Question
I have a magnolia tree that is about 16 feet tall and 6 feet around.  I would like to top it by cutting off about 4 feet from its main branch.  I prefer it to grow bushy rather than tall.  Can a magnolia tree be topped as such.

I also have a large river birch which is over taking my house.  About 35 feet tall.  Can I top it by 8 feet and cut back its some of the branches that are touching the house.

I know there is always a risk when cutting trees, but I just wanted to make sure that it is not absolutely fatal to the trees.  When is the right time of the year to cut these trees.  The birch has dropped its leaves.

Appreciate your guidance.  Thank you.  

Answer
Do not top the trees--this will cause major problems to the tree and could kill the trees. The branches that are hitting the house can be pruned back to the trunk and this will solve that problem. For birch prune during the dormant season to keep down sap leaking from the wounds.  Here is a web link to information on why not to top trees.
http://www.treesaregood.com/treecare/topping.aspx

and a web site that list many site for not topping trees.

http://www.steamboats.com/hobbies/treetoppinglinks.html