Environmental Engineering Reference
In-Depth Information
The most diverse temperate deciduous forests have five
distinct layers, ranging from a ground layer of mosses
and lichens through a layer of herbs, shrubs, and small
trees to the surmounting tree stratum, whose canopies
are mostly 15-30 m above the ground. Some of them
are also great accumulators of phytomass in various spe-
cies of oak, beech, maple, chestnut, and, on sandy soils,
pine. The richest cove forests of the Great Smoky Moun-
tains store up to 600 t/ha, but their typical phytomass is
around 250 t/ha, with 20% below ground. Energy limits
on tree productivity are evident in the life cycle of foliage.
In stressed ecosystems the investment in annual renewal
of leaves would far surpass the costs of their prolonged
retention. Life spans of expensive leaves are maximized
in order to amortize the high cost of their formation;
that is why all subxeric and boreal habitats are dominated
by evergreens (Chabot and Hicks 1982). Their leaves
have high specific weight (6.3-15 mg/cm
2
, compared
to the deciduous range of 2.9-7.8 mg/cm
2
) because
their photosynthetic tissues are diluted with structural
supports and protective coatings (to reduce herbivory,
leaching, and desiccation) to ensure their longevity.
With progressing deforestation the tropical grasslands
have become the world's most extensive terrestrial
biome, covering perhaps as much as 2.5 Gha, and the
second largest reservoir of phytomass after the tropical
rain forests. Their best-known regional formations are
East African savannas, Brazilian cerrado, and Venezuelan
llanos, and they exist either as pure perennial grass forma-
tions or as open woodlands with varying densities of ap-
propriately adapted (drought-, fire- and browse-resistant)
shrubs and trees. The presence of the woody phytomass
indicates that tropical grasslands are not climax ecosys-
tems. Their typical standing phytomass is 50-60 t/ha,
nearly one-third of it underground. Temperate grass-
lands (now largely converted to fields) were dominated
by species of Compositae and Leguminosae. Short-grass
formations consist of species growing no taller than 50
cm, and tall-grass prairies can exceed the height of 2 m.
Temperate grasses, such as spear grass and tussocky iron
grass, are admirably adapted to seasonal fire and drought,
with most of their phytomass below ground.
Root to shoot ratios in temperate grasslands can be
as high has 13:1, and bulky root mats retain water and
bind soils so tightly that they virtually eliminate water
and wind erosion. A representative mean for phytomass
stocks is 7.5 t/ha, two-thirds of it in roots (Roy, Saugier,
and Mooney 2001). Agricultural crops are the only veg-
etation category whose phytomass is known with a fairly
high degree of certainty (Smil 1999a; Wirsenius 2000).
During the late 1990s the aggregate annual phytomass
of field crops was 3.5 Gt C (35% in harvested parts, 48%
in crop residues, and the rest in roots and unharvested
phytomass), or just over 5 t DM/ha. Because of the
more frequent multicropping (the global mean near 1.5
harvests/ha) and staggered harvesting, the peak crop
phytomass is no more than two-thirds of the annual ag-
gregate, less than 1% of all terrestrial biomass.
The smallest oceanic monocellular autotrophs—
ultrananoplankton of bacteria and blue-green algae—
have diameters less than 2 mm. Nanoplankton, 2-20
mm, includes diatoms, coccolithophores, and silicoflagel-
lates, and diatoms and dinoflagellates (20-200 mm in di-
ameter) are the most common kinds of microplankton
(Falkowski and Raven 1997). Phytoplankton densities
peak near the coast and decline oceanward; the density
difference between the continental shelf and the open
ocean is up to 3 OM. Eutrophic waters, enriched by nu-
