Often we find, for example, that tree species survive best as seedlings when under the cover of specific shrub plants, as in the case of the coast live oak (Quercus agtifolia] from California [41. It has been found that 80% of the seedlings of
this tree are located beneath the cover of just two species of shrub. This "nurse" effect of one species upon another is not uncommon in nature and is an example of facilitation, which is an important element in the process of succession (see p. 142]. In all vegetation studies, the possibility of facilitation interactions that lead to positive associations between species must be balanced with the competitive interactions that lead to negative associations.