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have proved satisfactory, but they vary from one species to another (Le
Houerou, 1980). However these equations apply to unmanipulated trees
only and may therefore be of limited use in some cases and areas.
Although not expressively mentioned so far, there are other parameters
which are vital to an overall assessment of yields. The latter can only be
compared and properly evaluated if they are related to such basic information
as annual rainfall, rainfall distribution, temperature, altitude, soil texture and
soil reaction as prevailing in the area from which the yield data have been
obtained.