Distribution
In places where the climate is suitable, trees are the climax vegetation. In some of the cool temperate regions, conifers tend to predominate, but in much of the southern hemisphere, the tropics, or in warm-temperate climates, broad-leaved trees are more common. Shade tolerance in young trees varies between species, and may determine the pattern of forest succession. For example, pines are particularly intolerant of shade while many broad-leaved tree saplings in forests grow tall and thin but survive; even if pines dominate woodland at an early stage in its succession, they may eventually be suppressed by broad-leaved species.
More than half the species of terrestrial plants and animals on the Earth are thought to live in tropical rainforests even though these occupy just five percent of the land area. In tropical regions with a monsoon climate, where a drier part of the year alternates with a wet period, different species of broad-leaved trees dominate the forest, some of them being deciduous.
Tropical regions with a drier savanna climate have insufficient rainfall to support dense forests; the canopy is not closed and plenty of sunshine reaches the ground which is covered with grass and scrub. Acacia and baobab are well adapted to living in such areas.
In cool temperate parts of the world, particularly in the northern hemisphere, deciduous broad-leaved trees tend to be replaced by conifers. The long cold winter is unsuitable for plant growth and trees must grow rapidly in the short summer season when the temperature rises and the days are long. Light is very limited under their dense cover and there may be little plant life on the forest floor although fungi may abound. Similar woodland is found on mountains where the altitude causes the average temperature to be lower thus reducing the length of the growing season.
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