4
Methods of Estimating Belowground Net
Primary Production
William K. Lauenroth
Introduction
Despite much recent attention over the past
30 years, belowground net primary production
(BNPP) remains one of the poorest understood attributes of terrestrial ecosystems (Milchunas and
Lauenroth 1992; Nadelhoffer and Raich 1992). Estimates of the ratio of BNPP to net primary production (NPP) across ecosystem types range from
less than 0.20 to more than 0.80 (Bray 1963, Coleman 1976). Part of this range can be explained by
differences among ecosystems, but a portion of it
is the result of differences among methods. For
instance, Aber et al. (1985) reported ratios of fine
root production by two different methods for the
same sites ranging from close to 1 to more than
10. In addition to a wide range of estimates of the
importance of BNPP across ecosystem types, estimates within ecosystems also vary widely. For
instance, Sims and Singh (1978) found that BNPP
accounted for 24 to 87% of NPP over a range of
grassland ecosystems and Vogt (1991) reported
that fine root production accounted for 7 to 76%
of NPP over a range of forest ecosystems. How
much of this variability is the result of differences
among ecosystems and how much is the result of
differences among methods? While there is no
agreement on the answer to this question, there is
abundant evidence that all of the current methods
have important strengths and limitations. The objective of this chapter is to describe the most frequently used methods to estimate BNPP and their
strengths and weaknesses. The first section provides definitions of terms and descriptions of concepts. The next section describes the methods,
58
comments on their strengths and weaknesses, and
provides references in which they have been applied. The last section evaluates the degree to
which uncertainty (variability) in input data influences the uncertainty of the resulting estimate of
BNPP.
Concepts
Belowground net primary production is not a directly observable characteristic of an ecosystem.
One cannot go to the field and in a strict sense
measure BNPP. Consequently, approaches to assessing BNPP consist of a collection of methods
to approximate the amount of biomass or carbon
that gets assimilated belowground over a particular
interval. Each method consists of a technique to
collect the relevant information in the field and an
assumed mathematical relationship between the
variable(s) sampled in the field and BNPP.
Conceptually, BNPP during the interval from
t = 1 to t = 2 is:
BNPP = MJ + Mf + M + MJ (4.1)
where MJ is the change in belowground biomass
between t = 1 and t = 2, Mf is the amount of
biomass consumed by herbivores, M is the
amount of biomass lost to exudation and sloughing, and MJ is the amount of biomass lost to death
and detachment-all during the interval between
t = 1 and t = 2. The most common time interval
encountered in the literature for estimates of BNPP
is a year. The most frequently encountered quantities are annual BNPP (grasslands) or annual net
Methods of Estimating Belowground Net
Primary Production
William K. Lauenroth
Introduction
Despite much recent attention over the past
30 years, belowground net primary production
(BNPP) remains one of the poorest understood attributes of terrestrial ecosystems (Milchunas and
Lauenroth 1992; Nadelhoffer and Raich 1992). Estimates of the ratio of BNPP to net primary production (NPP) across ecosystem types range from
less than 0.20 to more than 0.80 (Bray 1963, Coleman 1976). Part of this range can be explained by
differences among ecosystems, but a portion of it
is the result of differences among methods. For
instance, Aber et al. (1985) reported ratios of fine
root production by two different methods for the
same sites ranging from close to 1 to more than
10. In addition to a wide range of estimates of the
importance of BNPP across ecosystem types, estimates within ecosystems also vary widely. For
instance, Sims and Singh (1978) found that BNPP
accounted for 24 to 87% of NPP over a range of
grassland ecosystems and Vogt (1991) reported
that fine root production accounted for 7 to 76%
of NPP over a range of forest ecosystems. How
much of this variability is the result of differences
among ecosystems and how much is the result of
differences among methods? While there is no
agreement on the answer to this question, there is
abundant evidence that all of the current methods
have important strengths and limitations. The objective of this chapter is to describe the most frequently used methods to estimate BNPP and their
strengths and weaknesses. The first section provides definitions of terms and descriptions of concepts. The next section describes the methods,
58
comments on their strengths and weaknesses, and
provides references in which they have been applied. The last section evaluates the degree to
which uncertainty (variability) in input data influences the uncertainty of the resulting estimate of
BNPP.
Concepts
Belowground net primary production is not a directly observable characteristic of an ecosystem.
One cannot go to the field and in a strict sense
measure BNPP. Consequently, approaches to assessing BNPP consist of a collection of methods
to approximate the amount of biomass or carbon
that gets assimilated belowground over a particular
interval. Each method consists of a technique to
collect the relevant information in the field and an
assumed mathematical relationship between the
variable(s) sampled in the field and BNPP.
Conceptually, BNPP during the interval from
t = 1 to t = 2 is:
BNPP = MJ + Mf + M + MJ (4.1)
where MJ is the change in belowground biomass
between t = 1 and t = 2, Mf is the amount of
biomass consumed by herbivores, M is the
amount of biomass lost to exudation and sloughing, and MJ is the amount of biomass lost to death
and detachment-all during the interval between
t = 1 and t = 2. The most common time interval
encountered in the literature for estimates of BNPP
is a year. The most frequently encountered quantities are annual BNPP (grasslands) or annual net