Environmental Engineering Reference
In-Depth Information
Wood Energy Development Programme in Asia found that during the late 1990s,
nonforest fuelwood provided more than 80% of all woody phytomass burned in
Bangladesh, Pakistan, and Sri Lanka (RWEDP 1997).
The second and much more obvious distinction is that any part of a tree can be
burned by households or local industries, including nonwoody phytomass (dry
leaves, palm fronds), bark, small twigs (suitable for kindling), and roots, whereas the
phytomass that is commercially harvested and traded as roundwood is limited to
stems. The standard American dei nition of roundwood restricts it to stems with
diameter greater than 12.7 cm (5 inches) at breast height above a 30 cm stump and
up to 10 cm diameter outside bark at the top. The same dei nition is also used for
growing stock, and in both cases it excludes live, sound, 10 cm diameter trees of poor
form (these cull trees typically amount to 5%-6% of the total live tree volume). As
a result, a great deal of phytomass either remains unharvested (roots and stumps,
which will gradually decompose or resprout) or is trimmed from the bole: in conven-
tional chain-saw felling, all branches and the treetop are cut off and left to decay on
the forest l oor (usually where it was produced, sometimes collected in windrows).
The bole used to be bucked into 1.2-2.5 m lengths for easier removal (dragged
to the nearest road by horse teams) but is now almost always removed uncut,
mostly by draggers and skyline assemblies, while particularly valuable logs may
be lifted by helicopters. Bark is stripped from timber only at a processing site and
is often used, together with sawdust, to generate heat and electricity. Depending
on species and age the bole is no more than 50%-60% of tree mass, while the
typical shares for branches and leaves or needles and for the stump and roots are,
respectively, about a quarter and a i fth of the total phytomass in temperate and
boreal forests.
While the quantii cation of crop harvests is done in mass terms (t/ha), foresters
have traditionally preferred to express the yields in volumes (m
3
/ha). This requires
knowledge of specii c conversion factors, and, because of a rather wide range of
specii c wood gravities, using a single approximate value (a practice common for
i rst-order estimates) can cause a substantial error (USDA 2010). The densest tem-
perate zone hardwood (white oak) has an absolutely dry density of 0.77 g/cm
3
, other
temperate hardwoods (birches, maples, oaks, walnuts) average mostly between 0.6
and 0.7 g/cm
3
, and the commercially most important softwoods (spruces, i rs, red-
woods) have densities between 0.45 and 0.5 g/cm
3
, while some pines and poplars
have densities as low as 0.35 g/cm
3
; both extremes are tropical woods, 0.11 g/cm
3
for balsa (
Ochroma pyramidale
) and up to 1.3 g/cm
3
for
lignum vitae
(genus
Guaiacum
).
