7. Decomposition and Soil Organic Matter Dynamics
(e.g., Melillo et al. 1982; Aber et al. 1990). Palm
and Rowland (1997) suggest a standard set of analyses for characterizing plant litter quality, as do
Harmon and Lajtha (1999).
Woody Detritus
Woody detritus includes both fine and coarse
woody debris. Fine fractions are defined operationally as dead trees and branches < 10 cm in diameter
and < 1 m long, as well as twigs and roots too large
for litter bags. Coarse fractions include boles,
branches, and roots larger than fine fractions. The
principal reason for dividing woody detritus into
these fractions is mode of study: fine fractions can
be weighed intact while coarse fractions must be
subsampled in the field.
Methods for studying woody detritus are analogous to litter bag methods but usually without the
need for litter confinement. Woody material can be
cut to standard lengths (a typical length is lO times
longer than the mean diameter of the material),
tethered and marked for later collection, and placed
in the field in microclimates that would be normally
encountered-on the litter layer, suspended above
the soil surface (for standing dead material such as
dead branches still attached to stems), or buried (for
coarse roots).
At intervals appropriate to the climatic environment and composition of the wood, tethered pieces
are collected and taken to the laboratory for drying,
weighing, and chemical analysis. A minimum of lO
sample collections will likely be needed, spaced at
1- to 3-year intervals. Any given study may take
decades to complete.
Decomposition of woody material is a function
of size and species as well as placement, so the
number of samples initially deployed will need to
be chosen carefully to provide a geometric range of
diameter classes and an appropriate range of genera. It is usually not necessary to sample below the
genus level except for genera with diverse heartwood contents. Because bark, sapwood, and heartwood decay at different rates, it is important to record the proportions of these materials in the pieces
initially deployed. As for fine litter, replication
should be organized to avoid pseudoreplication.
Sample preparation, chemical analyses, and calculations are similar to those for fine plant litter,
109
though analyses should be performed separately for
bark, sapwood, and heartwood.
Because it is impractical to weigh large woody
debris, one can remove trimmed end pieces from
the field for weighing and chemical analyses. Prior
to initial sampling, it is necessary to estimate the
mass of each piece that will eventually be removed;
this should be done at the outset of the study by
measuring in situ the volume of each segment that
will be removed and assuming an initial density
(g cm - 3) equal to the density of the first sample.
Formulas for determining the volume of complex
shapes are provided in Harmon et al. (1999).
Reciprocal Transplants
and Standard Substrates
The relative influence of climate versus litter quality for controlling decomposition rates can be evaluated using reciprocal litter transplants and standard substrates. Reciprocal litter transplants usually
involve exchanging litter bags or tethered wood
among two or more sites: litter from ecosystem A
is allowed to decompose in both ecosystem A and
ecosystem B, as is litter from ecosystem B. This
scheme effectively removes litter quality as an explanation for differences in the decomposition rates
of litter from the two ecosystems. For example,
ecosystem A litter should decompose at the same
rate in each ecosystem if the decomposition environment of ecosystem A is similar to that of ecosystem B; a faster rate of litter disappearance in
ecosystem B means a more favorable environment
in ecosystem B (see Fig. 7.2).
Different rates of decay are usually interpreted
to mean different abiotic conditions among sites,
that is, between-site differences in moisture, temperature, and (for buried litter) aeration regimes.
However, differences in the biotic environment can
also lead to different decomposition rates among
ecosystems, and these differences need also to be
kept in mind when interpreting transplant results.
For example, differences in the soil invertebrate
community and, perhaps to a lesser extent, differences in the microbial community may accelerate
decomposition in one ecosystem relative to another; separating abiotic from biotic factors requires a more elaborate experimental design. Notwithstanding the difficulty of separating the effects
of the abiotic from the biotic environment, recip-
(e.g., Melillo et al. 1982; Aber et al. 1990). Palm
and Rowland (1997) suggest a standard set of analyses for characterizing plant litter quality, as do
Harmon and Lajtha (1999).
Woody Detritus
Woody detritus includes both fine and coarse
woody debris. Fine fractions are defined operationally as dead trees and branches < 10 cm in diameter
and < 1 m long, as well as twigs and roots too large
for litter bags. Coarse fractions include boles,
branches, and roots larger than fine fractions. The
principal reason for dividing woody detritus into
these fractions is mode of study: fine fractions can
be weighed intact while coarse fractions must be
subsampled in the field.
Methods for studying woody detritus are analogous to litter bag methods but usually without the
need for litter confinement. Woody material can be
cut to standard lengths (a typical length is lO times
longer than the mean diameter of the material),
tethered and marked for later collection, and placed
in the field in microclimates that would be normally
encountered-on the litter layer, suspended above
the soil surface (for standing dead material such as
dead branches still attached to stems), or buried (for
coarse roots).
At intervals appropriate to the climatic environment and composition of the wood, tethered pieces
are collected and taken to the laboratory for drying,
weighing, and chemical analysis. A minimum of lO
sample collections will likely be needed, spaced at
1- to 3-year intervals. Any given study may take
decades to complete.
Decomposition of woody material is a function
of size and species as well as placement, so the
number of samples initially deployed will need to
be chosen carefully to provide a geometric range of
diameter classes and an appropriate range of genera. It is usually not necessary to sample below the
genus level except for genera with diverse heartwood contents. Because bark, sapwood, and heartwood decay at different rates, it is important to record the proportions of these materials in the pieces
initially deployed. As for fine litter, replication
should be organized to avoid pseudoreplication.
Sample preparation, chemical analyses, and calculations are similar to those for fine plant litter,
109
though analyses should be performed separately for
bark, sapwood, and heartwood.
Because it is impractical to weigh large woody
debris, one can remove trimmed end pieces from
the field for weighing and chemical analyses. Prior
to initial sampling, it is necessary to estimate the
mass of each piece that will eventually be removed;
this should be done at the outset of the study by
measuring in situ the volume of each segment that
will be removed and assuming an initial density
(g cm - 3) equal to the density of the first sample.
Formulas for determining the volume of complex
shapes are provided in Harmon et al. (1999).
Reciprocal Transplants
and Standard Substrates
The relative influence of climate versus litter quality for controlling decomposition rates can be evaluated using reciprocal litter transplants and standard substrates. Reciprocal litter transplants usually
involve exchanging litter bags or tethered wood
among two or more sites: litter from ecosystem A
is allowed to decompose in both ecosystem A and
ecosystem B, as is litter from ecosystem B. This
scheme effectively removes litter quality as an explanation for differences in the decomposition rates
of litter from the two ecosystems. For example,
ecosystem A litter should decompose at the same
rate in each ecosystem if the decomposition environment of ecosystem A is similar to that of ecosystem B; a faster rate of litter disappearance in
ecosystem B means a more favorable environment
in ecosystem B (see Fig. 7.2).
Different rates of decay are usually interpreted
to mean different abiotic conditions among sites,
that is, between-site differences in moisture, temperature, and (for buried litter) aeration regimes.
However, differences in the biotic environment can
also lead to different decomposition rates among
ecosystems, and these differences need also to be
kept in mind when interpreting transplant results.
For example, differences in the soil invertebrate
community and, perhaps to a lesser extent, differences in the microbial community may accelerate
decomposition in one ecosystem relative to another; separating abiotic from biotic factors requires a more elaborate experimental design. Notwithstanding the difficulty of separating the effects
of the abiotic from the biotic environment, recip-