492
F.I. Woodward and T.M. Smith
synthesis (Amax) at a global scale and to follow this by validating the model
against global scale observations of Amax in the field.
23.2 Philosophy
Field and Mooney (1986) have argued that the correlation between the
concentration of leaf nitrogen and Amax is a general case which applies to
plants across a wide variety of plant communities. Indeed they state that:
"From a purely predictive viewpoint, the correlation should prove a useful
tool in the preliminary characterization of photosynthetic capacities", although they move on to indicate that the mechanistic basis to the correlation
must be understood in order to establish a firm, functional foundation to
ecological understanding.
The ranges of the correlations between leaf Nand Amax are shown in Fig.
23.1 for the mean case (Field and Mooney 1986) and for the extremes of
observations (from Evans 1989). There is clearly a large range of possible
predictions of Amax from a single value of leaf N. This range is too large for
satisfactory prediction. Field and Mooney (1986) and Evans (1989) interpret
the large range of Amax predictions as variations in different patterns of N
partitioning between species and within plants, and as inter- and intraspecific
differences in plant responses to environmental constraints. As Field and
Mooney (1986) succinctly conclude: "Amax is not an ecologically meaningless
physiological parameter, but is an index of integrated natural constraints on
photosynthesis". However, species differ in their capacities to accumulate
and store N, particularly nitrate, (Millard 1988), in addition to the influences
of rates of leaf expansion on the concentration of leaf N (Evans 1989).
Therefore it is the complexity of function of leaf N which may account for
much of the scatter of the relationship between leaf Nand Amax and it is this
aspect which will be investigated further.
The particular aim of the work presented here is to provide a general
model which can predict the effects of changes in soil N on Amax, with a
simplicity which cannot be currently achieved from the traditional approach
relating the concentration of total N to Amax, such as shown in Fig. 23.1. In
this chapter, Amax is taken to be the highest rate of photosynthesis observed
in a vegetation, or biome type, and which is characteristic of the major
dominant species of the biome. Therefore Amax excludes the effects of
reduced irradiance, drought, vapor pressure deficit, and nonoptimal temperatures. These effects can be incorporated as environmental constraints
on Amax, always acting to reduce the realized value of Amax in the field, but
this aspect will not be presented in this chapter.
The overall philosophy is that as leaf N is mechanistically related to Amax
(Evans 1989) and that leaf N originates from the soil, then the first step in
predicting Amax should be based on the capacity of the soil to provide N to
F.I. Woodward and T.M. Smith
synthesis (Amax) at a global scale and to follow this by validating the model
against global scale observations of Amax in the field.
23.2 Philosophy
Field and Mooney (1986) have argued that the correlation between the
concentration of leaf nitrogen and Amax is a general case which applies to
plants across a wide variety of plant communities. Indeed they state that:
"From a purely predictive viewpoint, the correlation should prove a useful
tool in the preliminary characterization of photosynthetic capacities", although they move on to indicate that the mechanistic basis to the correlation
must be understood in order to establish a firm, functional foundation to
ecological understanding.
The ranges of the correlations between leaf Nand Amax are shown in Fig.
23.1 for the mean case (Field and Mooney 1986) and for the extremes of
observations (from Evans 1989). There is clearly a large range of possible
predictions of Amax from a single value of leaf N. This range is too large for
satisfactory prediction. Field and Mooney (1986) and Evans (1989) interpret
the large range of Amax predictions as variations in different patterns of N
partitioning between species and within plants, and as inter- and intraspecific
differences in plant responses to environmental constraints. As Field and
Mooney (1986) succinctly conclude: "Amax is not an ecologically meaningless
physiological parameter, but is an index of integrated natural constraints on
photosynthesis". However, species differ in their capacities to accumulate
and store N, particularly nitrate, (Millard 1988), in addition to the influences
of rates of leaf expansion on the concentration of leaf N (Evans 1989).
Therefore it is the complexity of function of leaf N which may account for
much of the scatter of the relationship between leaf Nand Amax and it is this
aspect which will be investigated further.
The particular aim of the work presented here is to provide a general
model which can predict the effects of changes in soil N on Amax, with a
simplicity which cannot be currently achieved from the traditional approach
relating the concentration of total N to Amax, such as shown in Fig. 23.1. In
this chapter, Amax is taken to be the highest rate of photosynthesis observed
in a vegetation, or biome type, and which is characteristic of the major
dominant species of the biome. Therefore Amax excludes the effects of
reduced irradiance, drought, vapor pressure deficit, and nonoptimal temperatures. These effects can be incorporated as environmental constraints
on Amax, always acting to reduce the realized value of Amax in the field, but
this aspect will not be presented in this chapter.
The overall philosophy is that as leaf N is mechanistically related to Amax
(Evans 1989) and that leaf N originates from the soil, then the first step in
predicting Amax should be based on the capacity of the soil to provide N to