I'm fairly certain that the wood fiber application is what killed them. It has not occurred in every playground where we have added 12 inches of wood fiber but certainly in enough playgrounds to raise concern. We have one playground where we lost 5 or 6 large Chestnut Oaks all over 50 years old and another where mixed species of oaks and maples died. Another has lost three White Oaks that were over 50 years old each. My training is as a horticulturist so I'm fairly certain that I am correct about what led to their death and that they did not die of other causes. It is generally accepted that adding any material over a trees roots that obstructs oxygen flow to the roots can kill roots and the tree. Normally I see this when soil is piled under a tree or grades are changed around the tree. I think some tree species will be more tolerant. For instance elms, Willow Oaks and Pin Oaks are adapted to areas that frequently flood and so are more tolerant of low oxygen conditions.
If on the other hand roots were killed because something from the decomposing mulch (an alcohol?) leached into the soil, that would be a problem for many species. Still unsure at this point- hence we are switching to the 6" of coarse rubber and a drainage system that moves water away from roots and oxygen to them.
I noticed a post in this thread asking about what depth is required for a specific surface. Fresh from a NPSI course (National Playground Safety Institute), I know of a couple of key concepts to keep in mind when evaluating the type and depth of a safety surface. Remember, the vast majority of playground-related injuries are due to a simple lack of proper safety surfacing. Using the right surfacing in the right amount can save thousands of children from unnecessary injury and can also have the added benefit of protecting the playground operator from liability.
1. ASTM (American Society for Testing and Materials) does the testing and sets the critical fall height for specific surfacing. The critical fall height is the maximum height from which a life-threatening head injury would not be expected to happen. Different materials have different critical fall heights at different depths, so it is advisable to reference ASTM F1292 for specific numbers for surfacing types.
2. Either reference the equipment manufacturer's installation documents or contact the manufacturer to figure out what the fall height is for the equipment on your playground (fall height is the highest designated play surface). The fall height of the equipment is NOT TO EXCEED the critical height of the safety surfacing.
3. In addition, always keep in mind ADA/ABA guidelines when selecting a safety surfacing. It must be accessible and allow wheelchair users to move freely on it. This is why sand is not compliant as a surface; wheelchairs just cannot get through it.
Once a surface has been selected and placed in the playground, make sure someone has been identified to maintain that surface and check it for appropriate depth. The initial installation of loose-fill material will often get compressed and need additional material added to keep it compliant and safe for the children using the playground.
I hope this information is useful to anyone thinking about what surfacing to use. Keep in mind that ASTM standards are often revised, so it is always good to check for the newest version of its publications.
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