Climate, topography, and soil—along with parallel influences in aquatic environments—determine the changing nature of plant and animal life and ecosystem functioning across the globe.
• Despite the fact that no two locations have exactly the same assemblage of species, we can categories biological communities and ecosystems based on climate and dominant plant form, which determine their overall character. Biomes are the names given to these classifications.
• Under similar environmental conditions, ecosystems belonging to the same biome type in different parts of the world develop a similar vegetation structure and ecosystem function, including productivity and rates of nutrient cycling.
• As a result, biomes serve as useful benchmarks for comparing ecological processes on a global scale.
• For example, Ecosystems of the Mediterranean woodland/shrubland biome, look and function similarly whether in southern California, southern France, Chile, South Africa, or Australia (cool, wet winters and hot, dry summers).
• The biome concept is a helpful tool that allows ecologists all over the world to collaborate in order to better understand the structure and function of large ecological systems.
• The fact that biomes can be distinguished reflects the fact that no single type of plant can withstand the entire range of conditions found on the earth's surface.
• The earth would be covered by a single biome if plants could tolerate such a wide range of physical conditions.
• Trees, on the other hand, cannot grow in the same dry conditions that shrubs and grasses can, simply because their physical structure, or growth form, necessitates a lot of water.
• The grassland biome exists because grasses and other herbs (known as forbs) can withstand the cold winters found in the Great Plains of the United States, Russia's steppes, and Argentina's pampas.
MAJOR DETERMINANT OF PLANT GROWTH FORM AND DISTRIBUTION
• We can divide ecosystems into biomes because climate, along with other factors, determines the best plant growth form for a given location, and plants with specific growth forms are limited to specific climates.
• The close relationship between climate and vegetation is established by these principles.
• Keep in mind, however, that areas of the same biome type share other, less obvious similarities, such as biological productivity, nutrient regeneration in soils, and the structure of animal communities.
• It is impossible to comprehend an organism's adaptations without first understanding the environment in which it lives. Each biome has its own set of physical conditions, and its inhabitants have evolved to cope with them.
• The broad and thin leaves of deciduous forest trees in the temperate seasonal forest biome provide a large surface area for light absorption but little protection from desiccation or frost.
• Many desert species, on the other hand, have small, finely divided leaves to dissipate heat, and some desert species have no leaves at all.
• The vegetation of the temperate seasonal forest and subtropical desert biomes differs dramatically as a result of these adaptations.
• Plant spacing and form are both affected by these differences.
• Trees in temperate forests form closed canopies that shade the entire ground surface.
• Trees and shrubs are more widely spaced in drier environments, such as deserts, woodlands, and savannas, due to competition among their root systems for limited water, and this spacing allows drought-resistant grasses to grow in the gaps between trees. Because the scarce water cannot support an uninterrupted expanse of vegetation in the most extreme deserts, much of the soil surface is bare.
3. PHYSICAL CONDITION
• Given that organisms are adapted to their biome's physical conditions, it's not surprising that many species' ranges are constrained by those same physical conditions.
• Temperature and moisture are the most important variables in terrestrial environments, especially for plants.
• The distributions of a number of maple species in eastern North America demonstrate how these factors interact.
• Cold winter temperatures to the north, hot summer temperatures to the south, and summer drought to the west limit the sugar maple (Acer saccharum), a common forest tree in the northeastern United States and southern Canada.
4. ROLE OF FIRE
• Fire has a distinct role in shaping plant communities at the drier end of the precipitation spectrum within each temperature range.
• Where moisture availability is intermediate and highly seasonal, fire has the most impact. Because deserts and moist forests rarely accumulate enough plant debris to fuel a fire, and moist forests rarely dry out enough to become highly flammable, they burn infrequently.
• The combination of abundant fuel and seasonal drought in grassland and shrub land makes fire a frequent visitor. Fire is a dominant factor in these biomes, to which all community members must adapt and, indeed, many are specialized.
• For the germination of their seeds and the growth of their seedlings, some species require fire. Frequent fires kill tree seedlings near the moister edges of African savannas and North American prairies, preventing the encroachment of forests that could be sustained by local precipitation if not for fire.
• Perennial grasses and forbs with extensive root systems and underground meristems (growth centers) benefit from burning. For the same reason, grasses tolerate grazing.) After a fire, grass and forb roots sprout new shoots, quickly establishing new vegetation above the soil's surface.
• Tree seedlings establish themselves in the absence of frequent fires, shading out savanna and prairie vegetation. Exceptions appear in all classification systems, and the boundaries between biomes are hazy. Furthermore, not all plant growth forms correspond to climate in the same way; for example, Australian eucalyptus trees form forests in climates that support only scrubland or grassland on other continents, as previously mentioned.
THE DISTRIBUTIONS OF SPECIES ARE LIMITED BY PHYSICAL CONDITIONS OF THE ENVIRONMENT.
• As a result, the sugar maple is restricted to roughly the northern half of North America's temperate seasonal forest biome. Sugar maples cannot tolerate average monthly summer temperatures above 24°C or winter Dryness determines the western limit of the sugar maple, which coincides with the western limit of forest in eastern North America.
• Sugar maples require less annual precipitation (about 50 cm) at the northern edge of their range than at the southern edge, because temperature and rainfall interact to control moisture availability (about 100 cm). The sugar maple's range ends abruptly at the Atlantic Ocean to the east
• temperatures below18°C, so attempts to grow them outside their natural range fail.
CLIMATE DEFINES THE BOUNDARIES OF TERRESTRIAL BIOMES
• The climate zone system, developed by German ecologist Heinrich Walter, is one of the most widely used climate classification schemes.
• The annual cycle of temperature and precipitation is the basis for this system, which has nine major divisions.
• The important climate and vegetation characteristics in each of these zones are depicted in the diagram below.
• Temperature and precipitation values used to define climate zones correspond to moisture and cold stress conditions, which are important determinants of plant form.
• The occurrence of water stress during a pronounced dry season, for example, distinguishes the tropical climate zone from the equatorial climate zone within tropical latitudes. Water is always scarce in the subtropical climate zone, which is found at slightly higher latitudes.
• Heinrich Walter divided the world into climate zones based on the annual cycle of temperature and precipitation.
WHITTAKER’S BIOME SCHEME
• The left-hand column lists the biome names assigned to these zones according to Whittaker's classification scheme.
• Robert H. Whittaker, a Cornell University ecologist, defined biomes first by vegetation type, then created a simple climate diagram on which he plotted the approximate boundaries of his biomes in relation to average temperature and precipitation.
• The biomes of Whittaker are classified based on average temperature and precipitation.
• Evergreen rain forest (equatorial), seasonal forest or savanna (tropical), and desert scrub (subtropical) are the typical vegetation types in these three climate zones. Below, we'll take a closer look at Walter's climate zones.
• Biomes can be classified in a variety of ways. Walter's is based first on climate, with climate zone boundaries drawn to correspond to changes in major vegetation types.
• Climate seasonality, fire, and soils determine whether woodland, grassland, or shrub land develops in climates that are halfway between forest and desert biomes.
• Inset: Average annual temperature and precipitation for a sample of locations distributed fairly evenly across the earth's land surface. The majority of the points are located within a triangular region that encompasses nearly the entire range of climates. Only high-mountain climates are not included in the triangle.
• As one might expect, the result is similar to Walter's scheme, and their nine biome types are identical.
• Most places on Earth fall within a triangular area on Whittaker's diagram, with three corners representing warm moist, warm dry, and cool dry climates. (Because water does not evaporate quickly at low temperatures and the atmosphere in cold regions holds little water vapor, cold regions with high rainfall are uncommon.)
• At tropical and subtropical latitudes, where average temperatures range from 20°C to 30°C, vegetation ranges from rain forest (Walter's equatorial climate zone), which is wet all year and receives more than 250 cm (about 100 inches) of rain annually, to desert (Walter's subtropical climate zone), which receives less than 50 cm of rain annually.
• Seasonal forests (150–250 cm rainfall), in which some or all trees lose their leaves during the dry season, or scrub and savannas (50–150 cm rainfall) thrive in intermediate climates. With respect to rainfall, plant communities at different latitudes follow the pattern of tropical communities, falling into four vegetation types: temperate rain forest (as found in the Pacific Northwest of North America), temperate seasonal forest, woodland/shrub land, and temperate grassland/desert.
• At higher latitudes, precipitation varies so little from one location to the next that climate does not distinguish vegetation types well. The boreal forest predominates where the average temperature is between 0°C and 5°C.
• When annual average temperatures are below 5°C, all plant communities can be classified as tundra.