Whitcomb, Establishment and Maintenance of Landscape Plants describes one in detail, beginning with the statement

"A long-term study was conducted to determine the effects of mulches on plant health and placing plastic beneath mulches."

The "long-term" in this instance was three years. Sounds like you are interested in something based on a significantly longer term. However, if you are interested I will relate the basics of what came out of the three years.

I think that the benefits of mulching continues on for the life of the tree. Since you are providing such an improvement to the soil/root system, a tree will always reap the benefits.

An organic matter mulch will buffer soil temperatures, provide a source of OM to microorganisms, decrease water evaporation from soil surface, improve water percolation, help prevent soil compaction, prevent weeds, suppress disease, improve porosity.....let's see, what else? ;-)

I don't know of any studies that have indicated a 'best' mulch material, though there are some products that should be avoided.

Rhizo,

it sounded like you were being really sarcastic...

I'm talking about hardwood mulch or wood chip mulch in the middle of grassy area for the young trees. I'm planting a lot of trees in the middle of grassy area and wondered when do i know that they are large enough to provide enough leaves for mulch. Much like forest...

Aged hardwood mulch seems to break down fast and would have to apply twice a year but they improve soil dramatically and quickly. Fresh cedar mulch last much longer but i don't think it does much to improve the soil for quite a while but easier on my wallet and still keep the soil moist and cool.

I'm being getting about 20 Taxodium mucronatum x distichum hybrid 'Nanjing Beauty' seedlings from a professor from my old college to do an evaluation. They are their own after they are planted. No supplemental watering. The only thing they are getting from me are the mulches. That's about it. The professor told me that they can do just fine when planted during the winter and receive spring rainfall before hot and dry summer comes..

I'd be looking for the leaf litter to be as thick and persisting as a mulch before I thought the mulch was no longer needed.

Here is a link that might be useful: Linda Chalker-Scott