C.4. Vegetation Classification
The vegetation classification from each model has been aggregated to ten broad
classes for MAPSS and nine for BIOME3. The models are most accurate in differentiating
the broad physiognomic divisions of Forest, Savanna, semi-arid lands (Shrublands
and Grasslands) and Arid Lands. These differences are largely based on the relationship
between leaf area and site water balance and the simulated changes in leaf area
and site water balance should be generally reliable (especially with respect
to the sign of the change). The aggregated vegetation classes used for this
analysis are as follows.
- Tundra is defined as the treeless vegetation which extends beyond
treeline at high latitudes and altitudes, regardless of whether it is dominated
by dwarf shrubs or herbaceous plants.
- Taiga/Tundra is the broad 'ecotonal' region of open woodland, which
occurs at higher latitudes or elevations beyond the 'closed' Boreal forest.
This type is not explicitly simulated by BIOME3, but rather is included in
Boreal Conifer Forest.
- Boreal Conifer Forest is the Taiga proper, i.e., relatively dense
forest composed mainly of needle-leaved trees and occurring in cold-winter
climates.
- Temperate Evergreen Forest encompasses the wet temperate and subtropical
conifer forests of the NorthWest in North America, as well as subtropical
evergreen broadleaf forests (e.g., in China) and the Nothofagus and Eucalyptus
forests of the Southern Hemisphere.
- Temperate Mixed Forest includes pure temperate broadleaf forests,
such as oak-hickory, or beech-maple. It also includes mixtures of broadleaf
and temperate evergreen types, such as the cool-mixed pine/fir and hardwood
forests of the northeastern United States or the warm-mixed pine/hardwood
forests of the southeastern U.S.
- Tropical Broadleaf Forest includes both tropical evergreen forest
and dense tropical drought-deciduous forests.
- Savanna/Woodlands encompass all 'open' tree vegetation from high
to low latitudes and elevations. The tropical dry savannas and drought deciduous
forests are contained within this class. So too are the temperate pine savannas
and 'pygmy' forests and the aspen woodlands adjacent to the boreal forest.
Fire can play an important role in maintaining the open nature of these woodlands;
while, grazing can increase the density of woody vegetation at the expense
of grass.
- Shrub/Woodlands are distinguished from the Savanna/Woodlands by their
lower biomass and shorter stature. This is a drier vegetation type than the
Savanna/Woodlands and encompasses most semi-arid vegetation types from Chaparral
to mesquite woodlands to cold, semi-desert sage shrublands. The actual vegetation
associated with this type is very susceptible to variation depending on soils,
topography, fire, grazing and land-use history. Distinctions between shrub-steppe
and grassland are sometimes difficult to quantify, given that each usually
contains elements of both grass and woody vegetation. The relative abundance
of the two functional types is considered in determining the classification,
but there are no generally accepted rules to indicate how much woody vegetation
is sufficient to label a region a shrubland, or conversely how much grass
is required to label it a grassland.
- Grasslands include both C3 and C4 grassland types in both temperate
and tropical regions. Much of the grassland type is a 'fire climax' type that
would be populated by shrubs either with the absence of fire, or with extensive
grazing.
- Arid Lands encompass all regions drier than grasslands, from hyper-arid
to semi-arid, ranging from the "waterless" deserts such as the Namib to the
'semi-deserts' of central Asia and Patagonia. The regions could be more or
less 'grassy' or 'shrubby' depending on disturbance and land-use history.
