Will different kinds of trees pollinate each other? (ex. Will an apple and pear pollinate?)

Generally, different species of trees will not pollinate each other, so an apple and a pear will not cross-pollinate. The main exception to this are European and Asian pears, although they are different species they will pollinate each other. There are a few random exceptions, but it's best to err on more pollination options than less with fruit trees.

It is good to keep the following in mind as well:

• Apples and crabapples will pollinate each other since they are the same species
• Crabapples are excellent pollinators as they have so many blossoms
• European and Asian pears will pollinate each other
• European and Japanese plums will not pollinate each other
• Sweet and Sour cherries will not pollinate each other