Environmental Engineering Reference
In-Depth Information
remain even at a species level. Second, not all publications make it clear whether
the biomass data are reported in terms of fresh or dry matter. Foresters, agronomists,
and compilers of wood and crop statistics almost invariably use fresh-weight values,
which require conversion to a common dry-mass denominator (g/cm
2
in basic sci-
entii c units, more often kg/m
2
in ecological studies and t/ha—tonnes per hectare—in
agriculture and forestry).
But there is an even more fundamental complication in assessing the density of
phytomass: what to count. “Everything” might be the most obvious answer, but one
that cannot be adduced so easily. Destructive sampling (harvesting of all phytomass
within a given unit area, reducing it to absolutely dry weight, and adding up the
components) is the most reliable way to count everything, but the approach is both
time-consuming and labor-intensive, and it has obvious spatial limits. Removing all
plants from small, randomly spaced squares is the best practical way to determine
the overall standing phytomass of a larger area of a uniform grassland but not of
a mature tropical forest, whose diversity of species would require sampling much
larger areas; a complete harvest, moreover, would require not only felling large trees
but also a particularly difi cult removal of all roots, while the sampling of small
plots may result in signii cant over- or underestimates once the results are used to
calculate phytomass totals for much larger areas.
Not surprisingly, most phytomass estimates, even for plant communities that are
fairly easy to sample, refer only to the
aboveground phytomass
and ignore the
belowground phytomass
of roots. But they should always state that clearly, and if
they claim to represent the total phytomass, then they should offer details as to how
the root mass was estimated. This is done most often by applying specii c correction
factors that are usually expressed as
root:shoot ratios
. These ratios are highest in
grasslands (commonly around 5), much lower in forests, and have the lowest values
(with the obvious exception of root crops) in croplands.
Grasslands are obviously easier to study than forests, whose belowground phy-
tomass is known accurately only from a limited number of sites sampled during
one-time investigations and mostly just to the depth of 1-2 m (Jackson et al. 1996).
Taproots and branching roots are relatively easy to sample, but accounting for all
the i ne roots is very difi cult: depending on the biome, their mass is between less
than 10% and more than 90% of the underground phytomass, and they have a
relatively fast turnover (Jackson, Mooney, and Schulze 1997). Different challenges
arise when assessing marine phytomass, whose components' brief life span, large
l uctuations, and unpredictable massive blooms make it difi cult to offer truly rep-
resentative longer-term (monthly or annual) averages.
