Predictions and Measurements of the Maximum Photosynthetic Rate
503
23.6 Conclusions: Global Scale Maps of Observed and Predicted Amax
The final approach of this chapter is to present a global map of predicted
Amax and observed values of A, by biome type. As for the estimates of soil
C and N, it has been assumed that the same biome type has the same value
of Amax or A on different continents, and even if field measurements are
only available from one continent or region.
The overall impression from the two maps (Figs. 23.7 and 23.8) is of a
general agreement between observations (Fig. 23.7) and predictions (Fig.
23.8). Amax tends to increase in an equatorial direction, for both observations
and predictions. Both sets of measurements are in agreement over the major
subtropical desert regions of the world, such as the northern Sahara. No
field observations of A in the tropical desert regions have been found, while
the predictions are for low rates in soils which are virtually skeletal with
little C or N (Post et a1. 1982, 1985).
The agreement between the high values of Amax in the tropical very dry
forests of the Sahel, India, and Australia may be fortuitous as grasses with
C4 photosynthesis form a very significant part of this savanna vegetatioll
(Ellis et a1. 1980; Livingstone and Clayton 1980). C4 grasses have high
values of Amax, compared with species with the C3 pathway of photosynthesis
(Pearch and Ehleringer 1984) and have not been modeled in the predictions.
However, a source of N is also required for species with C4 photosynthesis
and, in spite of higher nitrogen use efficiency (Pearcy and Ehleringer 1984),
it is to be expected that the rate of N supply from the soil is still the major
determinant of Amax (Wallace et a1. 1982). Therefore Amax could be very
similar for species with C3 and C4 photosynthesis co-occurring, a feature
which has been observed (Caldwell et a1. 1977).
The values of Amax (Figs. 23.7 and 23.8) have been observed and predicted
for conditions in which water supply by precipitation is least limiting. If the
effects of precipitation are included to provide an annual time course, then
the maps will appear very different from month to month and for the annual
average (Woodward 1987). This will be the next and future development of
the model, in parallel to predictions of leaf area index (Woodward 1987),
because it is the product of leaf area index and net photosynthesis which will
predict net primary productivity, a property which has also been measured
and which is a vital ecosystem property. Lange et a1. (1987) have the final
words for this endeavor: "However, photosynthetic carbon gain at the level
of the single leaf does not necessarily reflect plant characteristics at higher
levels of organization. At the whole plant or even at the community level
additional regulation occurs which may be of even greater significance.
Caution is warranted in generalizing about the ecological meaning of
leaf physiological processes for production or even plant success in
nature".
503
23.6 Conclusions: Global Scale Maps of Observed and Predicted Amax
The final approach of this chapter is to present a global map of predicted
Amax and observed values of A, by biome type. As for the estimates of soil
C and N, it has been assumed that the same biome type has the same value
of Amax or A on different continents, and even if field measurements are
only available from one continent or region.
The overall impression from the two maps (Figs. 23.7 and 23.8) is of a
general agreement between observations (Fig. 23.7) and predictions (Fig.
23.8). Amax tends to increase in an equatorial direction, for both observations
and predictions. Both sets of measurements are in agreement over the major
subtropical desert regions of the world, such as the northern Sahara. No
field observations of A in the tropical desert regions have been found, while
the predictions are for low rates in soils which are virtually skeletal with
little C or N (Post et a1. 1982, 1985).
The agreement between the high values of Amax in the tropical very dry
forests of the Sahel, India, and Australia may be fortuitous as grasses with
C4 photosynthesis form a very significant part of this savanna vegetatioll
(Ellis et a1. 1980; Livingstone and Clayton 1980). C4 grasses have high
values of Amax, compared with species with the C3 pathway of photosynthesis
(Pearch and Ehleringer 1984) and have not been modeled in the predictions.
However, a source of N is also required for species with C4 photosynthesis
and, in spite of higher nitrogen use efficiency (Pearcy and Ehleringer 1984),
it is to be expected that the rate of N supply from the soil is still the major
determinant of Amax (Wallace et a1. 1982). Therefore Amax could be very
similar for species with C3 and C4 photosynthesis co-occurring, a feature
which has been observed (Caldwell et a1. 1977).
The values of Amax (Figs. 23.7 and 23.8) have been observed and predicted
for conditions in which water supply by precipitation is least limiting. If the
effects of precipitation are included to provide an annual time course, then
the maps will appear very different from month to month and for the annual
average (Woodward 1987). This will be the next and future development of
the model, in parallel to predictions of leaf area index (Woodward 1987),
because it is the product of leaf area index and net photosynthesis which will
predict net primary productivity, a property which has also been measured
and which is a vital ecosystem property. Lange et a1. (1987) have the final
words for this endeavor: "However, photosynthetic carbon gain at the level
of the single leaf does not necessarily reflect plant characteristics at higher
levels of organization. At the whole plant or even at the community level
additional regulation occurs which may be of even greater significance.
Caution is warranted in generalizing about the ecological meaning of
leaf physiological processes for production or even plant success in
nature".