There is considerable debate over definitions for the word 'forest' and even for 'tree.' Most vegetation types fall clearly into the categories of forest or nonforest, but there is dispute at the margins. A similar debate rages over the classification of forests into natural and artificial types. On the one hand, we could say that totally natural forests do not exist. There is probably not a single hectare of the earth's surface that has not been modified to some extent by human activity. In some parts of the world, hominids have been a part of the ecosystem for perhaps a million years, often using fire or browsing mammals. Peoples have introduced new species or eliminated species from every land mass, and have even modified the air (which provides a tree with its most important nutrient by weight - carbon). On the other hand, even a 'monocultural' and monoclonal plantation contains a surprising variety of adventitious species and cannot be said to be entirely artificial.
[...] Effect on Wildlife and People The main difference between forests and other terrestrial ecosystems is that trees have a more pronounced vertical component. In terms of the volume of space bounded by the ground and the top of the canopy, forests contain considerably more volume than all other terrestrial ecosystems combined. Within this space, there are many biological niches and an abundance of plant and animal wildlife can develop. These species are interesting because they add variety to the world, and because some of them can be useful to humans. [...]
[...] So, although forests do not greatly influence the total quantity of atmospheric water moved from the ocean to the land, they may well affect the quantity and distribution of rainfall on that land. The effect of trees in a particular catchment is to reduce the yield of water, not to enhance it. Two effects cause this: interception and transpiration. Interception is where the rain wets the canopy and does not reach the ground. Readers will remember when they have stood under trees in a light shower and remained dry. [...]
[...] Forests play a vital role in mitigating erosion in most of its dozen forms. Soil erosion is a major global problem, which occurs naturally but has been exacerbated by human actions. It is caused by wind, rain, or mechanical damage (e.g., plowing or live- stock pugging (compaction and loss of soil structure in a clay soil)). Trees reduce wind speeds at ground level and thereby reduce wind erosion. They maintain the soil in a drier state, thus minimizing its mobility. [...]
[...] Water reduction from a forest cover depends on the proportion of the catchment that is forested, whereas water pollution is caused mainly by humans and animals having direct contact with the waterway. The reason why human pathogens (viruses, bacteria, plasmodia, etc.) are more likely to be found in agricultural as opposed to forestry catchments is that many domestic mammals share the same intestinal diseases. The reason why polluted water normally has low transparency is that it is either filled with sediment from erosion, or with micro- organisms fertilized by nutrient runoff. [...]
[...] So can forests act as a buffer, smoothing out flood peaks? That depends. It is easy to observe that a bucket of water poured on to the forest floor usually penetrates quickly. Holes left by dead roots and gaps around living roots may provide the mechanism for rapid and deep infiltration. In contrast, water poured on to a bare or grass- covered soil may run along the surface for a considerable distance. Often, the soil may have been baked hard by the sun or compacted by grazing animals. [...]
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