498
F.1. Woodward and T.M. Smith
23.4 Modeling Amax at the Global Scale
23.4.1 Introduction
Post et al. (1982, 1985) and Zinke et al. (1984) related estimates of soil
carbon and nitrogen to global patterns of climate and vegetation by classifying each soil profile by the Holdridge Life-Zone (1947, 1967) in which it
was found. The Holdridge Life-Zone Classification (Holdridge 1967) is a
bioclimatic classification scheme relating the distribution of major ecosystem
complexes or biomes to the climatic variables of biotemperature; mean
annual precipitation, and the ratio of potential evapotranspiration (PET) to
precipitation. The scheme defines a total of 36 biomes (or climatically
defined life zones) at a global scale. The classifications are geographically
independent, assuming that the same biome will occur on differing continents
given the same climatic conditions.
In the Holdridge scheme, each particular life zone (biomes/vegetation
types) is uniquely occurring in a zone of climate, defined by average annual
biotemperature and average annual precipitation. The average biotemperature is the average temperature over a year above a threshold of O°C. The
average biotemperature and average annual precipitation (both logarithmic
scales) form two sides of an equilateral triangle. A logarithmic axis for
the potential evapotranspiration (PET) ratio (effective humidity) forms the
third side of the triangle. By marking equal intervals along the three axes,
hexagons are defined within which a particular life-zone or biome occurs.
One additional division in the scheme is based on the occurrence of
potentially killing frosts. This division is along a critical temperature line that
divides hexagons between 12 and 24°C into warm temperate and subtropical
zones. Smith et al. (1992) provide a complete description and evaluation of
the Holdridge scheme.
Although errors, e.g., no consideration of seasonality, correlative not
mechanistic base, are involved in this simple classification (Woodward 1987),
it provides a broadly accurate method of representing the correlation between biome distribution and climate at a global scale (Smith et al. 1992). In
addition, it is a useful approach for extrapolating point data (e.g., soil C and
N) from various biomes to climate, in order to provide global coverage from
small data sets for representative biomes. The areal extent and global
classification of biome types are presented in Table 23.1.
23.4.2 Method of Predicting Amax from Soil C
The experimentally established relationship between soil C and Amax (Fig.
23.4) will be used to predict Amax without environmental constraints. The
procedure for providing a global coverage of Amax predictions uses the
global climate database of Leemens and Cramer (1990). The data base
F.1. Woodward and T.M. Smith
23.4 Modeling Amax at the Global Scale
23.4.1 Introduction
Post et al. (1982, 1985) and Zinke et al. (1984) related estimates of soil
carbon and nitrogen to global patterns of climate and vegetation by classifying each soil profile by the Holdridge Life-Zone (1947, 1967) in which it
was found. The Holdridge Life-Zone Classification (Holdridge 1967) is a
bioclimatic classification scheme relating the distribution of major ecosystem
complexes or biomes to the climatic variables of biotemperature; mean
annual precipitation, and the ratio of potential evapotranspiration (PET) to
precipitation. The scheme defines a total of 36 biomes (or climatically
defined life zones) at a global scale. The classifications are geographically
independent, assuming that the same biome will occur on differing continents
given the same climatic conditions.
In the Holdridge scheme, each particular life zone (biomes/vegetation
types) is uniquely occurring in a zone of climate, defined by average annual
biotemperature and average annual precipitation. The average biotemperature is the average temperature over a year above a threshold of O°C. The
average biotemperature and average annual precipitation (both logarithmic
scales) form two sides of an equilateral triangle. A logarithmic axis for
the potential evapotranspiration (PET) ratio (effective humidity) forms the
third side of the triangle. By marking equal intervals along the three axes,
hexagons are defined within which a particular life-zone or biome occurs.
One additional division in the scheme is based on the occurrence of
potentially killing frosts. This division is along a critical temperature line that
divides hexagons between 12 and 24°C into warm temperate and subtropical
zones. Smith et al. (1992) provide a complete description and evaluation of
the Holdridge scheme.
Although errors, e.g., no consideration of seasonality, correlative not
mechanistic base, are involved in this simple classification (Woodward 1987),
it provides a broadly accurate method of representing the correlation between biome distribution and climate at a global scale (Smith et al. 1992). In
addition, it is a useful approach for extrapolating point data (e.g., soil C and
N) from various biomes to climate, in order to provide global coverage from
small data sets for representative biomes. The areal extent and global
classification of biome types are presented in Table 23.1.
23.4.2 Method of Predicting Amax from Soil C
The experimentally established relationship between soil C and Amax (Fig.
23.4) will be used to predict Amax without environmental constraints. The
procedure for providing a global coverage of Amax predictions uses the
global climate database of Leemens and Cramer (1990). The data base