Environmental Engineering Reference
In-Depth Information
of total insolation of 0.42-1.81 compared to 0.20-0.50
for tropical forests), and the high proportion of phyto-
mass in the stem make the temperate forests, especially
their coniferous stands, the most suitable ecosystems for
management. Another important advantage for manage-
ment of these forests is their low nutrient-cycling inten-
sity, resulting from relatively large litter falls and slow
decomposition rates. Coniferous forests have commonly
accumulated litter at rates of 30-50 t/ha (1 OM higher
than the tropical stands), and a single application of fer-
tilizers may be effective for 10-25 years. Their NPP aver-
ages around 1.5 kg/m
2
(about 40% below ground), and
their LAI can be as high as 7. Environmental limits (inso-
lation, temperature, nutrients) keep the average global
NPP of boreal forests below 0.4 kg/m
2
, but there are
major differences between Europe (mean rate @0.9 kg/
m
2
) and Siberia (average
<
0.25 kg/m
2
), where after
logging the forest remains a net source or just a very
weak sink of carbon (Schulze et al. 1999).
But the knowledge of NPP (or even NEP) is of little
interest to those foresters who manage temperate and
boreal forests for timber. The concern is with the yields
of harvestable timber, and since the late nineteenth cen-
tury foresters have acquired a wealth of detail on wood
growth in pure and mixed stands (Assmann 1970; Davis
et al. 2001). Spruces have about 55% of their above-
ground phytomass in steam and bark, 24% in branches,
and 11% in needles, and their stumps account for about
20% of the whole-tree mass. In contrast, pines have 67%
of phytomass in steam and bark, and broadleaved trees
78% (Lehtonen et al. 2004). Merchantable bole may
thus amount to no more than half of the above-ground
phytomass (fig. 3.7).
The disparities between concepts are easily illustrated.
The annual NPP of a mixed temperate forest is most
3.7 Division of phytomass in a coniferous tree. From Young
(1979).
often about 1 kg/m
2
, its NEP is about 0.5 kg/m
2
, and
a forester's net annual increment (NAI) may be no more
than 150-200 g/m
2
(the latter rate is most commonly
expressed volumetrically, as 2-2.5 m
3
/ha). The NPP
is an academic concept, and the NEP is still a sub-
stantial overestimate of a potential harvest unless every
stump and leaf were removed. On the other hand,
the last value is unduly conservative for assessing more
complete wood harvests: whole tree utilization (includ-
ing stump removal) is now a technical possibility, but it
