Environmental Engineering Reference
In-Depth Information
Finally, cogent arguments can be made for both drastically reducing and greatly
expanding the dei nition of the Earth's phytomass. Restricting it to the living pro-
toplasm would make the biosphere even less substantial than it is; adding all recently
accumulated dead tissues would greatly enlarge its scope. Most of the terrestrial
phytomass is locked in structural polymers and in cell walls, tissues that play essen-
tial supportive, protective, and conductive roles but that are not alive. Correcting
for this fact is not easy. With forests containing roughly 90% of all phytomass, most
of this adjustment could be done by estimating the share of living cells in trees—but
no simple generalizations are possible.
The radial extent of the cambial zone, the generator of tree growth, is difi cult
to dei ne because of the gradual transition between phloem (the living tissues) and
xylem (which holds most of the dead carbon in a living tree). Some cells in axial
and radial parenchyma may remain alive not just for many months but for years
and decades, and there are substantial specii c differences in the shares of total
phytomass made up of fresh leaves, buds, young branches, and rapidly growing i ne
roots (Reichle 1981; Shigo 1986; Jackson et al. 1996). A very generous allowance
would be to assume that no more than 15% of all standing forest phytomass is
actually alive. Besides their dead stem wood, all trees and shrubs also have highly
variable shares of attached dead branches and roots. Cornwell et al. (2009) put the
total phytomass of this coarse dead wood at 36-72 Gt C, or on the order of 10%
of all forest phytomass.
In contrast, the standard inclusion of all nonliving structural cells in the total
weight of woody phytomass in living plants could be logically extended, i rst to the
inclusion of all dead but still standing trees, shrubs, and grasses (as well as their
roots), then to all accumulated surface litter, and eventually even to the huge stores
of soil organic carbon, whose mass, unlike the living phytomass, is composed largely
of long-lived and not easily decomposed compounds. Arguments in favor of such
inclusions are ecologically impeccable: if dead structural polymers in trees must
count because they provide indispensable plant services, then litter and soil organic
matter should count no less as harbors of soil fauna, as irreplaceable sources of
recyclable nutrients needed for plant growth, and as reservoirs of moisture.
Heterotrophs
are organisms that can survive and reproduce only by ingesting
fully formed organic compounds synthesized by autotrophs, either directly by eating
them (as is the case with herbivores and with the vastly more numerous bacterial
and fungal detritivores and saprovores) or indirectly by eating other heterotrophs
(as carnivores do). The imperatives of energy metabolism mean that the global
biomass of heterotrophs is equal to only a small fraction of that of all autotrophs.
