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As I'm sure you are aware, the roots of trees in nature normally experience a fair amount of temperature variation. As the feeder roots of trees tend to inhabit the upper 1 foot or so of the soil, they are affected by the increase in temperature that this layer of soil experiences from direct sun and ambient air temperatures. These effects rarely go below about 1 foot and the effects drop off with the depth.
In "Soils: An Introduction to Soils and Plant Growth" (Donahue, Miller, and Shickluna) Miller shows a chart of air temperature and soil temperatures at different depths throughout a day in July in Utah. At 12 noon the air temp was about 97F and fell to about 60F at midnight. By about 1pm the soil at a depth of 2 cm reached a peak of about 93F. At 10 cm deep a maximum of about 85F was reached at about 6pm. At 30 cm it only budged up to 75F at midnight. All the soil temps dropped off into the mid 60's to 70's overnight. This kind of temperature movement of the soil is fairly common in most areas. These authors offer a (not so accurate) rule of thumb to find the mean average soil temperature for your area: add 1 degree C to your mean average air temperature. This could be loosely applied to monthly averages also. This should give you an idea of what temperatures your local trees root's see in the soil in your area.
So it's obvious that some tree roots can see fairly high temperatures in nature. However, in a deep forest little sun reaches the forest floor and root temperatures are probably fairly low and more constant. This probably holds true for most lightly wooded areas such as my backyard also. (I'm sure all of us are aware of how much cooler the ground is when sitting in the shade of a tree). I would venture to say that many tree's roots are used to seeing moderate temperatures and are not used to high temperatures.
Dr. Carl Whitcomb in his book, "Plant Production in Containers" cites some research showing that very high temperatures can be experienced by roots in containers. One researcher in So. California found that soil temperatures in black plastic containers reached a maximum of 115F and remained at or above 100F for 5 hours each day that they were observed (Harris, R.W. 1967 "Factors influencing root development of container grown trees" Proc. Int'l Shade Tree Conf. 43:304-314). Some researchers in Mississippi also measured the temperatures of black plastic containers in a bed of plants and found temperatures in excess of 120F near the sides of the containers and 100F in the center of the containers on one edge of the bed. Containers located in the middle of this bed, for instance, had temperatures less than 90F. In this study the containers were closely arranged providing a solid canopy of foliage which shaded the inner containers. (Rauch, Fred D. 1969 "Root zone temperature studies" Miss. Farm Research) Generally, this will be the case in nurseries that typically arrange plants in beds to maximize space. Bonsai on the other hand are normally spaced far apart to assure that all branches get sun. Therefore the roots of trees grown as Bonsai will normally see the higher temperatures.
Although it is generally accepted in the nursery business that high temperatures are responsible for poor growth, there is little documentation on the species specific effects of high temperatures. There are some studies available though; for instance, Loblolly pine (Pinus taeda) was found to show maximum root growth at temperatures between 70-80F with growth reduced as much as 90% at 95F. This same study found that a number of conifers were killed in a few hours at 117F (Barney, C.W. 1947 "A study of some factors affecting root growth of Loblolly pine. Pinus taeda" PhD dissertation. Duke University School of Forestry). Although there are only a few more studies to go on, it is not a tremendous leap of faith to assume that most temperate tree roots probably have growth optima in the 70-80F range also. Higher temperatures probably slow or stop their growth, and maybe kill them as well.
Whitcomb states that "This study shows that heat stress on plant roots in containers is a serious problem. The rapid loss of roots following exposure correlates with the abrupt plant stress frequently observed when container-grown plants are spaced during the summer. Roots killed by heat are prime sites for the entrance of root-rot disease organisms. Root death from high temperatures may be a major factor in providing an easy entrance to root diseases".
I have thought about this issue many times over the years as I have noticed that some trees that I have kept up on my growing benches seem to languish in the hot summer sun where ones of the same species that I have crowded together on the (cooler) ground "seem" healthier. I have often suspected that summer heat damage might have been responsible for the loss of some of my stock plants in the winter.
Root rot usually occurs in the summer and is invariably blamed on poor drainage and overwatering. I wonder how much root death from overheating may also contribute to the summer root-rot phenomenon. (It's almost ironic that it's recommended to place Bonsai affected by root-rot in the shade after treatment - part of the cure?) Even some of the winter occurrences of root-rot may have had their origin in summer heat killed or damaged roots. (Now, there's some interesting studies to be performed).
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