Some trees are wild, others are cloned

It's easy to think that all trees of a species are alike.  This is only true, however, of certain species.  Most trees are wild-type trees, meaning that their genetic make-ups are as dissimilar as individual humans.  An example of a wild-type is an elm.  

Other trees have been cloned to produce many trees with identical genes.  This is usually done to guarantee the presence of certain favourable characteristics of the tree.  Examples include apple and pear trees.

This distinction is important when treating diseases.  In a wild-type tree, each tree will react differently.

Some trees are ring-porous, some are diffuse-porous

Trees can be divided into ring-porous or diffuse-porous types.  This refers to the structure of the vascular system.

This is significant because it affects a tree's susceptibility to vascular wilt diseases.  Ring-porous vascular systems are very efficient, but are much more vulnerable to blockage.  The elm's vulnerability to Dutch elm disease is an case in point.