BIOL 4120

Principles of Ecology

Phil Ganter

320 Harned Hall


Dry, coastal Brazil

Lecture 20 Terrestrial Ecosystems

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Overview - Link to Course Objectives

Biomes are the "types" of Terrestrial Ecosystems (the term is not often used for aquatic ecosystems) (23.1)

  • Large-scale communities characterized by climate type and dominant vegetation (terrestrial communities)
  • Original biome classifications were done by botanists and focused on terrestrial plant communities
    • Holdridge Life Zones puts terrestrial biomes into a scheme dependent on three factors:
      • Annual precipitation
      • Evapotranspiration ratio (a measure of the available moisture in the environment)
      • Biotemperatures (a division of the temperature scale into biologically relevant divisions)
    • Many other classification schemes are possible
        • a classification of terrestrial biomes based on vegetation types called the Ecoregion Domain Classification
  • Terrestrial environment
    • A graph of mean annual temperature versus mean annual precipitation reveals that not all climates are available
        • Remember (Lecture 5) that both rainfall and temperature fall as you go from the equator to the poles
          • Temperature decreases because of the curvature of the earth decreases the angle at which solar radiation passes through the atmosphere and strikes the Earth
          • Rainfall decreases because cooler air holds less moisture, so there is less rain to fall as you go toward the poles, and because of the areas of rising and descending air
            • rising air results in rainfall
              • at equator, this leads to tropical rain forests
              • at 60 N, this leads to terrible storms in the North Atlantic and a rainy climate for the British Isles
            • subsiding air is dry and little rain falls so at 30 N and S and at the poles there are dry regions
              • the famous 30° Desert Belt - The Sonoran, Chihuahuan, Atacama, and Saharan, and Namib deserts are all at this latitude (both north and south)
              • Most are on the west side of continents - got any idea why? Think gyre.
    • This means that there are no cold regions with lots of rain
  • Terrestrial Biomes are the result of climatic differences and below are the general climate types
    • Tropics
      • from 23.5°N (Tropic of Cancer) to 23.5°S (Tropic of Capricorn)
      • Day length varies little throughout the year
      • Warm, moist weather
    • Temperate Zone
      • ~30 to 50°, both north and south
      • Warm summers, cold winters
      • Moderate to Low rainfall
    • Boreal Zone
      • ~50 - 66.5°, both north and south
      • cold winter, short summer
      • relatively high levels of precipitation (area of rising air between Polar and Ferrell Cells)
    • Polar Zone
      • above 66.5° (can you think of why the line is so definite?)
      • low rainfall and very cold winters
    • Mediterranean Climate
      • dry, hot summers
      • cool, moist winters
      • Locations:
        • land around the Mediterranean Sea
          • portions of southwestern US (mostly southern California)
          • Central Chile
          • Cape region of South Africa
          • Southwestern and Southern Australia
          • Northern Argentina
    • Subtropics
        • region between tropics and temperate zone
        • warm and moist to warm and dry
  • There are also other "general influences" on the climate in a region
    • Continental and Oceanic effects
      • Land heats and cools faster than Oceans
        • Water has high specific heat so it takes lots of energy to heat it up and it loses heat slowly
        • Water has low surface complexity so winds can't cool it (think of how heat vanes cool computer equipment)
      • Continental Effect
        • at the center of continents, the summers heat and winter's cold are not moderated by proximity to the ocean
        • here is where you find the coldest winter and hottest summer temperatures for any latitude
      • Oceanic Effect
        • Ocean cools more slowly than land in Fall and warms adjacent land, heats more slowly in Spring and cools adjacent land, so the effect is to reduce seasonal variation
        • If the water comes from a different climatic zone, the Oceanic Effect can produce a general cooling or heating in coastal areas
          • Ireland is as far north as Hudson Bay in Canada but is heated by the Gulf Stream
          • Hudson Bay is famous for its polar bears
          • Some southern Irish cities use palm trees for shade along their streets
        • the oceanic effect can often be felt daily as well as seasonally as winds blow from
          • ocean during day as cool ocean air replace warm, rising land air
          • land to ocean in the night as cool land air replaces rising ocean air
          • still period in morning and evening when there is no great difference between land and sea
    • Montane effect - Mountains can cause climatic effects
      • Adiabatic Cooling - cooling of air as it rises - caused by expansion as pressure is reduced - means:
        • air is cooler at top of mountains
        • air loses the ability to carry moisture as it cools, so the relative humidity increases as air rises and cools (this leads to rainfall in areas of rising air like the Equator) and, once 100% relative humidity is reached, the moisture condenses out of the air as rain or mist
      • Adiabatic cooling causes rain to fall on side of mountains where moist surface air is pushed up the mountain
      • Adiabatic cooling causes rain shadows on the side of mountain where cool air is descending after being pushed over the mountain (it has lost it moisture)
        • western side of Olympic Mts in Washington and Oregon is a temperate rain forest, eastern side is a grassland (low rainfall, almost a desert)
      • mountains can change circulation patterns on a continental scale
        • North of Alps is cold climate, south is much warmer (warmer than predicted by latitude of Italy, Greece and southern France

      [NOTE - some common terms defined:

      Tree - a plant with woody secondary growth that grows as a single stem that branches closer to its upper end to hold leaves exposed to sunlight and usually reaches a mature height of over 50 feet

      Shrub - a plant with woody secondary growth that grows as multiple stems or from a stem that branches close to the ground and which rarely achieves a height of 50 feet at maturity]

The Biomes (note that we start with the wettest and move on to the driest in each of the Tropics, Temperate, and Boreal zones):

Tropical Rain Forest [image, image, image] (23.2)

  • Abundant rainfall in all seasons and warm temperatures
    • Long growing season, little seasonal variation
  • Soils are leached by rainfall (soluble nutrients removed)
    • Soils are thin (bedrock not far below surface
    • Most inorganic nutrients are part of the biomass (both living and decomposing)
  • High rate of photosynthesis (primary productivity high)
  • Competition for light leads to tall forest trees
    • Vertical Zonation of plants with five layers
      • Canopy is closed layer of leafy portions of trees that are exposed to sunlight
      • Emergent Trees stick up above the canopy
      • Understory Trees - trees that are not tall enough to be part of the canopy
      • Shrub Understory
      • Ground Layer of Herbs (non-grass flowering plants without woody secondary growth), Grasses, and Ferns
    • Trees often develop Buttresses for support as root systems are shallow
  • Litter does not accumulate on forest floor as the rate of decomposition is high under warm, moist conditions
  • High diversity of plant and animal life
    • Many tree species with no group numerically dominant
      Epiphytes (plants that grow on other plants) and Lianas (vines that grow on trees so they can get to the light) are common
    • Lots of species of all kingdoms and lots of biomass/acre

Tropical Deciduous (Dry) Forest [image, image, image] (23.2)

  • Abundant rainfall only in one season and warm temperatures
    • Shorter growing season than rain forest, little seasonal variation in temperature
    • dry period increases as the latitude increases (up to 8 months)
  • Trees are drought deciduous, dropping their leaves when the soil dries out
  • High rate of photosynthesis (primary productivity high)  during wet season
  • High diversity of plant and animal life
  • Found in Central and South America, northern Australia, India, Africa (south of rain forests) and portions of southeast Asia
    • Agriculture has replaced much of the Tropical Deciduous forest where rainfall is sufficient or other water sources are available

Tropical Savannas [image, image, image] (23.3)

  • Dry areas with grass as the ground cover and occasional trees and shrubs
    • Referred to as Cerrado or Caatinga (Brazil - caatinga shrub dominated) or Chaco (Argentina), moimbo or mopane woodlands (Africa), and Mulga or Brigalow in Australia
  • Warm climate with seasonal rainfall insufficient for  forest development
    • Differ from Mediterranean Climate in that the rainfall comes during the warmer season
  • Often occur on flat lands with nutrient-poor soils
  • Ants are often important herbivores and termites can be important detritivores (on the woody portion of fallen plants)

Desert [image, image, image] (23.4)

  • Abiotic characteristics
    • Sunlight abundant
    • High daytime temperatures (Warm Deserts), except at high latitudes or elevations (Cold Deserts)
    • Water limiting
      • lack of rainfall due to latitude or to Rain Shadow effect of mountains
    • Large daily temperature variation
  • No trees
  • Plants dominated by:
    • Annuals - grow only during wet periods (may not be "annual")
    • Succulents that store water in stems or leaves
    • Root Succulents that store water in underground swollen roots
    • Deciduous Shrubs - drop leaves when water is limiting
    • Plants often have thorns or spines to protect water resources
  • No canopy
  • Plant cover may cover less than 10% of soil (none in some areas)

Temperate Deciduous Forest [take a drive in Middle Tennessee to see this ecosystem] (23.6)

  • Abiotic characteristics
    • Moderate rainfall
    • Temperature goes below freezing, but not to extremes
    • Soils are full of organic materials
  • Trees species dominated by a small number of species (or related species, like the oaks or maples or hickories)
    • Growing season ends with onset of cold weather and loss of leaves
    • Trees shorter than in rain forest, but canopy is closed
  • Four layers in Vertical Zonation
    • Canopy
    • Tree Understory
    • Shrub Understory
    • Ground Layer
  • Plant and animal diversity lower than in tropical rain forest
  • Found in Eastern North America, Mid-latitude Europe, Japan, Northern China, India
    • Not found in the Southern Hemisphere except for small area in southern Chile and Argentina

Temperate Shrublands [image, image, image] (23.5)

  • Occur in areas with a Mediterranean climate
  • Xeric (dry) evergreen shrubs and small Sclerophyllous (with tough leaves having thick cuticles) trees
  • Called Chaparral in North America, Matorral in South America, Fynbos in South Africa
  • Often subject to a constant fire regime, burning during the dry season

Grasslands [see below for lots of images] (23.7)

  • Abiotic characteristics
    • Temperate climate
    • Rainfall too little for trees, enough to support 100% ground cover
    • grasslands often found between deserts and forests
  • Fire a factor in maintaining grasslands
    • Most of the biomass of many grassland plants is below ground, where most fires will not kill the plant
    • For millennia, Humans have kept some landscapes grassland rather than dry forests by setting fires at regular intervals
  • Grazing mammals also a factor
    • Large herds (buffalo) kill young trees
  • Lack of moisture slows decomposition so the soils are very rich in organic material
    • Best soils for grain agriculture
    • Little natural grassland left
  • Grasslands are treeless except for Riparian (river-edge) forest sometimes called Bosques in North America [image, image, image]
  • Found on most continents
  • Much of biomass is below-ground, where it loses moisture more slowly, is insulated from cold and hot air temperatures, and is fire-resistant (although a hot fire can kill off root stock)
  • Flooded Grasslands (and Flooded Savannas) regions that are regularly flooded (covered in Lecture 21)

Boreal Forest (Taiga) [image, image, image] (23.8)

  • Abiotic characteristics
    • Moderate rainfall during the short, cool summer
    • Severe winters, drier than the summer season
    • Soils often wet, especially when Permafrost is found under the forest
      • Permafrost is a layer of frozen soil.  Soils freeze and thaw from the top, where they are warmed by the sun.  If the warm season is too short, the lower portion of the frozen soil doesn't melt - this is the permafrost.
      • Permafrost is water-proof and so no moisture that falls on soils underlain by permafrost percolates into the lower groundwater and regions of permafrost often have many small wetlands or pools where the melting soil water and rainfall collects
    • Fire can burn large areas during dry spells
  • Forest dominated by Conifer Trees
    • Low diversity, but population numbers can be very high
    • Spruce (Picea), Fir (Aibes), Pines (Pinus) dominate boreal forests
    • short growing season means the annual productivity of these forests is lower than temperate or tropical forests
  • Bordered by Temperate Forest or Grasslands to the south and Tundra to the north
  • Herbivores may show strong population cycles with outbreak years
    • Insect outbreaks
    • Vole and lemming cycles
  • Found in
    • Taiga of Russia and northern Europe
    • Canada, Alaska and at high elevations in the Rocky Mts south to northern Arizona and southern California
    • Not found in southern hemisphere
      • there is very little landmass that is far enough south to have the right climate
      • Southern portion of South America has temperate coniferous forests dominated by members of the Araucariaceae (if you have seen Norfolk Island Pines as potted plants or growing in yards in southern Florida, you have seen a member of this family)

Tundra [image, image, image] (23.9)

  • Abiotic characteristics
    • Low rainfall, although Permafrost keeps soils moist
    • Extreme cold, short growing season
    • Permafrost melting during the summer often water logs soils in low areas, forming swamps and bogs
  • Shrub and small trees only, much open grassland
  • Repeated freeze-thaw cycles over many years push the soil into regular shapes.  Click on the name to see a website with pictures
    • Stripes - long, parallel folds
    • Frost Hummocks (here the hummocks fill larger polygons) - a checkerboard of mounds, sometimes called Pingos
    • Polygons - regular flat patches of soil (look here too) with trenched edges
    • Solifluction Terrace - hillsides in slow motion, flowing down and spreading out of wet soils on hillsides
  • Many migratory animals present during summer months only (many birds nest in the tundra)
    • Many biting insects
  • Found
    • North of the boreal forests in the Northern Hemisphere
    • No southern tundra as there are no continents far enough south

Some other terrestrial biomes

Coastal Pine Forest

  • Sandy, low nutrient soils, fire common
  • Southern Alabama, Mississippi, coastal Carolinas and Georgia

Temperate Rainforest

  • Very wet temperate climate, trees dominated by conifers
  • Found in portions of southern Chile (the Araucaria forests), New Zealand, Australia, Olympic Peninsula in the Pacific Northwest of US


  • these communities occur as you ascend mountains and are similar to biomes you find as you go north
  • Sometimes montane is used instead of alpine
  • Air is cooler as you ascend (adiabatic cooling)
  • Rainfall increases on windward side, decreases on leeward side (Rain Shadow)


Biome, Holdridge Life Zones, Evapotranspiration, Biotemperature, Tropics, Temperate Zone, Boreal Zone, Polar Zone, Mediterranean Climate, Continental effect, Oceanic effect, Montane effect, Adiabatic Cooling, Tree, Shrub, Tropical Rain Forest, Vertical Zonation, Canopy, Emergent Trees, Understory Trees, Shrub Understory, Ground Layer, Herb, Buttress, Epiphytes, Liana, Tropical Deciduous (Dry) Forest, Tropical Savannas, Desert, Warm Deserts, Cold Desert, Annuals, Succulents, Root Succulents, Deciduous Shrubs, Temperate Deciduous Forest, Temperate Shrublands, Xeric, Sclerophyllous, Chaparral, Matorral, Fynbos, Grasslands, Riparian forest, Bosque, Tallgrass Prairie, Shortgrass Prairie, Veldt, Pampas, Steppe, Boreal Forest (Taiga), Conifer Tree, Tundra, Permafrost, Stripes, Frost Hummocks, Pingos, Polygons, Solifluction Terrace, Coastal Pine Forest, Temperate Rainforest, Alpine, Montane, Rain Shadow

Last updated April 14, 2007