Question
 I have a large Magnolia tree in my backyard.I am putting concrete around the entire area that is around the tree that is all grass.I have no other grass in the yard.I know there must be standard measurements for the space around the tree so it can get rain water ect... Do you know how much dirt area I need to leave around the tree?
                              Thank You,
                                   cami

Answer
Sorry, Cami - There is no magic number that concrete should be kept back 搙?feet from the trunk.  Most of the tree抯 roots are located within the top 3' of soil.  However, because the majority of fine roots are concentrated in the top several inches of soil, even minor soil disturbances can injure or remove a large portion of the water & nutrient-absorbing tree roots, as is common in construction activity.  

Magnolias have a very unusual root system.  It has tap-roots, but their feeder roots are soft and close to the surface. The roots like to spread wider than most trees, just under the soil surface.  Unlike most other trees and shrubs, the roots are largely un-branched and rope-like. For these reasons, magnolias suffer more than many other trees, if you were to attempt moving it after it has reached a larger size (over 4?in trunk diameter). Older magnolia trees sometimes develop a fluted base of roots.  

With lawn, there is irrigation pipe and trenching, which injures the roots. A concrete slab is NEVER just 4 inches thick ?A contractor will over-excavate, cut or rip out roots, and compact a base for the concrete to sit on, so the effective area of disturbance depth can easily be 2+ feet.  

So, if possible, disturb no more than 25 percent of the roots within the drip-line.  Magnolias like a cool root zone, too, so adding a slab of concrete that will then heat up the soil is not a good idea.  Porous pavers would be a better solution ?many manufacturers across the country.  One we use locally is the Belgard Environmental series 揂quabric? or similar  (oldcastlestockton.com).  

The University of Minnesota has a good article on tree & construction disturbance:
http://www.extension.umn.edu/distribution/housingandclothing/DK6135.html

The University of Georgia ranks the Magnolia grandflora as 搈oderate?tolerance for root disturbance, and Ohio State University has observed 揼ood?tolerance to urban conditions. It will adapt to a low-oxygen environment. It is intolerant of trunk wounding ?so watch out for bashing it with the backhoe. It tolerates some soil compaction & prefers acid soil.   

Trees are like people ?some adapt better than others to change.  There are so many variables to your tree's potential for adapting to a new patio & lawn ?watering, soil condition, tree condition, etc.  

If you absolutely want a number, I would personally keep any new paving back a MINIMUM 5-7 feet from the trunk flair, depending on the size and condition of the tree.  Consult a local arborist IF this is a prized 60-footer that absolutely must be preserved.  Again, if possible, disturb no more than 25 percent of the roots within the drip-line, and just expect that the tree will be 搒tressed?for the first two to three years, and possibly decline over the next 15 years ... OR, it will adapt.

~Marc