Sunday 9 September 2007

One Mans Junk Another Mans Treasure

While traveling recently I was reflecting on people’s relationship with trees.

Quite often I will hear people describe this or that tree as a "junk tree".

It’s easy to attach labels to trees that for one reason or another have developed a bad reputation. Professional tree care people are often the worst culprits in this practice.

However one must proceed with caution, because as the title of this articles eludes "one mans junk is another mans treasure”.

In the Western Plains cottonwoods are revered as giants of the riparian forest. Riding along the banks of the Missouri river at the height of summer one can’t help but marvel at the majesty of these stately stands. Yet in the eastern parts of North America these very trees are ridiculed as being weedy, messy and totally undesirable. Similarly the box elder, also known as the Manitoba Maple is a very fast growing hardy tree that graces the yards and streets of many a prairie town. Again it is much maligned in the Midwest and New England.

It can be said that is that a weed is simply the wrong plant in the wrong place. This is true for trees as well. The Norway maple is fast growing and comes in varieties with beautiful burgundy foliage. This tree can become weedy and often escapes into the woods, successfully out competing native maples. There is some merit to calling invasive species less desirable or at the least not recommended for planting.

I have often considered many of the Asian elms to be in this category. In fact I once recommended the removal of one such tree to a client, due to its prolific seed production and numerous broken, decayed small branches over the patio in the court yard of the house.

“I would recommend taking this junk tree down, it will be nothing but trouble. “ I stated confidently to my client.

Listening closely to what he said provided an invaluable lesson. The potential client paused and then told me how he and his son had grown the tree from a small sapling in the woods to its present size. The tree had a very special sentimental value.

This illustrates how often, in Urban Forestry, you are dealing with the person’s feelings and emotions towards a tree and not just its botanical attributes. Junk or treasure, it is truly in the eye of the beholder. I’m sure many readers have similar stories. Please feel free to forward them to the address below. They may be featured in a future article. And I always like a good tree story.

No Sweat !

“Do trees sweat”? The answer is yes, in their own way.

Trees use the evaporation of moisture to cool themselves when moisture is easily available.

Moisture in the leaves comes up from the roots through the trees vascular system.

This process is both passive in the form osmosis and active with cells moving the water along like a vegetative bucket brigade. This all works well in a particular set of limited circumstances. For instance, when ground water is available and the tree is in relatively

dry air. Only then can the tree release tremendous amounts of water. Some estimates run as high as 350 gallons per day with optimum conditions. 95% of this water is lost to the atmosphere in the process called evapotranspiration. Trees don’t exist to emit water vapor into the air and cool the globe in the process. However this seemingly altruistic process does occur as a result.

Transpiration uses water to move minerals from the root zone to the leaves where they are combined in the process of photosynthesis to create sugars. When trees release moisture into the atmosphere it does indeed cool the leaf. This is important because the process of photosynthesis in most trees occurs in a relatively narrow temperature range. This range is between 15’c and 25’ c or 59 ‘f and 77’f. Within this small range of temperatures the whole process works in an optimum fashion. Too low a temperature and the process is not efficient. At high temperature it begins to slow down as carbon dioxide becomes a limiting factor. Evapotranspiration works to keep the leaf in this optimum range.

Leaves are also able to dissipate heat directly into the atmosphere using a process called convection. If the leaf is hotter than the surrounding atmosphere heat will waft away from the leaf on air currents. Typically a leaf in sun light is slightly hotter than the surrounding atmosphere. This temperature gradient in itself allows moisture to evaporate from the leaf even if the relative humidity in the surrounding air is high. Trees are not immune to loosing more water than they can replenish from the roots. This causes a water deficit. If the tree is able to replenish its supply of moisture before the start of the next day it is able to continue growing even though this situation does limit the overall growth of the tree. This condition is called a daily water deficit. If the tree is not able to re-supply its water needs it will become desiccated and die.

Tree leaves regulate the loss of water vapor through small openings in the leaves called stomata.

These specially modified groups of cells are found on the outer surface of the leaf, top and bottom. As moisture exits the leaf it creates a humid boundary layer around the leaf. Wind moves the moisture away and allows the leaf to continue transpiring. If the wind speed becomes too great the leaf will close the stomata to reduce moisture loss also reducing the cooling effect losing moisture creates. For a more detailed explanation of tree water relations contact an I.S.A. Certified Arborist.