropical and subtropical grasslands
are, in aggregate, the biosphere's second most productive
biome, accounting for nearly 30% of the global NPP.
Highly rain-dependent NPP can be quite variable, aver-
aging about 1 kg/m
2
with extremes ranging from less
than 500 g/m
2
in semiarid locations to more than 4.5
kg/m
2
in subhumid tropics, and with above-ground pro-
duction accounting for more than below-ground NPP in
a minority of studied cases (Coupland 1979). Similarly,
the NPP of temperate grasslands, averaging mostly 1-
1.5 kg/m
2
(but surpassing 2 kg/m
2
in the richest mead-
ows), comes mostly from often dense and deep root
mats. Lawns account for a large part of temperate urban
areas; their NPP is mostly 1-1.8 kg/m
2
, comparable to
the fixation of natural grasslands (Falk 1980).
At the beginning of the twenty-first century, cultivated
land—according to FAO (2006) about 1.54 Tm
2
,or
about 12% of ice-free land, under annual and permanent
crops—produced about 14% of the global NPP. The spe-
cific rates (given the great variety of crops, growing con-
ditions, and agronomic practices) differ substantially, and
they also vary with the availability of water and the inten-
sity of fertilization. Conversion of readily available yields
to NPP must take into account the changes of harvest in-
dex, the ratio of grain to total above-ground production
(Donald and Hamblin 1976; R. K. M. Hay 1995). In
1900 harvest indices of cereals were 0.25-0.35 because
the bulk of their DM phytomass was in long stalks and
numerous leaves. Modern high-yielding cultivars have
short stems with fewer and narrower leaves, and harvest
indices 0.40-0.42 for wheat, 0.47-0.50 for corn, and
about 0.5 for rice. With optimum water supply, fertiliza-
productivity.
The global average of forest growing stock is about
110 m
3
/ha. European rates are close to this figure, Latin
America's rate tops 150 m
3
/ha, and the highest rates in
tropical rain forests are about 200 m
3
/ha (FAO 2005).
The latest survey of U.S. forest resources puts the nation-
wide average at just over 113 m
3
/ha (Smith et al. 2004).
Reliable European data show nationwide NAI rates rang-
ing from less than 0.9 m
3
/ha in Greece to nearly 6 m
3
/
ha in France (Lehtonin et al. 2004). But these rates are
determined by the density and composition of forest
stands, and hence a better indicator of economically
useful productivity is the ratio of NAI/1000 m
3
of
growing stock. Finland, with its exemplarily managed
boreal forests, has a very high NAI/1000 m
3
of grow-
ing stock, about 39 m
3
; the rates are about 32 m
3
for
France and Germany and just over 20 m
3
for Italy and
Greece.
In energy terms, typical sustainable worldwide wood
harvests thus range from a mere 4 GJ/ha in many tropi-
cal stands to over 20 GJ/ha in North American or Scan-
dinavian coniferous forests (dry weight equivalents are
@525 kg/m
3
for hardwoods and 440 kg/m
3
for soft-
woods). The productivity of temperate and boreal forests
shows a general decline with the stand age. The ratio of
bole to leaf production, a good indicator of wood pro-
duction efficiency and stand vigor, shows an especially
striking decline in boreal coniferous forests, where cold
climate shortens the period during which the trees can
maintain positive phytomass and energy balance. Age-
related decline is also conspicuous in even-aged stands
of temperate conifers: a 70-year-old stand of Douglas fir
produces annually the same mass of needles as its 20-
