Owners of bluff properties have many questions about site development, erosion control, view clearing and beach access. Often, these questions are asked too late: after the damage is done and possible options are eliminated. Even when a property owner is aware that his or her decisions are critical to the long-term stability of a site, it can be difficult to judge the best course of action.
In preceding chapters the complexity of the shoreline environment and the role of vegetation has been discussed. By now you realize that it is important to consider all the factors involved before acting. This chapter addresses some of the most common questions asked by shore property owners and offers generalized answers.
This simple question generates a range of sometimes contradictory answers. There are many factors to consider before reaching a decision. These factors include: stability of the slope, species, age, health, current stability of the tree, position on the slope, surrounding vegetation, rooting habit/soil type, density of the stand, and the ability of the tree to sprout. Before we discuss these factors, it is necessary to mention some general considerations that apply to tree removals on steep slopes.
Tree Roots. The root systems of trees form an interlocking network, especially on many shoreline sites where rooting can be shallow. Often rooting is only two to three feet deep. The depth of root penetration is largely a function of soil depth and type, soil moisture, and the presence or absence of a dense layer of clay or till. These factors have a greater influence on rooting than any tendency of a tree to develop a characteristically deep or shallow root system.
Trees compensate for shallow rooting by increased spread of root systems. Recent research has indicated that a tree's root system will extend considerably beyond the dripline, often as much as two to three times as far. Extensive lateral root systems are common where soil moisture is excessive, soil is shallow, and impervious soil layers impede vertical growth. Where soils are porous, well-drained, deep, and no impervious layer exists, deeper rooting will occur.
Generally, the influence of a tree's roots on a given site will be related to the tree's age and size. Larger trees will have more extensive, often deeper and better developed root systems. Dominant trees, those larger and taller than the surrounding ones, have been more subject to wind and usually have developed stronger root systems as a result. Before clearing trees, consider the effects of removal on tree rootmass over time. Roots of dead trees decay, their stabilizing influence diminishing over a three to nine year period. As a result of the gradual loss of root strength after tree removal, barely stable slopes may fail several years after clearing or thinning.
Trimming debris can contribute to stability problems by smothering vegetation and by causing damage to the slope in sliding or rolling downhill. It is difficult to offer general recommendations for dealing with this material due to the wide range of site characteristics and debris volumes that might be generated.
Since regulations regarding the disposition of trimming debris vary it is advisable to check with local planning or engineering departments for advice.
Disposing of bluff top clearing debris over the edge of a slope will be discussed later in the guide.
Do Not Remove Trees Without Cause. People tend to remove many more trees than are necessary during site preparation. The value of a healthy, strong tree on a slope or bluff far outweighs its value as lumber or firewood. A tree should be retained unless it is a hazard to life or property, is growing on the proposed house site or drainfield area or has some other major problem. Do not clear a reserve drainfield area before it is needed. Explore alternatives to removal thoroughly before deciding to cut. The location of trees and other factors involved should be considered carefully. Do not remove trees on slopes until home construction is complete. You may find that the trees do not need to be removed.
On Choosing a Tree Service
The tree care industry is currently undergoing something of a revolution. Many common practices, such as tree topping, are no longer recommended. There has been a great deal of recent research regarding how trees grow and react to environmental changes. New equipment and techniques are continually being developed.
Groups like the Seattle-based Plant Amnesty actively lobby to abolish topping and poor pruning practices. Professional associations such as the International Society of Arboriculture support research and provide certification programs for tree care practitioners. They are good sources of assistance in finding a tree service. See Web Links.
Choosing a tree service can be a bewildering experience for a property owner. For an owner of shore property, making the wrong choice can have serious consequences. Beware of bids that seem "too good to be true." The money saved initially may pay dividends of disaster within a few years.
When hiring a tree service to work on a potentially unstable site, require proof of the following:
Most of the pruning practices described later in this guide are hazardous operations. They should only be performed by qualified and well-equipped personnel. Most property owners should not attempt to perform the work themselves.
Different species have different characteristics. The growth habit, rooting habit, height, shape, longevity, strength, durability, resistance to salt and climatic stresses, and tolerance to pruning all differ among species. Refer to the plant lists in the Appendix for a relative comparison of characteristics for trees commonly encountered on Puget Sound shorelands.
Age. Tree age in relation to expected longevity of a particular species, can be an important consideration when deciding whether or not a tree should be removed. For example, should you cut down a 65 year-old, large Red alder that is obscuring your view? Because alder is a fairly short-lived species that seldom survives beyond 70 years of age, it is probably not going to survive much longer. In this case, expensive view pruning would not be warranted. The advisability of the tree's removal would be dependent on its role in stabilizing the site. If the tree in question were a Pacific madrone, which can live for well beyond 200 years, then removal would not be advised. Alternatives such as pruning would be an excellent investment for the Pacific madrone. This simple example does not take into consideration other factors that may bear upon a decision to remove a tree in a particular location.
Health of the Tree. Tree health and vigor are important considerations when deciding on removal. Root rots and stem or trunk diseases are the most serious defects. In dense, single species stands infested by root rot, removal may be your only choice. It is advisable to confer with a knowledgeable professional, such as a forest pathologist or arborist if widespread forest health problems are observed.
Current Stability. An assessment of the stability of a tree in relationship to surrounding trees is important. Before landscape alterations begin, determine if the tree is part of an inter-dependent group or can be managed as an individual. Generally, if mature trees grow within 10 feet of each other and share crown canopy space, they are functionally a group. If rooting in the area is shallow due to high water table, impervious or impermeable layers, or shallow soils, then inter-dependence will be greater. If tree trunks lean away from each other (Illustration 8) it is probable they are "balanced" and the removal of one will predispose the other to windthrow.
It is often difficult to evaluate how inter-dependent a grouping is when considering a dense stand. Normally, the denser the stand and the younger the trees, the more can be removed safely. Again, consider all pertinent factors.
When a tree on a slope has become undermined or is otherwise in danger of falling over it should be cut. Determine if an individual tree is losing anchorage or if the lean is the result of soil movement as shown in Illustration 6. Exercise extreme caution when cutting trees on slopes.
Position on Slope. Consider a tree's location on the slope before removal. Illustration 9 depicts a situation where various conifers and deciduous broad-leaved trees are obscuring the view. They are also protecting the residence from the full force of prevailing winds, as well as stabilizing the site of an old slide. Tree cover can often reduce the height of brush. If trees are removed, the brush grows higher thereby requiring constant trimming.
One solution would be to remove some or all of the trees to access a view. However, upon considering the benefits these trees provide and some of the possible adverse impacts that could result, a landowner might seek ways to enhance the view without removing the trees. This might include interlimbing, cutting windows, and skirting-up as discussed later in the question, "What are alternatives to tree removal and topping?" (See illustrations 12 and 13.)
Surrounding Vegetation. All factors should be considered together. This is especially important in regard to the vegetation surrounding trees being considered for removal.
As mentioned, some brush species thrive and flourish when a tree overstory is removed, creating a view management problem. This is particularly true for species such as elderberry, oceanspray, and salmonberry. Alder, wild cherry and some willow species may become maintenance problems when tree canopies are removed and additional light is able to reach the ground. Another species encouraged by increased light levels is Himalayan blackberry which is difficult to control. Invasive species such as Scot's broom prefer disturbed sites with abundant light, and can require constant control to maintain a view.
Native shrub species such as Oregon grape, salal, snowberry, and Evergreen huckleberry are excellent groundcovers that are often common under conifers. They are sometimes over-stressed when trees are removed and can be replaced by less desirable or weedy species.
Most brush problems occur in the area of the bluff between the uplands, the crest, and the upper margin of the slope face. Lower down on the slope, brush is not a consideration in view obstruction. When contemplating the removal of trees high on the bluff, consider the response of surrounding vegetation so as not to create subsequent problems.
Stability of the Slope. An analysis of slope condition by a geologist or geotechnical engineer is strongly advised and in many counties is required. Vegetative clues should be used in conjunction with the geotechnical data and an assessment of the role of the vegetation on the site should be made.
In situations where soil and hydrological conditions promote well-rooted, healthy, mature trees, the trees should be left insofar as is possible. As mentioned, the practice of removing a majority of trees on a slope can greatly increase the probability of a slope failure in the future as the trees roots decompose and their soil-binding capacity declines.
Some geologists or geotechnical engineers routinely recommend the removal of trees because of concerns that: 1) large trees exposed to wind can transmit that force to the slope, thereby causing slope failure; 2) soil moisture is reduced by evapotranspiration of trees, thereby creating cracks in impermeable layers and promoting water infiltration to lower soil layers; and 3) the weight of trees on the slope may cause landslides.
These concerns have been addressed in recent research and the overwhelming conclusion is that in the vast majority of cases, vegetation (especially well-rooted, mature trees) helps to stabilize a slope.
Density of the Stand. The implications of dense stands of short-lived species such as alder and willow have been discussed. In the case of dense stands of conifers such as Douglas-fir, Western hemlock, Red cedar, Grand fir, Sitka spruce or mixed stands of these species, the situation can be quite different. On stable sites with no serious ground water or surface runoff problems, the landowner has several options.
When trees are fairly young (between 5 and 30 years old) they are still capable of vigorous growth in response to thinning. It is possible to remove enough trees to attain a view and even improve the strength and growth of existing trees without creating a potentially hazardous situation. If the crowns of the trees are "crowding" each other and receiving light only from the top, then a thinning could be done. Caution should be exercised not to predispose the remaining trees to windthrow by altering the wind-deflecting properties of the windward trees or allowing wind to be channeled into the interior of a stand that was previously protected.
Removal of trees from a dense stand without damaging those remaining can be difficult and expensive, but the extra care required is a good investment in maintaining the health of the trees that protect your property. Broken tops and branches, as well as trunk scars left by falling trees can serve as entry ports for disease and insects. Consult with a qualified tree service when low-impact falling and removal of trees on a slope is necessary.
There are many other possible situations where stand density could be a consideration. Most of them require good judgement and compromise.
Ability of the Tree to Stump-sprout
The ability of a tree to sprout from a cut stump can be an important characteristic when a property owner is concerned about securing a view without jeopardizing the stability of a slope. The maintenance of a vigorous, live root system insures soil-binding benefits.
Though most tall brush species common to our area will readily sprout when cut, there are relatively few tree species that do so. All of these are broad-leaved deciduous trees. Careful cutting of the species listed offers a means of view clearing without jeopardizing slope stability. The following common trees are capable of sprouting when cut. (See the question "When is the best time to cut back vegetation?" on the Frequently Asked Questions page.)
Most conifers will not successfully stump-sprout when cut.
Remember that cutting back of brush and trees near the crest will be required periodically to maintain your view. If you find that brush must be cut more often than once every two to three years you may want to consider planting a lower-growing species to replace the existing brush. Kinnikinnick, an evergreen, forms a dense, low mat and has good erosion control properties. Allow at least three years for its establishment and provide protection from animal damage for the new plantings as required. The offending brush will eventually die if cut back repeatedly after two or three years. Under no circumstances should herbicides be applied to kill unwanted brush. The value of the root system far outweighs the inconvenience of maintenance when slope stability is a concern.
Copyright © Washington State Department of Ecology|
Privacy Notice | Site Info | Accessibility | Contact the web team |