One of the new dogmas seems to be that turfgrass and groundcovers are bad for trees. They compete for water and nutrients. So the new wisdom and knee-jerk deal seems to be 'let's rip up the grass and pachysandra and English Ivy and mulch the area; what you don't want to mulch the whole yard?; well then let's at least do the 20' around the tree.'

A) So, if we accept that the roots nearest the tree are mostly large, butress/support/compression/transport roots and the absorbing roots are mostly farther away from the trunk and the volume of soil to accomodate then increases geometrically farther away from the trunk, and the absorbing roots (maybe a majority of them) may extend well beyond 'dripline,' are these 20' mulch circles really accomplishing that much?

B) If, as Peter Torres has pointed out, moisture limitation has the effect of slowing growth and inducing defense mechanisms, maybe competition is not such a bad thing. There was some research out of Cornell that showed some trees were bringing water up from rather low levels, to the extent that herbacious understory material did noticably better under those species! (They could measure ionic differences in the water in sampled plants between surface and sub-surface water). Maybe at least some combinations are not competitive, maybe even beneficial.

Reply to post by Scott Cullen, on May 18, 1999 at 06:22:24:

Regardless of the water regime issues, keeping grass away from the base of the tree makes sense merely as a means of keeping lawnmowers and weed eaters away from thre trunk. Constant lawnmowing eventually causes soil compaction, and loss of soil cover leadfing to mower blight on the roots. That in itself is a good reason to mulch.

Julian

Reply to post by Scott Cullen, on May 18, 1999 at 06:22:24:

A)- And if we don't accept that the near roots are only large, non-absorbing? Try digging through the mat of mulch or soil under a maple!

Since we can't do as thorough a job of lion-tailing the root system, most trees have a well dispersed root system, with absorbing roots all throughout the system (allowing for soil variations, shade factors, etc. The roots are not only in an annulus beyond a certain point. The roots grow, die, and regrow throughout the soil.

So, IF we assume this premise, then applying mulch to an area surrounding the trunk will benefit the tree (for all the usually proclaimed reasons).

Does competition matter? I think it does, to some extent. Turf, for one, is known to be agressive in water demands, and nutrients. I don't know if that alone is enough to affect the trees. But many grasses have allelopathic effects on other plants, and that has been demonstrated in a few studies.

Like the rest of what we do, we have to be careful of what becomes dogma, and what is simply reasoned fact. Recognizing the degrees of sensitivity and tolerance of trees can keep it from becoming dogma.

Reply to post by Scott Cullen, on May 18, 1999 at 06:22:24:

I have to stop myself once in a while and remind myself that its all too easy to steriotype rootsystems because of the difficulty in actually seeing them. All roots do not divide equally into nifty zones of what absorbs and what conducts as we all know. I think Julian has an good point about the mowers and with larger trees the 20 foot circles keep the large machinery away from the buttress and any exposed roots. I am sometimes amazed at the ability of our machine operators here to hit a single tree in an open lawn..... Mulch in these instances to me is the same as rumble strips along the interstate: its designed to wake up the operator before he drives off the road.

I do percieve a difference in establishment rate on new transplants if the grass is kept back and a mulch bed established. It may simply be that the colonization of the mulch itself by tree roots gives the tree an accelerated start.

Wayne

Reply to post by Scott Cullen, on May 18, 1999 at 06:22:24:

Richard Harris has a wonderful set of slides from U.C. Davis from some years ago. Young trees were given 3 treatments. First, in turf up to their necks. These trees appeared to be 4-6 feet tall at the end of the alloted period. 2nd treatment was in the middle of 4X4 square (bare soil, no mulch as I recall) turf outside. These trees were 12-15 ft, I think? 3rd treatment was in the middle of 16 foot squares, 8 ft. to the nearest turf. These trees were 20-25 feet, I believe. I don't have the actual numbers, but I recall the dramatic nature of the photos. Not a test of mulch benefit, of course, only of competition/allelopathy. Trees may have been Monterey pines. Don't know the turf.
Joe McNeil

Reply to post by Joe McNeil, on May 18, 1999 at 06:22:24:

Interesting to note that if trees grow as fast as possible, they really might not be building their defensive infrastructure. That is a "strategy"- as in cottonwoods- grow fast and put more material over the rooten spots, rather than like the oak, which is to avoid susceptibility in the first place. If the cottonwood were forced to grow slower, I don't think it has the genetic make-up to build defensive infrastructure. But, if the oak were forced to grow full-out without investing in the defensive market, I'd be expecting problems from pests - pit scale to armillaria.

Reply to post by Wayne Cahilly, on May 18, 1999 at 06:22:24:

Lots of interesting points introduced by all here. It's all too easy to stereotype treatments too, that was my point. As Peter points out, maximizing fiber production rate may not be the best goal. There certainly is a difference in helping trees get established and maintaining large, mature trees.

Wayne points out that in a public garden a varied workforce may be kept from damaging butresses simply by keeping them away from the butresses; at the same time creating a consistent 'look' throughout the garden. But a homeowner may prefer the turfgrass or groundcover look; may carefully hand trim around those exposed roots or flares; and with a root system spread over perhaps thousands of square feet do we really suggest that mulching the few hundred square feet closest to the trunk has any meaningful effect on tree health or performance?

Reply to post by Scott, on May 20, 1999 at 08:18:03:

In light of Elaine Ingham's work (Oregon State U.), I say yes, we do gain health factors from mulching. [I pause to qualify that I agree with concerns about ripping out stuff just to get mulched, and the associated fanaticism.]

Elaine has discovered that the type of plant that dominates the ecosystem is an indicator of the microbial community in that system. An old growth Douglas fir forest (that is healthy) has been shown to be inhabited by one part bacteria for 100 parts fungus species. Grass land soils have been measured at pretty close to a one to one ratio of bacteria to fungus. The trend is that the woodier the dominant species becomes, the more fungal driven the soil ecology must be in order to support the health needs of the dominant woody species there.

Fire sites have been reviewed, and speed of woody species recovery has been tied to the ratio of live and thriving fungus in the soil.

Turf grass thrift can be tied to an idealistic ratio of 1-1, tree thrift can be tied to 40, 50, 60 ,70 to one ratios. Legumes, alders, and others who are known to associate strongly with bacterial driven communities tolerate turf grass competition more readily than conifers, for instance.

So, to round up the thought; 1)turf and fleshy plant soil ecosystems are bacterial driven, trees are fungal driven, so there is a bit of a conflict. One group or the other is not likely to thrive. 2) Tree sensitivity is species dependant.

Best regards,
KWK

Reply to post by Kerry, on May 21, 1999 at 06:44:52:

Fascinating. It makes a great deal more intuitive sense to me than the argument based on pure quantitative competition for moisture and soil mineral.

It certainly supports or explains improved establishment rates.

I'm left, however, with the same question about how effective (setting aside the lawnmower - stringtrimmer issue) a little halo around the trunk is when the root system spreads in a 40' radius to the dripline and who knows how much beyond. That brings us back to the quandry about the distribution of absorbing roots.

This discussion and the thread I posted about the roots invading the wooden barrel right next to a 30" trunk suggest an interesting possibility: 1) maybe narrow mulching bands can encourage additional growth of absorbing roots in a favorable microbial environment. 2) maybe there's a theraputic measure to be made out of putting some number of above grade soil masses (like a planter or large tub) within an undisturbed part of root zone to compensate for root loss in an unprotected zone.

Reply to post by Scott, on May 21, 1999 at 11:45:39:

Here's the link to Elaine's web site. She has a side business of sample analysis, although she's a hot enough speaker right now, I can't imagine how she gets any samples processed.

As far as the above ground soil masses and mulch halos and trenching is concerned:

It seems to me that tree guys in the "olden days" may have accidentally been doing something right in what has become vertical mulching. That technique lost some credibility with me when perlite was being used, I thought it would be pretty important to get organic matter in there. As I recall information from conferences, 7% organic matter is a good target for soil contents. Of course, urban soils are a long way away from that.

OM fosters microbial activity and protects them by holding moisture and sheltering from UV light.

It seems to me,the concept of creating little islands of soil ecology which are beneficial to the tree would help to bridge the gap between grass needs and tree needs.

On the other hand, one of the things Dr. Ingham emphasizes is that cultural activity SELECTS for bacteria. That would make trenching or other large scale disturbance a bad idea for tree health. To select for fungus, she says to leave it alone.

Interesting stuff eh!

Best regards,
KWK

Reply to post by Kerry, on May 22, 1999 at 06:38:25:

Here's another interesting link to Ingham's work that adds some elements I forgot to mention.

Best regards
KWK

Reply to post by Scott Cullen, on May 18, 1999 at 06:22:24:

Scott,

Kew Gardens in London, England have been conducting trials on the type and area of mulch reqiured to achieve the best plant health care (PHC)for stressed mature trees. In general an area to the dripline is best. In addition to this a compressed air charge containing advantageous fungi has been used to see if it helps trees recover from soil compaction. If you would like any more information get in touch.

Regards

David Partridge