Question

Rigolets Lot   I purchased a lot on Lake Pontchartrain last year and there are two oak trees on this lot.  We will be building soon and one tree will need to be removed but the second may be able to be saved.  The lot will need 2 - 3 feet of fill but someone recently told us that if we covered the trunk of the tree with that much fill that it would kill the tree.  Is that accurate?

Answer
The short answer is yes this much soil will kill the trees.

Fill dirt frequently is added around existing mature trees so that a level or more visually desirable lawn can be established. Fill dirt changes the ratio of oxygen to carbon dioxide around tree roots and the roots may subsequently die if too much is added.

In general, roots grow where the resources of life (water, oxygen, and mineral nutrients) are available. They usually will not grow where there is no oxygen or where the soil is compacted and hard to penetrate. This need for oxygen explains why a majority of tree roots are located in the top 12 to 18 inches of soil. Root systems are also extensive. They often extend outward from the tree trunk to occupy an irregularly shaped area 1 1/2 to two times larger than the crown (branch) spread. It is easy to see why any type of soil disturbance near trees can, and usually does cause damage. As trees mature in the landscape they attain a rather delicate balance with their surrounding environment. In fact, trees grow best in an environment of minimal change.

Soil additions reduce the oxygen supply to roots, compact the soil, and often raise the water table. Soil additions six inches or less will probably not harm "fill-tolerant" trees  especially if the fill material is good topsoil, high in organic matter and loamy in texture. But, irreparable damage will result if as little as two inches of clay soils are used as fill, particularly around "fill-intolerant" trees (oaks fit into this category of fill-intolerant trees).  Never pile the fill up on the trunk of the tree.

Now you can add up to two inches of sand over the roots without causing any damage to the tree. Do not compact the sand and DO NOT fill within a foot of the trunk.