I think everyone has contributed a lot of information. Some is anecdotal, some are simple facts based on peoples experience or other published articles, but I think I am the only person who posted references to a scientific study of any sort. Even that reference was completely inconclusive on the topic of my original post, so we seem to have exposed an area for much needed research.
I think this thread needs some summary statements, which I encourage comments on:
1. There is the widely held belief that the bigger the mulch ring, the better it is for the tree. While this is commonly believed, I have not seen any scientific evidence for this. It is certainly true that the further you keep the lawn maintenance away from the trees, the safer they will be, given the quality of most lawn workers.
2. It has been suggested in a couple of articles that there is a greater root density under mulch beds, and less dense roots in turf areas. While I will not deny that this is likely true, how does that translate to improved health for the tree? There is a VASTLY greater root density in container grown plants (or trees planted in root impenetrable soil), but this certainly does not translate to improved health.
In my experience, the general vigor of any plant is related to how far and how deep it roots, not so much as to how "dense" it roots. It is entirely possible that the trees are rooting more densely under the mulch to their detriment, not their gain. It is well known that turfgrass which is NOT irrigated send its roots deeper and invariably survives drought much better than irrigated lawns when irrigation stop orders are issued due to low water supply. Perhaps by encouraging greater root density under the mulch, you might be reducing the trees tendency to seek water, thereby reducing it's survivability during drought (or even localized compaction). I will guarantee that the trees growing in non-irrigated areas survive drought better than irrigated trees once the water supply gets turned off by system malfunction or "stop irrigation" orders.
3. Granted, it is provable that DEEP accumulations of mulch protect the soil from compaction in construction areas. We all know that mulch piled deep enough to protect from construction equipment is not good for the trees either, so let's not use that as an argument for giant mulch rings against soil compaction.
I think that any reduction of soil compaction by sensible mulch depths of 3"-4" compared to turf are arguable and unsupported. I have driven a lot of heavy equipment through every type of terrain, including mulch beds. NOTHING supports the passage of equipment better than turf except for pavement. (Thick mulch definitely offers greater traction when wet, however) I can assure you unbelievers that if you drive across a yard and venture across a mulch bed (unsupported by tree roots or insanely deep mulch, of course) you will go down deeper into the soil. If you drive repeatedly across wet turf, you end up tearing the roots and working up a trench filled with water & mud.
4. It is provable that mulch increases soil moisture compared to turfgrass. It is also a scientific fact that increasing the moisture in the soil predisposes the soil to compaction. Hmmm...it would seem that there is a strong argument here that mulch might make compaction more likely by increasing the soil moisture compared to turf. I think it is safe to say that mulch rings will prevent soil compaction from turf maintenance operations, since they won't be in the area at all.
5. Soil compaction from turf maintenance: Clearly an issue with big heavy machines like tractors, not too much of an issue with lawn mowers. So what solutions exist for this? The lawn maintenance folks often do aeration of the lawn surfaces, particularly when compaction is an issue. Rather than suggesting giant mulch rings, I would suggest a better solution: stop any practices that cause compaction (good for both trees AND lawn), and relieve any compaction with aeration.
6. Competition from turf: As requested at my first posting, no one has offered any credible evidence that turf competes unfavorably with trees to their detriment. Comparisons to a woodland setting are ridiculous, because then the trees are competing with each other as well, provably to their mutual decline. All you need to do is look at all the dead lower branches in the forest, the misshapen trees overshadowed by superior trees, and the relative rate of growth to a tree located in a clearing.
There are many examples of allelopathy from trees against other plants, and I have no doubt that some exists in small ways from the turf to the trees. No documentation offered in this thread to support grass as the bad guys.
7. Soil fertility: While it is certainly true that hauling the leaves and branches away during grounds maintenance prevents return to the soil of those nutrients, that is a ridiculous argument favoring mulch over turf.
The leaves that fall on the ground are not replaced in "nutrient value" by ground up wood. Leaves do not have the same nutrient value that wood does, so they are not exactly interchangeable. Very few people that clean up all the leaves in the turf leave them in the mulched areas, so there is no gain to the mulch area from fallen leaves. On the other hand, there are a LOT of people that chew up the leaves with the mowers and just leave it on the ground. Net gain for the tree, turf offers the preferred results over mulch on this issue alone. There are always the people that do nothing with the leaves, in which nature is undisturbed by our actions.
It is well known that mulch often forms layers of impermeable mycelia, which require MAINTENANCE (not NATURE) to disturb and prevent. Although less common, the same thing can happen in the soil, killing off the turf, too. It is also well known that the fungal growth that decomposes the mulch prevents those nutrients present in the wood from reaching the tree until it is all reduced to compost; probably about the same time the landscaper will be asked to rake up all that old mulch and replace with newer, fresher looking mulch. Grass clippings, on the other hand, are more often left where they fall. Oops. No gain to the tree while considering leaf disposal by favoring mulch over turf, either.
[Side note: mulch does NOT "rob the soil of nutrients". It does have an absorbing effect on man-made fertilizers because the fungi that are munching on the carbon energy in the wood are starving for nitrogen. Eventually, the fungi decompose too, releasing all the proteins they captured.]
8. NATURE mulches the trees in the woodland, so why shouldn't we in a lawn: This is a silly argument. Since the conditions in a landscape are completely different than in the woods, why do you think that attempting to duplicate one facet of that ecosystem will be preferred over another facet from a different, equally successful ecosystem? One thing is pretty sure: where the prairie meets the forest (naturally growing together: grasses and trees!), the grass is growing under the trees (unmowed) and the trees have a completely different shape than they do in "the woods". Not surprisingly, this shape is what we see in our landscapes, rather than the tall, no-lower-branches, top heavy trees that are found in the forest.
I don't recall ever noticing the mulch under any prairie trees I ever walked around under, either. What I did notice was reduction in the height of the grass and a trend toward other varieties of plants at the base of the tree.
9.Erosion: When the pioneers came west across the plains, they found deep soil and very thick sod. The depth of the topsoil had accumulated over millenia, and has since been shown to be diminishing with each passing year of farming. So the soil building characteristics of grasses are well known. Unfortunately, I am not aware of any information that documents the relative rate that forests "build" topsoil. I can guarantee that turfgrass is vastly better at erosion control than mulch, since I have never seen a mulch layer stand up to running water, whereas turfgrass is better than many erosion control products sold, once established. It is pretty well understood by even the giant mulch ring proponents that once the landscape trees have eliminated the grass, soil erosion occurs and the tree roots are exposed. Their solution: add mulch! Why not add topsoil and a better erosion control product...say turfgrass?
10. More on different ecosystems: The prairie is mostly found in areas where there are two prevailing conditions: relatively low rainfall and alluvial soils that hold water. Where there is less water,you don't get sod, you get sparse collections of hardier species of plants, both clump grasses and tougher woody plants; when it really gets dry, you get cactus. Where there is only rocky soils (and sufficient water), you get trees and forest; almost no grass. Where there is lots of water, you only get forest. The grasses are only able to compete with the forests on two areas: they do survive pretty well in climates that have an unpredictable water supply, and they can survive fires. In fact, the prevalence of prairies is almost exclusively attributable to periodic drought that encourages the fires that kill off the trees. Too much rainfall, the trees take over. Too many fires, the grasses have free reign over the land. Even in forested areas following a fire, the grasses and soft stem broadleaves move in first, then get whipped later by the trees.
Where do comparisons to prairie and forest ecosystems fit with our concerns over giant mulch rings? They don't. Neither prairie nor forest really match the conditions of the modern landscape, so arguments that draw conclusions based on facts drawn from one setting and applied to another are likely to be flawed.
