62
to differential herbivory and decomposition of 14C
compared with l2C. They subsequently adopted a
14C turnover method first described by Dahlman
and Kucera (1967). This method is similar to the
14C dilution method in that it requires an initial
pulse labeling. The decrease in 14C in the root system is then followed for a period sufficient to describe the rate of loss. After 2 years, Dahlman and
Kucera (1967) fit a linear regression to their loss
data and concluded that the root system would tum
over every 4 years. Milchunas and Lauenroth
(1992) fit a linear regression to their loss data after
3 years and estimated that the root system would
tum over every 5 to 7 years. In addition to information about the dynamics of 14C in the root system, the carbon isotope methods require an estimate
of belowground biomass. The turnover coefficient
calculated from the tracer data is multiplied by the
biomass data to obtain an estimate of BNPP.
Dahlman and Kucera (1965, 1967) estimated
root turnover using biomass methods and 14C turnover and found good agreement between the two
methods. Milchunas and Lauenroth (1992) calculated both ANPP and BNPP using the 14C turnover
method and found excellent agreement between the
estimate for ANPP using 14C turnover and an estimate based upon biomass. Milchunas and Lauenroth (1992) interpreted this agreement as support
for the use of 14C turnover to estimate BNPP. The
limitations of the 14C turnover method are largely
related to the difficulty of introducing the label and
working with radioactive materials. The requirement to be able to label the vegetation limits the
applicability to ecosystems dominated by shortstatured plants, such as grasslands and shrublands.
Use of 13C would solve the radioactive materials
problem, but it is substantially more expensive than
14C. This is an excellent method for grasslands and
shrublands if the problems with the labeled material
can be resolved.
Carbon Balance
This and the following method using nitrogen balance exploit knowledge about element budgets to
estimate BNPP. The carbon balance method uses
the law of conservation of mass to derive the following conceptual model (Nadlehoffer et al. 1998):
William K. Lauenroth
where TRCA is total root carbon allocation, Rs is
soil respiration, P a is aboveground litterfall, E is
export as a result of leaching or erosion, L\Cr is the
change in root carbon (coarse + fine), and L\C s is
the change in soil carbon. Site-specific application
of the method relies on estimates of Rs and P a and
either measurements of E, L\Cr and L\Cs' or an assumption of steady state (Binkley and Ryan 1998).
Raich and Nadlehoffer (1989) developed a statistical model for ecosystems at or near steady state
by assuming that:
(4.4)
where Rh is heterotrophic respiration and Pb is
belowground litter production. Recognizing that
soil respiration comprises both root (Rr) and heterotrophic respiration, estimates of TRCA (P b
+ Rr) can be calculated by the relationship:
Pb + Rr = Rs - Pa
(4.5)
This relationship effectively shifts the problem of
estimating BNPP from estimating the change in
belowground plant biomass to estimating aboveground litter production (see Chapter 2) and soil
respiration (Chapter 7). Both Pa and Rs have been
measured for many ecosystems, and Raich and N adelhoffer (1989) reported significant regressions
between P a and Rs (,-2 = 0.71) and between TRCA
and P a (,-2 = 0.52). Their final statistical model is
TRCA = 1.92 P a - 130
(4.6)
Accepting these relationships further reduces the
problem of estimating BNPP to one of only estimating aboveground litter production (P a ).
Raich and Nadelhoffer (1989) suggested that
their relationships were useful to place an upper
constraint on the amount of carbon allocated to
roots (for tissue production and respiration), and
therefore NFRP, and Nadelhoffer and Raich (1992)
recommended that estimates of NFRP always be
compared with TRCA. They also warned that the
large scale of their analysis may have overlooked
important smaller-scale variability (Raich and Nadelhoffer 1989) and seasonal and yearly variability
within sites (Nadelhoffer and Raich 1992). Gower
et al. (1996) tested the utility of the Raich and
Nadelhoffer (1989) statistical model to estimate
TRCA and NFRP at a range of temperate forest
sites and concluded that it should not be used at the
TRCA = Rs - Pa + E + L\Cr - L\Cs (4.3) individual site scale. In a rejoinder to the Gower et
to differential herbivory and decomposition of 14C
compared with l2C. They subsequently adopted a
14C turnover method first described by Dahlman
and Kucera (1967). This method is similar to the
14C dilution method in that it requires an initial
pulse labeling. The decrease in 14C in the root system is then followed for a period sufficient to describe the rate of loss. After 2 years, Dahlman and
Kucera (1967) fit a linear regression to their loss
data and concluded that the root system would tum
over every 4 years. Milchunas and Lauenroth
(1992) fit a linear regression to their loss data after
3 years and estimated that the root system would
tum over every 5 to 7 years. In addition to information about the dynamics of 14C in the root system, the carbon isotope methods require an estimate
of belowground biomass. The turnover coefficient
calculated from the tracer data is multiplied by the
biomass data to obtain an estimate of BNPP.
Dahlman and Kucera (1965, 1967) estimated
root turnover using biomass methods and 14C turnover and found good agreement between the two
methods. Milchunas and Lauenroth (1992) calculated both ANPP and BNPP using the 14C turnover
method and found excellent agreement between the
estimate for ANPP using 14C turnover and an estimate based upon biomass. Milchunas and Lauenroth (1992) interpreted this agreement as support
for the use of 14C turnover to estimate BNPP. The
limitations of the 14C turnover method are largely
related to the difficulty of introducing the label and
working with radioactive materials. The requirement to be able to label the vegetation limits the
applicability to ecosystems dominated by shortstatured plants, such as grasslands and shrublands.
Use of 13C would solve the radioactive materials
problem, but it is substantially more expensive than
14C. This is an excellent method for grasslands and
shrublands if the problems with the labeled material
can be resolved.
Carbon Balance
This and the following method using nitrogen balance exploit knowledge about element budgets to
estimate BNPP. The carbon balance method uses
the law of conservation of mass to derive the following conceptual model (Nadlehoffer et al. 1998):
William K. Lauenroth
where TRCA is total root carbon allocation, Rs is
soil respiration, P a is aboveground litterfall, E is
export as a result of leaching or erosion, L\Cr is the
change in root carbon (coarse + fine), and L\C s is
the change in soil carbon. Site-specific application
of the method relies on estimates of Rs and P a and
either measurements of E, L\Cr and L\Cs' or an assumption of steady state (Binkley and Ryan 1998).
Raich and Nadlehoffer (1989) developed a statistical model for ecosystems at or near steady state
by assuming that:
(4.4)
where Rh is heterotrophic respiration and Pb is
belowground litter production. Recognizing that
soil respiration comprises both root (Rr) and heterotrophic respiration, estimates of TRCA (P b
+ Rr) can be calculated by the relationship:
Pb + Rr = Rs - Pa
(4.5)
This relationship effectively shifts the problem of
estimating BNPP from estimating the change in
belowground plant biomass to estimating aboveground litter production (see Chapter 2) and soil
respiration (Chapter 7). Both Pa and Rs have been
measured for many ecosystems, and Raich and N adelhoffer (1989) reported significant regressions
between P a and Rs (,-2 = 0.71) and between TRCA
and P a (,-2 = 0.52). Their final statistical model is
TRCA = 1.92 P a - 130
(4.6)
Accepting these relationships further reduces the
problem of estimating BNPP to one of only estimating aboveground litter production (P a ).
Raich and Nadelhoffer (1989) suggested that
their relationships were useful to place an upper
constraint on the amount of carbon allocated to
roots (for tissue production and respiration), and
therefore NFRP, and Nadelhoffer and Raich (1992)
recommended that estimates of NFRP always be
compared with TRCA. They also warned that the
large scale of their analysis may have overlooked
important smaller-scale variability (Raich and Nadelhoffer 1989) and seasonal and yearly variability
within sites (Nadelhoffer and Raich 1992). Gower
et al. (1996) tested the utility of the Raich and
Nadelhoffer (1989) statistical model to estimate
TRCA and NFRP at a range of temperate forest
sites and concluded that it should not be used at the
TRCA = Rs - Pa + E + L\Cr - L\Cs (4.3) individual site scale. In a rejoinder to the Gower et