It means of different ages, of disparate ages. You can refer to any type of population. For example in humans, animals, puppies, trees. In the case that Furoya comments, it is very valid : dissytanous forest is one that its individuals have different ages (e.g. a natural forest) and contemporaries, is the same age or similar ages (artificial forest or plantation). They are effectively terms used in Dasonomy and Dasometry.
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