Biology 2483A Lecture Notes - Temperate Deciduous Forest, Seasonal Tropical Forest, Biome
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Ecology Notes – Sept. 18/12
- The biosphere is the zone of life on earth. It is mostly restricted to the surface of the earth. It is
located between the lithosphere (earth’s surface crust and upper mantle) and the troposphere (the
lowest layer of the atmosphere). Biomes are large-scale biological communities shaped by the
physical environment, particularly climate. Biomes are categorized by dominant plant forms, not
taxonomic relationships. They are not characterized by animals because animals are mobile. They
can avoid local conditions by moving around and are more adept at surviving by choosing a more
favourable microclimate. Plants can’t move around, so categorizing biomes based on them is better
and more accurate. Plants occupy sites for a long time and are good indicators of the physical
environment, reflecting climatic conditions and disturbances.
- Terrestrial biomes are characterized by growth forms of the dominant plants, such as leaf
deciduousness (seasonal shedding of leaves) or succulence (think, leathery leaves, development of
fleshy water storage tissues). The growth form of a plant is an evolutionary response to the
- Plant growth forms: Deciduous trees – Environment is moist, seasonally warm/cool on fertile soils or
warm and seasonally wet/dry. Cacti and shrubs, succulent stems or leaves – Environment is dry,
seasonally hot/cool. Needle-leaved evergreen trees (retain photosynthetic tissues year-round) –
Environment is moist, seasonally warm/cool or cool/cold on infertile soils. Grasses, sedges (grow
from the base of their leaves, can tolerate cold conditions by sitting under snow) – Environment is
moist, seasonally warm/cool, with fire. Evergreen broad-leaved trees (carry out photosynthesis year-
round in tropical regions) – Environment is wet, warm year-round. Forbs (broad-leaved
herbaceous/nonwoody plants) – Environment is seasonally cool/cold. Sclerophyllous shrubs (have
tough, leathery leaves) – Environment is seasonally dry/moist and warm/cool.
- Plants have taken many forms in response to selection pressures of the terrestrial environment such
as aridity, extreme temperatures, intense solar radiation, grazing by animals, and crowding. Similar
plant growth forms can be found in similar climatic zones on different continents, even though the
plants are not genetically related. The evolution of similar growth forms among distantly related
species in response to similar selection pressures is called convergence.
- The locations of terrestrial biomes are correlated with the variations in temperature and
precipitation. Temperature has direct physiological effects on plant growth form. Precipitation and
temperature act together to influence water availability and water loss by plants. Water availability
and soil temperature determine the supply of nutrients in the soil. Soil microbes slow down when the
temperature increases and soil is dry. Biomes vary with mean annual temperature and precipitation.
These two factors predict biome distributions fairly well, but this does not account for seasonal
variation in temperature and precipitation. Climatic extremes are important in determining species
distributions. Tropical rainforests are found in warm and wet places. Deserts are found in places with
low precipitation and many different temperatures. Tundra is found in cold and dry places.
Temperate deciduous forests are found in places with moderate precipitation and temperatures.
Trees in deciduous forests lose their leaves because of the cold.
- Global biome distributions: The potential distributions of terrestrial biomes differ from their actual
distributions due to human activity. Human activities influence the distribution of biomes. Land use
change is the conversion of land to agriculture, logging, resource extraction, and urban development.
The potential and actual distributions of biomes are markedly different. We live in the temperate
deciduous forest biome, and this biome is also found in Europe and parts of Asia. This biome has
been affected by human activity – converted to agriculture farmland. It is still considered deciduous
forest because trees would still grow here if we didn’t affect it. Temperate grassland and tropical
seasonal forest biomes have also been affected. The least affected biomes are tundra, boreal forest,
- There are nine major terrestrial biomes. Climate diagrams show the characteristic seasonal patterns
of temperature and precipitation at a representative location in that biome.
- Tropical rainforests: Are found between 10ᵒ N and S. There is low pressure here. Tropical rainforests
experience warm, seasonally invariant temperatures and abundant precipitation. Plants grow
continuously throughout the year. Tropical rainforests contain high plant biomass and high diversity
– contains about 50% of earth’s species. This biome occurs in Central and South America, Africa,
Australia, and Southeast Asia. This biome is characterized by broad-leaved evergreen and deciduous
trees. Light is a key factor in determining the vegetation. Plants must grow very tall above their
neighbours or they must adjust to low light levels. Emergent trees rise above the canopy. The canopy
consists of leaves of evergreen trees. Lianas (woody vines) and epiphytes (plants that grow on tree
branches) use the canopy and emergent trees for support. Understory trees grow in the shade of the
canopy, and shrubs and forbs (broad-leaved herbaceous plants) occupy the forest floor. It is dark
under the canopies. Tropical rainforests are disappearing due to logging and conversion to pasture
and croplands. About half of the tropical rainforest biome has been altered by deforestation. The
recovery of rainforests is uncertain. Soils are nutrient-poor and recovery of nutrient supplies may
take a very long time.
- Tropical seasonal forests and savannas: Is found at the tropics (23.5ᵒ N and S). Band of low pressure
shifts up and down. Rainfall is more seasonal here with pronounced wet and dry seasons associated
with shifts/movement in the ITCZ. There is water stress/shortages in the summer months. The
temperature is stable and is warm all the time. In this biome there are shorter trees, lower tree
densities, and deciduousness in dry seasons (trees drop leaves during the dry season). There are
more grasses and shrubs and fewer trees relative to rainforests. Fires promote the establishment of
savannas, which are communities dominated by grasses. The frequency of fires increases with the
length of the dry season. Some fires are set by humans. In Africa, large herbivores (wildebeests,
zebras, elephants, and antelopes) also influence the balance of grass and trees, and are important in
promoting the establishment of savannas. On the Orinoco River floodplain, seasonal flooding
promotes the establishment of savannas because trees are intolerant of long periods of soil
saturation. Less than half of seasonal tropical forests and savannas remain. Human population
growth in the biome has had a major influence. Large tracts have been converted to cropland and
- Hot deserts: This biome is located in the high pressure or descending air area of Hadley cells, making
it very dry. High pressure inhibits the formation of precipitation. Hot deserts contain sparse
vegetation and animal populations and are characterized by high temperatures and low moisture.
Low water availability constraints plant abundance and influences their form. The major desert
zones are the Sahara, Arabian, Atacama, Chihuahuan, Sonoran, and Mojave. Some plants species can
flower when it does rain. Many plants have succulent stems that store water. This is an example of
convergence in plant form. Cacti in the western hemisphere and euphorbs in the eastern hemisphere
both have succulent stems which were evolved independently in response to similar environments
(they species are not related). Human use deserts for agriculture and livestock grazing. Agriculture
depends on irrigation and results in soil salinization. Long-term droughts and unsustainable grazing
can result in desertification, which is loss of plant cover and soil erosion.
- Temperate grasslands: Is found between 30ᵒ and 50ᵒ N and S in North America, Europe, Asia,
Australia, and South Africa. This biome has greater seasonal temperature variation and plants
growing here have to deal with subfreezing temperatures in the winter months. This biome has
warm, moist summers and cold, dry winters. Grasses dominate and are maintained by frequent fires
and large herbivores such as bison (these things prevent the establishment of trees). Grasses grow