Environmental Engineering Reference
In-Depth Information
To get fairly accurate accounts of the actual standing forest phytomass is even
more challenging. Such accounts are available only for those afl uent countries that
conduct periodic forest inventories and refer to standing forest phytomass only in
its narrowest dei nition, the growing stock of timber. One standard way to ascertain
this resource is by measuring i rst the diameter of growing trees (all of them in a
small area, otherwise by sampling) at breast height (1.4 m above the ground), using
either a special tape calibrated in
(giving directly the diameter reading) or calipers;
the next step is to i nd the average height of major species that make up the forest
and then to use the two measures in formulas designed to express the roundwood
volume (West 2009).
Foresters quantify the growing stock of timber in volume measures; in temperate
regions it can go as high as 90-100 m
3
/ha, but the rates are considerably lower
when averaged over larger areas. The U.S. data show a growing stock volume
of less than 40 m
3
/ha in both the northern and southern regions and more than
60 m
3
/ha in the West, with the annual increment averaging almost 3% of the stand-
ing stock and with tree mortality amounting to about 0.7%/year (USDA 2001).
Species-rich tropical rain forests with closed canopies have a much higher growing
stock, up to about 180 m
3
/ha. Broader measures of forest resources—that is, data
on all aboveground phytomass and the all-encompassing total of all above- and
belowground tissues on a national or a biome scale—remain a matter of uncertain
estimates.
π