We have observed that plants require light, water, carbon dioxide, soil minerals and some reasonable temperature to survive. We have considered most of these factors and it is time to consider temperature, itself tied into all of these other factors in some way. This one environmental factor strongly determines the distribution of plants on the planet.
The Equator. Most of us are generally aware that the earth spins about its axis which includes the north and south pole. Midway between the poles the earth bulges to its greatest extent and this is called the equator.
The Tropical Zone. As the earth is exposed to light from the sun, the sun passes directly overhead at the equator and up to 23 degrees of latitude north and south of the equator on at least one day per year. The lines on either side of the equator delineating where the sun goes overhead on JUST one day are called the Tropic lines. The one on the north of the equator is the Tropic of Cancer, the one south of the equator is the Tropic of Capricorn. The zone between the two Tropic lines is the Tropical Zone or the Torrid Zone. Some of the world's deserts occur in the Tropics, however the Tropics also include the finest examples of rainforests on Earth.
To help you understand why it is warmer near the equator the diagram below shows the sunlight rays striking the earth in the tropical zone and then also at a point higher in latitude. You can see that the five rays striking the equator are spread over a very small surface area, and this will heat that local environment strongly. The five rays shown striking high in the temperate zone show how the same amount of light ends up being spread over a greater surface area, not generating as much heat locally.
Two SubTropic Zones. Two band-like zones both north and south of the Tropical Zone are warm too, but at these latitudes (North and South) the sun never passes straight overhead. Summers are warm and winters are mild and generally frost-free in these SubTropical Zones.
The Temperate Zones. There are two bands further to the north and south of the subtropics. In these zones the sun always stays in the southern (North Temperate Zone) or the northern (South Temperate Zone) sky. Summers are warm enough, but winters include freezing temperatures in both the air and soil. Plants surviving here must be able to tolerate frosts and freezes.
The Polar Circles. At each pole is a circle of land: the Arctic Circle including the north pole and the Antarctic Circle including the south pole. For each of these circles there is at least one summer day when the sun never sets and one winter day when the sun never rises. At the poles the number of days without sunset, and days without sunrise is maximum. Considering the low sun angles at each pole, the solar gain is minimal so winters are long and quite severe, while summers are mild and brief. The soil near the poles thaws for just a short time each year at the surface, beneath this the ground is permanently frozen (permafrost). This of course limits root growth and, ultimately, plant size.
Plants and Temperature. Tropical plants are most competitive in the tropical zone and warmer parts of the subtropical zones. These plants generally cannot survive winters, and so are restricted to their homelands. Subtropical plants can tolerate some very light frost, but cannot tolerate any kind of "hard" frost. They survive in the subtropical zones, and in some areas in the tropical zone. Plants that can survive hard frosts or freezes are called Hardy Plants. These plants can handle varying levels and durations of freezing, explaining their distributions up into the high latitudes toward the Polar Circles. Where permafrost exists the thin layer of soil that thaws briefly in summer supports the distinct vegetation known as tundra. These are among the toughest plants on earth. They go through their life cycle in just a few weeks of summer and then tolerate extreme cold for the rest of the year. They tend to be short in stature.
Plants that are sold commercially are rated according to a hardiness zone system devised by the USDA (United States Department of Agriculture). Zone one is in the northern reaches of Alaska, Zone 2 is located in Alaska and Canada, and Zones 3 to 11 are found in the "lower 48" states. Hawaii is Zone 11. To be sure that a plant is suitable for a particular area, you need to be sure it is rated for your zone or for zones with a lower number than yours. So a Zone 3 plant will do very will in Connecticut (Zone 6). Below is a map to assist you in understanding this. Notice how the zones are milder in the southern latitudes and winters are harsher in the higher latitudes. You will also notice that the zones are not strictly following the latitude lines, but wrap up along the coasts. This shows the moderating influence of the adjacent oceans. In fact some coastal areas of Alaska are Zone 6 for this very reason.
The diagram below shows how winds and precipitation patterns form globally. In addition to the simple equatorial heat and polar cold caused by the sun, these air movement patterns (driven by solar heat and earth's rotational momentum) add a pattern of drought at 30 degrees north and south latitude and precipitation at 60 degrees north and south latitude. The patterns also show us the movement of water in the oceans. These movements alter terrestrial environments quite strongly. Cactus and succulent plants can tolerate the deserts in the zones found 30 degrees north and south of the equator. Plants that live high-precipitation environments 60 degrees north or south of the equator will not tolerate desert conditions. So precipitation can alter the distribution of plant species.
The diagram below demonstrates the relationship between altitude and climate. The gradient of latitude that alters the temperature and season length variables is mirrored in altitude. As one moves from low elevation to high altitude the environment changes from tropical (or perhaps temperate depending on which mountain you consider) to arctic. So the same gradient of organisms that exists from equator to poles is expected to be reflected from base to mountain top.
A Longitude Variable. The diagram above also shows why, in the north temperate zone, the western slopes hold rainforests and the eastern slopes hold deserts. The humid air at low altitude moves with the prevailing westerly wind up the western slope and is cooled. The water vapor condenses and it rains upon that western face of the mountain. As the now-drier air passes over the peak of the mountain the air goes from cool to warm as it cascades down the eastern slope. Here the water vapor expands and so does not condense or precipitate. So we have rainforest on the western slopes of the coastal mountains in California and we have exceedingly dry desert lands to the east of this range.
All of these factors of climate ultimately influence the distribution of organisms. These are demonstrated in two figures below. One view is of the "New World" and the other the "Old World."
We will briefly discuss some of the major biomes including our own: The Eastern Deciduous Forest Biome. You have seen that biome first-hand in our trip to the arboretum. A critical table below describes key biotic and abiotic features of each biome.
|Biome||Temperature||Precipitation||Key Plants||Other Features|
|Rainforest||High||High||Broadleaf evergreen trees, epiphytes, lianas (vines)||The soils are infertile and the species diversity is very high|
|Savannas and Deciduous Tropical Forest||High||Seasonal Drought||Grasslands with scattered broadleaf deciduous shrubs and trees||Periodic fire is common, C4 photosynthesis|
|Desert||High||Low but a "wet" season||Succulents and some annual herbs||Small/no leaves, thick waxy cuticles, hairy epidermi, CAM photosynthesis|
|Grasslands||Temperate||Moderate/Low||Perennial bunch and sod grasses||Land exploited for crops|
|Temperate deciduous forest||Temperate||Moderate/even||Deciduous trees and perennial herbs||Obvious herbaceous plants vary with season|
|Temperate mixed and conifer forests||Moderate||Moderate||Mixtures of broadleaf and conifers||Transition between temperate and taiga, soils are infertile|
|Mediterranean scrub||Temperate||moist winter/dry summer||Evergreen and summer deciduous trees and shrubs, thicket/coppice||Also known as chaparral (NW) and maquis (OW)|
|Taiga||Severe temperate||Moderate/low||Coniferous forest||Soils are acid and infertile, possible permafrost|
|Tundra||Extreme temperate||Low||Very low shrubs, grasses, lichens, herbs||Permafrost present in soil, much plant biomass is below ground|