500
F.1. Woodward and T.M. Smith
23.4.3 Validating Amax Predictions
The experimental points presented for 14 species (Fig. 23.4) were the source
of the relationship between soil C and Amax. The scheme described in
Section 4.2 provides the capacity to Amax from soil C, at a global scale. This
approach can be tested against field observations of Amax (A). Unfortunately,
reliable measurements of A are not available for all biomes. The Appendix
shows the sources of field measurements of A which have been used to test
the model predictions. Data have been found for 29 of the 36 biome types
classified by the Holdridge scheme. In order to provide a global coverage, it
must be assumed that measurements of A in a particular biome on one
continent are equally applicable to the same biome on another continent.
The points presented on Fig. 23.4 indicate field observations of A in 29
biomes (data listed in Table 23.1). It may be seen that the experimentally
derived line describes the upper boundary of the relationship between A
and soil C. However, at low values of soil C (usually semi-arid and arid
areas), observations of A are much lower than expected from soil C alone.
It was considered that the total pool of available N in these soils may also be
a limitation to A. Therefore this limitation was also devised to be included
in the model.
23.4.4 Predicting Amax from Soil C and Soil N
The soils used in the experiments all possessed soil N levels greater than
800 g m -2. However in arid regions total soil N may be considerably less
(Post et al. 1985), imposing a limitation on Amax. Therefore the experimentally derived relationship between soil C and Amax needs to account for
the total pool of soil N. In this case, theoretical estimates of the required N
supply for growth (Moll et al. 1982; Evans 1989) and field observations of
soil N limitations of plant growth (Wild 1988; Rendig and Taylor 1989;
Schlesinger 1991), have been used to establish a simple relationship between
soil N and the multiplier of Amax (Fig. 23.5). These theoretical and experimental approaches indicate a simple linear relationship between increasing
Amax and soil N, up to a total soil N of 600 gm- 2 . Beyond that concentration,
there is little evidence for any further responses to total soil N. Therefore
the simple model for Amax is modified so that the estimate of Amax from soil
C (Fig. 23.4) is reduced, if soil N is less than 600 g m -2, by a soil Ndependent multiplier (Fig. 23.5).
The ratio between field observations of A and Amax predicted from soil C
and N indicates that the theoretical relationship between the multiplier
and soil N defines the upper boundary of the relationship, indicating the
suitability of the approach for at least defining the maximum potential
photosynthetic rate based on soil C (i.e., mycorrhizal supply rate) and soil N
(total N pool).
F.1. Woodward and T.M. Smith
23.4.3 Validating Amax Predictions
The experimental points presented for 14 species (Fig. 23.4) were the source
of the relationship between soil C and Amax. The scheme described in
Section 4.2 provides the capacity to Amax from soil C, at a global scale. This
approach can be tested against field observations of Amax (A). Unfortunately,
reliable measurements of A are not available for all biomes. The Appendix
shows the sources of field measurements of A which have been used to test
the model predictions. Data have been found for 29 of the 36 biome types
classified by the Holdridge scheme. In order to provide a global coverage, it
must be assumed that measurements of A in a particular biome on one
continent are equally applicable to the same biome on another continent.
The points presented on Fig. 23.4 indicate field observations of A in 29
biomes (data listed in Table 23.1). It may be seen that the experimentally
derived line describes the upper boundary of the relationship between A
and soil C. However, at low values of soil C (usually semi-arid and arid
areas), observations of A are much lower than expected from soil C alone.
It was considered that the total pool of available N in these soils may also be
a limitation to A. Therefore this limitation was also devised to be included
in the model.
23.4.4 Predicting Amax from Soil C and Soil N
The soils used in the experiments all possessed soil N levels greater than
800 g m -2. However in arid regions total soil N may be considerably less
(Post et al. 1985), imposing a limitation on Amax. Therefore the experimentally derived relationship between soil C and Amax needs to account for
the total pool of soil N. In this case, theoretical estimates of the required N
supply for growth (Moll et al. 1982; Evans 1989) and field observations of
soil N limitations of plant growth (Wild 1988; Rendig and Taylor 1989;
Schlesinger 1991), have been used to establish a simple relationship between
soil N and the multiplier of Amax (Fig. 23.5). These theoretical and experimental approaches indicate a simple linear relationship between increasing
Amax and soil N, up to a total soil N of 600 gm- 2 . Beyond that concentration,
there is little evidence for any further responses to total soil N. Therefore
the simple model for Amax is modified so that the estimate of Amax from soil
C (Fig. 23.4) is reduced, if soil N is less than 600 g m -2, by a soil Ndependent multiplier (Fig. 23.5).
The ratio between field observations of A and Amax predicted from soil C
and N indicates that the theoretical relationship between the multiplier
and soil N defines the upper boundary of the relationship, indicating the
suitability of the approach for at least defining the maximum potential
photosynthetic rate based on soil C (i.e., mycorrhizal supply rate) and soil N
(total N pool).