the earth's surface is covered by diverse vegetation, from tropical and polar deserts, to alpine and arctic tundra, to coastal temperate and tropical rainforests. Forests cover a large fraction of the Earth's terrestrial biosphere (approximately 4 billion hectares) and differ from other vegetation types in several important ways. Forests tend to occur where soils and climate are more favorable to plant growth. As a result, forests tend to store larger quantities of carbon then other terrestrial ecosystem types. Forests are taller than non-forest vegetation types (such as grasslands), and as a result, forests more profoundly affect microclimate at the ground level through canopy shading and reduced effects of wind.
Forests tend to use more water than nonforest vegetation, which together with higher rain interception rates, results in less water entering soils and leaving forests through ground and surface channels. Forests have very different effects on soils compared to other vegetation types. Forests tend to return larger inputs of litter to the soil surface, resulting in the accumulation of a sometimes-thick organic layer, including large woody debris. There is more soil-mixing in forests when large trees fall over and root systems tip up. Disturbances such as fire or hurricanes are more intense, as larger amounts of biomass are consumed or displaced.
Forests occur across a remarkable diversity of soil types, climates, elevations, and aspects. They range from monotypic stands of pine, to highly-diverse tropical rainforests with many hundreds of tree species per hectare. Together, compositional, environmental, and physical/chemical diversity results in very wide ranges in forest productivity, with differences in species composition and productivity altering the chemical and physical properties of soils. Overall, rates of net primary production for forests can range from 200 g. or less per sq. m. per year in cold climates on infertile sites, to several kg. per sq. m. per year in warm, wet climates on fertile sites. Of particular interest to ecologists is how forest composition affects the factors that regulate forest productivity, including nutrient availability, water-holding capacity, and carbon content. For all these reasons, forests receive substantial attention from global change scientists.
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