Geography Reference
In-Depth Information
the woodland is evergreen. The differences may be related to plate
tectonics where deciduous habitats only evolved in the north after
the large land mass of
Gondwanaland
split up. There is a fairly
short transitional zone between the deciduous forest and the more
northerly Boreal forest biome. The climate in the deciduous forest
biome is moist but temperate all year, with over four months having
a mean temperature above 10°C. Well-mixed soils are typical with a
rich soil fauna and the soil holding plenty of nutrients. In the best
areas for the forest there are four main layers to the structure of the
vegetation. The upper canopy at up to 30 metres high has broad,
rounded tree tops. There is a shrub layer below around 5 metres in
height, with a third layer of grasses and a ground layer of mosses and
liverworts. The vegetation cover is seasonal with dramatic changes to
the appearance of the system through the year. The loss of leaves in
winter reduces transpiration and frost or snow damage. Flowering of
trees is usually very early in the spring providing as much time as
possible for fruits to develop. Some animals may hibernate during
the more dormant period or burrow to avoid winter cold. Deer and
bears were common but humans have modified these landscapes
through hunting and deforestation.
The southern evergreen forest tends to have a similar climate to
the deciduous forest in the north but generally there are two upper
tree canopies and a shrub layer. As the forest is evergreen then
ground level plants are less common as there are limited opportun-
ities for good light to reach the forest floor.
Tropical biomes
The tropical biomes cover highly productive tropical rainforests, less
productive savanna and low productivity hot deserts. Tropical forests
tend to be consistently moist and warm, typically occurring where
average annual temperatures are around 25°C with little seasonal
variation, and where there is around 2,000 millimetres of regular
rainfall each year. The regular water supply creates plentiful stream
networks and major rivers such as the Amazon and Congo. Soils in
tropical forests are deep, but relatively infertile since most of the
nutrients are stored within the above-ground structure of organisms,
although an amazing number of soil organisms also store biomass.
The cycling of nutrients is extremely rapid so plant litter does not
