For years now I have checked or not checked off the little box (on the tree-hazard rating sheet provided in Jim Clark and Nelda's guide to hazard trees in urban areas) that says "trunk wetted" under site conditions. And then, after having checked on not checked it, was non the wiser.

What's the issue with irrigation wetting tree trunks? In the northeast we water in the early morning (turf) and the grass and tree trunks are dry by 10 or 11 am on an average summer day if not long before that. I wonder if this issue has some regionallity such as developing rootrots in the southwest or something like that.

Any thoughts?

Wayne

Reply to post by Wayne Cahilly, on October 06, 1999 at 08:28:20:

Wayne, since you and I discussed this a few weeks ago, Nelda answer the question on The UKTC site.

In a nutshell, I hope accurately, in CA the Summers are dry, that's when temperatures are high enough to promote the growth of root and trunk rots but they don't do well in absence of moisture. So the native trees have not evolved resistance to these organisms and are very susceptible in the presence of artifical moisture.

The check box on the rating sheet reflects that situation. It may be less important in other climates. If Nelda is reading maybe sheet can fill in the blanks.

To answer your other question what - to do about checking yes or no - I guess is "nothing" if it's not a factor in the study area.

Reply to post by Wayne Cahilly, on October 06, 1999 at 08:28:20:

In the Pac.NW, with Douglasfirs, summer is the dry season. When the soil dries and water is a limiting factor th growth, the metabolic pathways that build new structural tissues switch to the pathways that produce defensive compounds. This would include the various allelochemicals, and "hardening" with lignin, in short- reinforce the new spring wood and foliage against insect and pathogen sampling. Given water, the growth phase lasts longer, less defense is created, annd that works for some species e.g. Populus sp., so it wouldn't matter there. but for oak species and Douglasfir it is going to be eroding the viability of the tree.

Reply to post by Peter Torres, on October 06, 1999 at 08:28:20:

Peter, it sounds like the mechanism you're describing has to do with supplemental moisture presumably absobed by roots and affecting metabolic function. Is this a different issue, albiet from the same source, than water hitting the trunks and root crowns?

Reply to post by Scott, on October 06, 1999 at 22:46:58:

I think it is the same issue, because if water is hitting root cron and trunk, the tree is getting out-of-season, supplemental moisture, which will alter the normal seasonal metabolic changes.
Also, the other obvious point would be creating a warm, moist environment for fungi e.g. armillarias to colonize root crown areas.
I did see some Dougfirs in a park that literally had the thick bark eroded off due to water spraying sprinklers, too. I think it was mechanical erosion. Didn't seem to be injurious to the trees' health, although susceptibility to fire and mechanical injury would increase.

Reply to post by Peter Torres, on October 07, 1999 at 11:16:44:

I too have seen powerful spray heads erode the bark on nearby trees, and also a staining effect from direct spray,
whether from mineral deposits or bleaching from chlorinated water I don't know. I never see the staining (white)
on the trunks of trees irrigated with untreated well water.

Reply to post by Mark Goodwin, on October 11, 1999 at 00:11:44:

I'm told that water utilities like to maintain an alkaline pH in water that goes into mains... it's kinder to the pipes. Wonder if that might have a bleaching effect.

Reply to post by Scott, on October 11, 1999 at 11:07:57:

I think you have to look at the water chemistry as a whole to determine what effect it might have on the soils and the trees. pH is just one aspect, but there may be minerals and other chemicals that could, over the long term, affect local conditions.

Reply to post by Scott Cullen, on October 06, 1999 at 08:28:20:

Thanks all for the thoughts. I posted this and then sat back to listen, I think its significant that westcoast explainations came in, and Scott and I had discussed this earlier. It's apparently not a national issue but it's on a form that is broadcast nationally.

Perhaps in the next generation of discussions of hazard rating sheets (the actual value of which is still open for discussion in my opinion) we should look at a generic first page and regional supplements to cover important issues that are not as widespread.

Wayne