tropics) is highly differentiated relative to
variation in length of the dry season. A
southern (coniferous) forest climate and
northern (forest-tundra) climate can be distinguished within the Subarctic Division of
the Polar Domain on the basis of temperature differences.
7. The arrangement of ecoclimate zones
depends largely on latitude and continental
position with location-specific influences
derived from elevation. Mountain ranges
that cut across latitudinally oriented
ecoclimate zones support their own
ecosystems. Elevation creates characteristic
ecological zones that are variations of the
lowland climate. Mountains show typical
climatic characteristics depending on their
location in the overall pattern of global
ecoclimatic zones. The mountain ranges of
Central America, for example, experience
the same year-round, high-energy input and
seasonal moisture regime consisting of relatively dry winters and rainy summers that
are typical of their neighboring lowlands.
Beyond these principles, an additional point
must be made. Many people think of climate as
weather elements (e.g., mean annual temperature
or precipitation) that go into a database or constitute a data layer in a geographic information
system (GIS). Ecological climate is emphatically
more than that. It is an integrated concept and
consideration should be given to how climate
affects all other elements of the ecosystem, both
at a site and across the landscape.
The most frequently used climate classification that follows these principles was developed
and proposed by Wladimir Ko ¨ppen in his seminal book on the subject (Ko ¨ppen 1931). Building on ideas of an earlier bioclimatologist, de
Candolle (1874), Ko ¨ppen carefully observed
the climatic conditions required for the growth
of various groups of plants, and he related
variations in vegetation to temperature and precipitation. Ko ¨ppen’s system is based on the
concept that native vegetation is the best
expression of climate. Perhaps the main shortcoming of this classification lies in the fact that
the boundaries of certain climate types do not
correspond with the observed boundaries of
natural landscapes. This led Trewartha (1968)
to slightly modify the Ko ¨ppen scheme by
establishing more realistic criteria to distinguish
climate types.
I developed an approach to mapping
ecoregions for the United States (Bailey 1976,
revised 1994), North America (Bailey 1998), and
the world’s continents (Bailey 1989) based on
these principles. The boundaries of these
ecoregions follow the subdivision of the Earth
into climatic zones established by Ko ¨ppen as
modified by Trewartha (1968, Table 4.2),
2
although some modifications have been made to
maximize correspondence of the regions with the
vegetation (plant formation classes).
4.2.2 The Ko ¨ ppen–Trewartha
Classification of Climates
The Ko ¨ppen–Trewartha classification identified
six main groups of climate, and all but one—the
dry group—are thermally defined. They are as
follows:
Based on temperature criteria
A. Tropical: Frost limit in continental
locations; in marine areas 18
C for the coolest
month
C. Subtropical: 8 months 10
C of above
D. Temperate: 4 months 10
C or above
E. Boreal: 1 (warmest) month 10
C or above
F. Polar: All months below 10
C
Based on precipitation criteria
B. Dry: Outer limits, where potential evaporation equals precipitation
These groups are subdivided into 15 types
based on seasonality of precipitation or on degree
of dryness or cold. They range from the icecaps
2 Other methods for mapping zones at the global scale are
those of Thornthwaite (1931, 1933), Holdridge (1947),
and Walter and Box (1976). All methods appear to work
better in some areas than in others and to have gained their
own adherents. The Ko ¨ppen system was chosen as the
basis for ecoregion delineation because it has become the
international standard for geographical purposes.
36
4 Continental Types and Their Controls
variation in length of the dry season. A
southern (coniferous) forest climate and
northern (forest-tundra) climate can be distinguished within the Subarctic Division of
the Polar Domain on the basis of temperature differences.
7. The arrangement of ecoclimate zones
depends largely on latitude and continental
position with location-specific influences
derived from elevation. Mountain ranges
that cut across latitudinally oriented
ecoclimate zones support their own
ecosystems. Elevation creates characteristic
ecological zones that are variations of the
lowland climate. Mountains show typical
climatic characteristics depending on their
location in the overall pattern of global
ecoclimatic zones. The mountain ranges of
Central America, for example, experience
the same year-round, high-energy input and
seasonal moisture regime consisting of relatively dry winters and rainy summers that
are typical of their neighboring lowlands.
Beyond these principles, an additional point
must be made. Many people think of climate as
weather elements (e.g., mean annual temperature
or precipitation) that go into a database or constitute a data layer in a geographic information
system (GIS). Ecological climate is emphatically
more than that. It is an integrated concept and
consideration should be given to how climate
affects all other elements of the ecosystem, both
at a site and across the landscape.
The most frequently used climate classification that follows these principles was developed
and proposed by Wladimir Ko ¨ppen in his seminal book on the subject (Ko ¨ppen 1931). Building on ideas of an earlier bioclimatologist, de
Candolle (1874), Ko ¨ppen carefully observed
the climatic conditions required for the growth
of various groups of plants, and he related
variations in vegetation to temperature and precipitation. Ko ¨ppen’s system is based on the
concept that native vegetation is the best
expression of climate. Perhaps the main shortcoming of this classification lies in the fact that
the boundaries of certain climate types do not
correspond with the observed boundaries of
natural landscapes. This led Trewartha (1968)
to slightly modify the Ko ¨ppen scheme by
establishing more realistic criteria to distinguish
climate types.
I developed an approach to mapping
ecoregions for the United States (Bailey 1976,
revised 1994), North America (Bailey 1998), and
the world’s continents (Bailey 1989) based on
these principles. The boundaries of these
ecoregions follow the subdivision of the Earth
into climatic zones established by Ko ¨ppen as
modified by Trewartha (1968, Table 4.2),
2
although some modifications have been made to
maximize correspondence of the regions with the
vegetation (plant formation classes).
4.2.2 The Ko ¨ ppen–Trewartha
Classification of Climates
The Ko ¨ppen–Trewartha classification identified
six main groups of climate, and all but one—the
dry group—are thermally defined. They are as
follows:
Based on temperature criteria
A. Tropical: Frost limit in continental
locations; in marine areas 18
C for the coolest
month
C. Subtropical: 8 months 10
C of above
D. Temperate: 4 months 10
C or above
E. Boreal: 1 (warmest) month 10
C or above
F. Polar: All months below 10
C
Based on precipitation criteria
B. Dry: Outer limits, where potential evaporation equals precipitation
These groups are subdivided into 15 types
based on seasonality of precipitation or on degree
of dryness or cold. They range from the icecaps
2 Other methods for mapping zones at the global scale are
those of Thornthwaite (1931, 1933), Holdridge (1947),
and Walter and Box (1976). All methods appear to work
better in some areas than in others and to have gained their
own adherents. The Ko ¨ppen system was chosen as the
basis for ecoregion delineation because it has become the
international standard for geographical purposes.
36
4 Continental Types and Their Controls