Winter, 2001 Newsletter

Pruning Young Shade Trees

“As the twig is bent so grows the tree.” This insightful old bromide about children might just as well serve as the cardinal principle for pruning young shade trees. What you do to your tree in its first few years of life will affect its shape, strength, and even its life span. In importance, early pruning must rank just after selecting the right tree for the site, and careful planting.

As TREE CITY USA Bulletin editor Jim Fazio skillfully explains, pruning young shade trees is a simple, straightforward task. It is a job that needs to be done, whether by you for the trees in your yard, or by your community for the trees on public property. We do recommend you attend a local pruning class whenever possible. In any case, proper pruning will save money in the long run, and give you safer, more beautiful, healthy, easy-to-maintain trees.

Keys to Good Pruning

1. Prune early in the life of the tree so pruning wounds are small and so growth goes where you want it.

2. Begin your visual inspection at the top of the tree and work downward.

3. Identify the best leader and lateral branches (scaffold limbs) before you begin pruning and remove defective parts before pruning for form.

4. Don’t worry about protecting pruning cuts, the evidence is that tar or paint does not prevent or reduce decay.

5. Keep your tools sharp and clean. One-hand pruning shears with curved blades (secateurs) work best on young trees.

6. Make safety a number one priority. For high branches use a pole-pruner. Some, like the one pictured, have both a saw and shears on the same tool. A major job on a big tree should be done by a professional arborist.

7. When you prune back to the trunk or a larger limb, branches too small to have formed a collar (swollen area at base) should be cut close. Otherwise follow the rules of good pruning of larger limbs by cutting just outside the branch ridge and collar and at a slight down-and outward angle (so as not to injure the collar). Do not leave a protruding stub.

8. When simply shortening a small branch, make the cut at a lateral bud or another lateral branch. Favor a bud that will produce a branch that will grow in a desired direction (usually outward). The cut should be sharp and clean, and made at a slight angle about 1/4 inch beyond the bud.

Davis’ Urban Forest: A Personal Viewpoint

by David Adams

Davis is fortunate to have such a wide variety of trees that seem to tolerate our climate and soil. Our trees come in an amazing variety of sizes, shapes, and fall colors that together very effectively break up the rectangular skyline of buildings and homes. Among these are columnar conifers, mostly pines, and the rounded hardwoods comprised of oaks, elms, ashes, sycamores (plane trees), alders, lindens and many others. Street trees, shade and ornamental trees around the home, throughout downtown, within our many fine parks, and over the campus, give Davis a special feeling of community togetherness. Our trees, with our bike trails, tie the community together in a way no other features could do. Many species of trees have been used in Davis and many have failed to fulfill their promise under the demanding conditions that the out-of-place ornamental trees must endure. Those species that have survived are special to us. We must give due respect to our city arborists and interested community groups and citizens who for so many years have recognized and cared about those trees capable of giving so much to the community in return for the little care they require.

An urban forest is largely an artificial forest, created in communities where native trees are scarce, by people who appreciate the value offered through tree form, foliage, and just something as innocent as their presence. The few very large valley oaks scattered around town are aging remnants of the once very extensive valley oak forest that existed here. Our urban forest has been in the making for many years and continues to enlarge as Davis expands. Early Davisville records show non-native trees planted in the late 1800s for various enterprises and as ornamentals. Essentially all Davis trees are distanced from their native ecosystem. Some such as the ash are near-local, while most are far away from their native land; many from distant continents. Biologically speaking, this means that very likely they are not living under environmental conditions that simulate their needs. For example, the soil they are rooted in is undoubtedly different in soil quality characteristics (depth, texture, structure, nutrients, pH, and seasonal water content) in comparison to their native soil. Climate differences such as summer heat and winter cold are undoubtedly different too and affect the whole tree. By whole tree I mean not only the part we see, but also the root system. The root system is the heart and soul of a tree, for it is through the health of a tree’s roots that the whole tree survives. For a tree, what you see is only as good as what you don’t see.

Remember that trees are literally 'rooted to the spot' they are planted in and don’t have opportunity to move to a better place. Fortunately, for the trees and for Davisites alike, these perennial plants have within their broad genetic systems an amazing ability to tolerate a wide variety of atmospheric and soil environmental conditions. Yet, to do so, for non-native and native trees alike living in an urban forest there is a cost to be paid. Sometimes this cost is moderate, sometimes very high, but it is always there impacting their growth and life expectancy. The price is paid in what biologists call stress. Severe stress impact on the whole plant will significantly shorten a tree's useful life.

Mostly, without actually looking at a tree, we just look at the trunk of a tall street tree or seek its shade from our hot summer sun. Put on a biologist’s hat for a moment and take the time to look up into these trees, especially when the leaves have fallen, and you'll see a whole new world. It's the world of birds, squirrels, parasitic leafy mistletoe, foliar disease and wood decay, and insect herbivore activity, and even sometimes an arborist-the human caretaker of trees of the urban forest.

Arborists do many things in the tops of our urban forest trees. Among them, unfortunately for the form of the tree, is to provide and keep clearance between the high voltage wires at the tops of the poles and any nearby trees. Wherever high voltage lines run, whether along the street shoulder, through an alley, or between streets along the common property backyard fence line, the high voltage lines must be kept clear of potential tree damage. This is the law. It is an important law, one designed to protect the public against fire and personal hazard.

Now look up again, critically this time, wearing an arborist’s hat. What do you see? I look to see how well the tree has been taken care of. Has it been pruned by necessity to avoid overhead power lines, or roofs of nearby homes and buildings? Have dead branches been removed or are they still there hanging over your head? Does the pruned shape approximate that of a natural tree of the same species, or does the tree look misshapen? Is the foliar crown in balance? Do those heavy branches hanging over the sidewalk, street and your parked car look stable or questionable, i.e., are they poorly or strongly attached? Are weakly-attached water-sprouts, those fast-growing stems sticking straight up near stubbed-off branches, present? I look especially at the wound left where a large branch as been removed. Was the cut properly made, is the wound healing vigorously or not? The former tells you something about the arborists skills in making a proper pruning cut; the latter about tree health. If it was a good cut, the wound should be healing over (callusing) equally all around to eventually seal over the wound. The bark on the bottom of the cut should not have been ripped off as the branch was removed. If it was a poor cut the callus tissue growth will be out-of-round in appearance. It is important for a wound to callus over quickly to prevent undo exposure to entrance by those ever-present, opportunistic decay fungi. Rate of wound healing is an important criteria of tree health. The callus tissue should have the appearance of vigorous growth, somewhat smooth and shiny and at least looking like the wound will be sealed over within a few years. By the way, research has decided that sealing over a wound with tar or other material is not good; the wood underneath remains moist longer than if it were exposed thereby allowing decay fungi greater opportunity to become established.

Certified arborists are not just any tree climber with a chainsaw and pruning shears working out of the back of a pickup. To be a certified arborist one must obtain and hold a certificate from the American Society of Arboriculture. The applying individual must pass classroom tree knowledge and tree work ability tests. Continuous education credits are required. Their proper training is paramount, for it is the arborist who tends to tree form and tree safety, and it is the arborist who piecemeal takes down a large tree when it is no longer believed to be safe around the public and their property. In most instances a badly pruned tree is definitely worse than no pruning at all. The cause and effect of bad pruning are separated by time. Bad pruning unnecessarily exposes a tree to entrance of decay fungi that will, at some later date, create a hazardous situation. It is sad, but true, not all tree climbers are certified arborists. This means that before you have work done in the upper crowns of your personal trees, be sure to check out the workers qualifications, and property damage and public liability insurance carried for any person or business you might hire. Get referrals and check them out. Incidentally, tree topping represents the pinnacle of bad tree work. The newly exposed stem becomes sunburned and dies, opening up opportunity for decay fungi. Topped trees produce sprouts from buds on the sides of the stem just below the point of the cut. These new leaders are poorly attached to the side of the decaying trunk and will fall away after several or more years. Topped trees generally ‘melt down’ over a number of years suffering from extensive decay and continuous branch falling.

Impacts facing urban forest trees are constant right from the very beginning. Replanting the urban forest, especially public trees, is challenging. The first years are critical for the tree’s success. Most often, we plant ‘specimen’ sized trees, and for good reason. We want something we can see. Something that says “a tree is planted here.” We select a nice-looking young tree in a 5-gallon can. What do we know about the ‘whole’ tree; what about its roots? A tree lives or dies by its roots. Always check out the roots before planting a tree. Circling roots, sharply bent roots, and trees having diseased and dead root ends are unacceptable. Circling and bent roots will not ‘straighten themselves out’ after being planted. Difficulties for nutrient and water transfer from root tip to upper tree locations will forever remain-if the tree lives. Diseased roots will only continue to die. Examples abound of nice looking trees in a nursery having terrible root conformation. It’s exactly like buying a ‘good looking’ two year old car without examining its most important parts-the mechanical features and kicking a few tires.

Once a tree is selected, and assuming there is a space selected that is large enough to accommodate its growth over the next 30-50 years, we dig a hole for it. This is where it will spend the rest of its life. In goes the tree, malformed roots and all. We water it in and pound in a stake right beside it. We use that green tape that never seems to break to tie it tightly in place. There it is; it is expected to grow and strapped tightly to the stake it won’t give us any trouble about bending inappropriately or falling over. I’ve seen trees, green tape and all, cut up and thrown into the gutter for recycling. That’s not what anyone wants, but how do we really take care of a newly planted tree?

Once a good whole tree has been selected, early care is easy once the fundamentals are understood. Water management is especially critical during a tree’s formative years. Those unseen root tips reaching a few inches to several feet into the soil must be taken care of as though the tree’s health depended upon it-which it does. Root ends breathe. They respire, requiring oxygen and releasing carbon dioxide into the soil atmosphere. Roots have several basic functions. Among these is continuing enlargement of lateral roots and root spread for support for the developing tree, the intermittent uptake of nutrients and water through their root tips, to act as a conduit for the movement of nutrients and water from the soil to all the tree parts, and water and carbohydrate storage. Contrary to popular thought, lateral root spread can be large. A rule-of-thumb is to the drip line (outer edge of the foliar crown) of the tree. Some trees have roots that go far beyond that edge given good soil and no competition from roots of other trees. Roots grow under driveways, sidewalks, and even somewhat under streets. Don’t put the young tree in a basin. The philosophy behind this seems to be twofold: roots always grow only directly under the tree and filling up the basin allows the tree to take a ‘drink’ when it needs one. Both concepts are wrong! Irrigation needs to be intermittent, and away from the young stem just far enough to encourage roots to grow into the moistened soil. Flood irrigating at the tree base can lead to crown rot and tree death at worst and a stunted root system at best.

Continuously flooded soil drowns actively growing roots. Dormant trees can have roots flooded with little harm, provided the water recedes when they become active. Soil water uptake is intimately connected to the plants foliage. Deciduous trees take up very little water; evergreen trees will take up water throughout the year, but mostly during the warm spring and summer months when photosynthesis and respiration is most active. Most water taken up by roots is expelled (transpired) into the atmosphere through openings (stomata) in the tree’s leaves. Think of roots as taking turns ‘breathing’ and ‘swallowing;’ breathing when soil moisture is low and swallowing when soil moisture is high. Of course it is not as simple as this, but this scenario does represent the overall effect of water uptake and root respiration. Neither circumstance, flooding or drying, can dominate during the times of plant growth, otherwise stress will occur. Having said that, I must say too that some stress occurrence is a necessary part of healthy plant life. Stress occurrence, in its many and varied forms in conjunction with hormone (phytohormones) production, provides biochemical signals to the tree to perform its seasonal functions of dormancy breaking, bud enlargement and breaking, stem diameter increase, flowering, seed formation and seed maturity, seed germination, controlling of opening and closing of stomata, slowing growth in the summer, and winter dormancy. This brings us to consider the soil trees are planted into. Good soil is not a solid; it is full of very small diameter pores called micro- and macro-pores. Worms and soil insects create relatively much larger pores and gophers create much larger still. However, it is the micro- and macro-pores that are the bread and butter of soil ‘breathing.’ Yes, metaphorically of course, soil breathes. Oxygen is taken in to be utilized by soil organisms and root ends during respiration, and carbon dioxide as a waste product is expelled from these same sources and through the same soil pores. The fine-textured clay soils of Davis make it difficult at times for soils to breathe. Compacting soil over tree roots severely compounds the problem. Compacted soils become low in biological activity and organic nutrients and consequently low in the organic part of soil nutrition. Compacting also kills the fragile root ends and their symbiotic fungal associates-the mycorrhizae, and the myriad of soil microorganisms that render soil organic matter into forms available for root uptake. Most trees become severely stressed or die when their surrounding soil is compacted.

Early tree support or staking is a critical process. Young specimen trees (whips) often need to be staked until they can support themselves through stem enlargement and root development into the soil around the planting hole. A staked tree must be allowed to bend somewhat to help the tree build basal diameter (caliper as it is called) to support itself. Staking can be either a single stake, i.e., the newly designed metal stake that has an arm with a tree support on its end, or two stakes separated some distance from the tree with the tree’s stem loosely tied between the stakes. You’ll find both systems in use by the City.

Many people expect 'their' tree to be there forever. That would be wonderful and goes right along with not having to rake leaves, trim or prune branches, roots not pushing up sidewalks and driveways, and always having the right tree in the right place. Trees come with a price of care, maintenance, and cleaning up after them. More importantly, trees are not forever, as with all living things they have their times of immaturity, maturity, old age, and finally death. Trees do have the potential to live longer than humans, but under the stresses imposed by living in an urban environment their life spans are often significantly decreased. For example, Monterey pine in its native coastal habitat may live 125-150 years; in the Sacramento Valley, 35-40 years is usually its maximum. Unfortunately, trees of the urban forest cannot be allowed to live out a full life span as do trees in the natural forest. This is because they eventually become a safety threat to humans and property within reach of their height. Branches break, trunks split apart, and trees fall; that is a fact of life not only in the artificial urban forest, but in the natural forest too. In the natural forest trees tend to support each other by passing the wind mainly through and over their tops, rather than the tree itself fending off the wind. This is not necessarily what happens in the urban forest where individual trees are more exposed to wind. Combined with poor root structure, urban forest trees are relatively more inclined to break apart or fall over than are their native forest counterparts.

The urbanized tree, as a member of the urban forest, is a remarkable tree. The urban forest is best described I believe, as a community-important, artificially created forest, that behaves as a vital organism, and which lives precariously in a foreign land. It is besieged by humans, diseases, herbivorous insects, and parasitic plants, both above and below ground. It’s origin may lie in the form of plants having poorly formed root systems, often poorly planted and supported, subjected to foreign soil conditions and soil water regimes, and sometimes shaped above with disregard for natural tree form. It is no wonder that this forest needs to be constantly renewed. Old decrepit trees must be appreciated for what they have given to the city, however, they must be recognized for what they are and removed to make way for young trees that will provide the amenities for our heirs that today’s trees provide for us. Foresight and practical decisions will sustain our urban forest legacy.