On the other hand, the figures may omit the numbers of trees planted by farmers from their own seedlings. Plantations cannot provide the full range of goods and services supplied by a natural forest. They are tree crops, analogous to agricultural crops, with a simplified ecology of one or, at most, a few species usually chosen for their yield and ease of management. The primary purpose of most plantations is to produce wood or other products quickly and cheaply. Their role, which is a highly valuable one, complements national or global forestry management strategies.
Plantations can be highly productive.
The increment of timber from a tropical plantation may be 30 ml per hectare compared with 2 to 8 ml per hectare from a managed natural forest. Annual yields of up to 70 ml per hectare have been attained in Brazil from clones of hybrids of eucalyptus species. A recent review of tropical plantations concluded that planning is generally poor, particularly for vital issues such as matching species and site. The study also noted that plantation projects are often designed in haste, with scant attention paid to important issues because of time and financial constraints. Case-studies describe improperly managed temperate plantations as degrading key natural habitats, increasing soil erosion, modifying local hydrological cycles, intensifying pest and disease attacks and elevating levels of agrochemical pollution.