Trees  in  the  Santa  Catalina  Mountains.  The  distribution of tree
   species  at  one  elevation in the Santa Catalina Mountains of Arizona
   supports  the  individualistic  hypothesis.  Each  tree species has an
   independent  distribution along the gradient, apparently conforming to
   its  tolerance for moisture, and the species that live together at any
   point  along  the gradient have similar physical requirements. Because
   the   vegetation  changes  continuously  along  the  gradient,  it  is
   impossible to delimit sharp boundaries for the communities.