Environmental Engineering Reference
In-Depth Information
alive, then the terrestrial phytomass would be less than
80 Gt C, and the real total might be less than 50 Gt C.
On the other hand, it can be argued that the definition of
phytomass could be extended to include not only all the
standing dead structural matter but also plant litter and
the stores of organic soil carbon (the latter inclusion
brings the problem of separating autotrophic and hetero-
trophic contributions). Such inclusions would multiply
the aggregate storage. For example, the FAO's latest
forest assessment put the average phytomass at 71.5
t/ha in living trees, 9.7 t/ha in dead trees, 6.3 t/ha in
litter, and 73.5 t/ha in soil up to 30 cm deep (FAO
2005a).
Forests store the bulk of all phytomass (at least 80%-
85%), with tropical rain forests dominant. Tropical rain
forests can be monospecific, or dominated by a single
family, such as Dipterocarpaceae in Southeast Asia and
in the northeastern basin of the Congo River (Connell
and Lowman 1989). But high biodiversity is the hall-
mark of this biome, with many large tree families entirely
or largely restricted to the biome. Destructive sampling
of an Amazonian site near Manaus ended up with nearly
95,000 plants/ha belonging to more than 600 species
stratified in six distinct layers (Klinge et al. 1975). Nine-
tenths of all plants were in the light-starved layers, and
hence their phytomass contributed less than 1% of the to-
tal. In contrast, 50 emergents and more than 300 canopy
trees, altogether just some 50 species dominated by
Leguminosae and Euphorbiaceae, contained 85% of the
standing phytomass.
Almost two-thirds of above-ground phytomass was in
the stemwood of dicot species (palms contribute merely
a fraction of 1%), just over one-quarter in branches and
twigs, a mere 2.5% in leaves, and a small remainder in lia-
nas, epiphytes, and parasitic plants. Total dry phytomass
can be as high as 450-500 t/ha, but the global mean is
just short of 400 t/ha, with about 22% of all phytomass
in roots (Roy, Saugier, and Mooney 2001). The forest's
enormous diversity means that a single tree species will
store no more than 5% of all phytomass, most commonly
just 2%-3%. High diversity increases the spacing among
the adult trees of the same species, and this lowers the
high mortality of juveniles caused by concentrated heter-
otrophic attack (Janzen 1970). Clark and Clark (1984)
confirmed this hypothesis, finding the seedling survival
positively correlated with distance to adult trees and neg-
atively with local conspecific seedling density. Tropical
trees also have such protective mechanisms as smooth
barks and coated leaves, chemical defenses, and symbiotic
myrmecophily (ants guarding the huge energy stores in
stems).
In contrast, in boreal ecosystems a single species may
store the bulk of the site's phytomass. This is most pro-
nounced in the case of the world's highest accumulation
of phytomass, the old-growth coniferous forests of west-
ern North America (Edmonds 1982). These ecosystems
shelter the oldest living plants (bristlecone pines, Pinus
longaeva, some 4,600 years old) as well as the bio-
sphere's most massive living creatures (although most of
their phytomass is dead wood): giant sequoias (Sequoi-
dendron giganteum) growing over 100 m tall and able
to live for over 3,200 years. Phytomass accumulations in
these forests are enormous. A century-old forest domi-
nated by Sitka spruce (Picea sitchensis) or noble fir (Abies
procera) can store about 900 t/ha, older stands of Doug-
las fir (Pseudotsuga menziesii) and noble fir may have up
to 1700 t/ha, and the maximum rates for coastal red-
woods are at 3500 t/ha (@68 TJ/ha), with all of these
values excluding roots. Even the richest tropical rain for-
ests will store less than one-quarter of this mass.
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