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Climate and vegetation
The dominant influences on vegetation type and productivity are temperature and availability of water. Extremes of temperature and lack of water tend to go together so ice deserts ocur at the poles and hot deserts near the tropics.

Two factors determine temperature: available sun (angle and daylength) and altitude. There is a tendency to see similar vegetation changes progressing south or north from the equator or rising in altitude:

  1. rain forest
  2. temperate forest
  3. taiga (boreal forest)
  4. tundra.

This pattern is modified by availability of water, particularly in the tropics where prevalent winds carry moisture towards the equator. The winds converge at the equator; the rising air cools depositing rainfall and maintaining rainforest. The interiors of continents close to the tropics tend to be very dry - deserts.

Looking at the vegetation map of the world we are struck by the fact that much of the land surface is unfavorable for plant growth. Desert is the largest of the biomes.

Tropical rainforest
It is only in the tropical rainforest that water and temperature are continuously favorable for plant growth. This makes it the most diverse and productive biome. There can be as many as 10,000 species of plant per hectare. In this environment nutrients are mostly in living organisms and cycle rapidly; so that soils are deficient in nutrients. Microbial activity is very rapid and little organic matter accumulates in the soil.

Fertility declines after clearance and the soils quickly become useless for agriculture. The loss of diversity, productivity and fertility make clearance a threefold tragedy. (Countries are driven to clearance by their burden of debt not by wickedness.)

Mixed deciduous forest
When temperature and water supply are temporarily favorable for plant growth we tend to get mixed deciduous forests. These conditions exist mostly in the Northern hemisphere. In the summer these systems approach tropical rain forest in terms of productivity and species diversity can be up to 2,000 per hectare. In winter water is often frozen and with low temperatures most plants become dormant (after leaf fall or death of topgrowth).

 This is the climax vegetation for most of Ohio. In the South-East on rolling drier areas (white, scarlet, shingle) oaks and hickories dominate. In the North-West maples, beech, linden, ash, tuliptree, red oak are more common. Beneath the dominant trees small trees and shrubs occur: buckeye, flowering dogwood, and viburnum in oak- hickory woods; service berry, hornbeam and witchhazel in maple-linden woods. 

The trees take most of the light in late spring and summer so annual plants have little chance to get established. Herbaceous perennials have two strategies:

  1. rapid growth before the trees come into leaf: (bulbs and spring flowers)
  2. slower shade-adapted growth and flowering in late summer (Asters etc.)

The litter layer is often quite deep in temperate forests and the soils may have a higher organic content, but nutrients are mostly tied up as they are in the tropical rainforest. Soils tend to be acidic and do not support crops for many years after clearance. Agriculture on these soils tends to require high inputs. Nearly 95% of the original forest in the US has been cleared in the last 200 years.

Moving west from Ohio the climate becomes drier passing through a transition zone of oak savannah to the prairies from Montana and Minnesota to West Texas and Oklahoma. First comes the tall-grass prairie - a region of higher rainfall - and then the short grass prairie which grades into desert. Perennial grasses dominate and flowering perennials (Liatris), coneflowers, golden rod, asters) can compete but there are few annuals. These plants survive cold winters and fires through underground storage organs and thick stem bases.

Historically these areas were kept free of trees and shrubs by periodic fires and they were home to roaming herds of grazing animals such as the American bison (buffalo). Without grazing and periodic burning, tall-grass prairie tends to become deciduous woodland.

Smith Cemetery Prairie (near Plain City OH)

Relict savannah near Marion (OH)

The deep, organic-rich soils built up by tall grass prairie are well adapted to agriculture - they retain fertility and good structure. The corn-belt from Kansas to Oklahoma is probably the best agricultural land in the world. The short-grass prairie is not so well-suited to agriculture. Its use as grazing for static herds of cattle carries the risk of degradation and invasion by cactus. Much of the desert of Arizona was short- grass prairie before Europeans arrived.


World wide, no biome is more threatened than grassland; in the US only 1% of the prairie remains.


Wetlands are not a biome, but areas with high water table tend to occur in many different biomes. They are often perceived as useless or undesirable by developers and agriculturalists who have set about draining them for their own uses. Wetlands, as we are beginning to realise, are important for several reasons:

  • They are buffer zones that hold back water, preventing sudden overload and flooding of rivers.
  • In dry seasons they hold water that can delay drying out of surrounding areas.
  • They provide refuge and food reserves for animals and birds
  • They remove nutrients and sediments from water moving through them. This cuts down pollution and silting of watercourses.

Thus wetlands can be a stabilizing component preventing collapse of a biome. Nationally about 50% of wetland has been reclaimed; most of what remains is in Alaska. Of all the states, Ohio is second to California in terms of percentage of wetland drained, although Ohio had more wetland in the first place so that a greater area has been drained than in California.

The deliberate reconstruction of wetlands is now part of urban and agricultural land use planning. Constructed wetlands are one solution to the problems of fertilizer run-off from horticultural and agricultural operations.


Human-manipulated ecosystems (including agricultural areas) now cover 37% of the land surface of the world. Although not generally regarded as a biome, we cannot ignore the ecology of these areas.

Area (million ha)






















(1 hectare (ha) is about 2.5 acres)

The Russian botanist N.I. Vavilov pioneered investigation of the origins of crop plants and came to the conclusion that most came from a few centers of origin that we now know as the Vavilov Centers. The crops that we grow today originated mainly in a "fertile crescent" extending from the Mediterranean to India in five centers of origin. Other centers were in the Horn of Africa and in the New World Tropics. Eighty percent of our food calories derive directly or indirectly from just six species of plant that were brought into cultivation in these areas 4 to 8,000 years ago:

  • corn (Zea mays - Poaceae)
  • wheat (Triticum aestivum - Poaceae)
  • rice (Oryza sativa - Poaceae)
  • white potato (Solanum tuberosum - Solanaceae)
  • sweet potato (Ipomea batatas - Covolvulaceae)
  • cassava. (Manihot esculentum - Euphorbiaceae)

The fields in which these crops are grown have become the dominant ecosystems of the world

As population has increased, production has intensified and people have become dependent on a food supply with a dangerously narrow genetic base. Plant breeding struggles to keep up with the challenge to increase food supply with growing danger of crop failure through disease or climatic disaster. In order to improve crops breeders often return to their centers of origin in search of wild species that can broaden the genetic base.

As agriculture develops, these wild species are displaced and genetic resources are lost. Marginal land is brought into production and all too often becomes degraded under the pressure of use. Along with crop production, animals have been domesticated and the attempt to intensify their production has usually been disastrous. It is only possible in a country like the US with abundant land and natural resources at its disposal. Reflecting on the extent of habitat loss in the US, how can we expect poor countries to hold back from the exploitation of their remaining natural areas?

Human beings now consume 40% of the earth's primary photosynthetic productivity and population is set to double in the next 50 years. The future of plant life and all life on the planet, including our own is bound up with our cultivation of the earth and its biomes. Agriculture has traditionally focused on a narrow range of plant species that are likely to remain the dominant food crops, whereas horticulture has maintained a plurality of edible and ornamental plants (and may have seemed something of a luxury).

Agriculture is necessary to maintain the food supply. As natural areas and their genetic diversity disappear under increasing pressure from the human population, horticulture offers the only hope of preserving our genetic heritage and providing an environment that people will want to live in.


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The Ohio State University
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