Chapter 2: Vegetation on Shore Bluffs
When property owners become aware of the dynamic and fragile nature of shore areas through an understanding of the landscape's origins and the processes continually shaping it, they are better able to answer some of the questions listed at the end of Section 1. A knowledge of the nature and functions of the vegetation growing on these sites is no less important if they are to avoid the sometimes disastrous consequences of ill-advised development practices.
Illustration 4 shows ways vegetation protects soil from surface erosion. Live plant foliage and forest litter (partly decomposed leaves, twigs, etc.) break the force of falling rain and reduce the impact of raindrops, which can loosen soil and transport it downslope. Absorptive capacity of the soil is increased substantially by the presence of forest litter, which acts as a sponge by holding water and releasing it slowly over an extended period. Low-growing plants catch and slow rainfall and allow some moisture to evaporate from leaf surfaces. Groundcovers and forest litter also help reduce surface water runoff velocity and act as a filtering system for soil particles in suspension. Plants draw water up through their stems or trunks and branches to their leaves and into the air by the mechanism of transpiration, thereby removing water from the soil.
Plant roots, especially the smaller feeder roots, provide a fibrous web that stabilizes and anchors soil. They function much like reinforcing steel in concrete structures, increasing the cohesive strength within a soil horizon. The roots of many brush and tree species penetrate deeply across the contact zone between two soil layers, thus increasing the soil's shear strength and reducing risk of shallow landslides.
Several layers of plant foliage multiply the benefits discussed above. Ideally, a site will support low groundcovers, small shrubs, taller shrubs, and small and large trees.
Vegetation, though more effective in protecting against surface erosion than in controlling mass soil movements triggered by groundwater, can still be valuable in sustaining slope stability. As mentioned, many bluff sites are barely stable and the removal of vegetation on some slopes can precipitate a landslide or reactivate an old one. Due to the complex root network formed by trees and shrubs, potentially unstable slopes are held together and the resistance of the soil to slipping, sliding, and washing away is increased. Slopes susceptible to soil creep (Illustration 5) are also held in check to some degree by the presence of vegetation. The ability of plants to absorb water and slow its velocity also allows time for soils to "meter" the absorption and discharge of water more effectively.
The type, age, health, and abundance of vegetation growing on a shoreline bluff site can offer valuable clues to determine slope stability. Even the presence of stumps and fallen trees can tell a story to a knowledgeable observer. This section discusses these clues and what they may indicate. Vegetative indicators are best interpreted in combination with soil and geological data.Curved Trunks
Trees on a slope curved as shown in Illustration 5 are usually the result of a slow, gradual soil creep. Care should be exercised in clearing sites like this because you may de-stabilize an already marginally stable area."Jackstrawed" Trees
Illustration 6 shows the jumbled appearance of trees after a slump or earthflow. In situations like this, groundwater problems can cause a mass of soil and the vegetation on it to move downslope. If the trees are dead, this may indicate that the roots were sheared or broken loose.Trees Tipped Downslope
On sites with shallow soils and steep slopes, this may indicate mechanical shifting of materials and signal the potential for a slope failure.Groups of Trees Growing Across the Slope in a Line
Lines of trees growing across a slope may indicate two conditions. If the trees are species such as Red alder or willow, a slide may have caused bare ground in the recent past, subsequently offering a site for germination and growth of these fast-growing trees. Chances are good that the slide is active and periodic. The age of trees growing in this manner can be a clue to when the slide occurred.
A line of trees may also indicate an area of perched water or groundwater seepage that in turn may indicate a layer of impervious material underlying a deposit of sandy soil (Illustration 7). These sites usually are unstable and should be investigated geologically.Bluff Faces Without Vegetation
Shorelands with slopes or sections of bluffs devoid of vegetation can indicate many different situations. Generally, a bare bluff face suggests a site is either too steep to support vegetation or that recurrent erosion precludes the establishment of plants (Illustration 2). The first case is common on exposed bluff faces comprised of glacial till. These sites are often vertical. They are difficult places for vegetation to become established. Of more concern to property owners are steep, erodible sandy bluffs that are actively eroding or retreating. These sites are usually not able to be stabilized by vegetation.
Bare areas may also be indicative of recent or active slope failure. These sites are usually obvious. If the toe of the slope is protected from wave action, signs of debris will be seen. However, wave action will often remove the evidence of erosion.Old Stumps
Stumps from past logging and clearing are often found on shoreline sites. These remnants can offer much information about the stability of a site and the history of an area. Most shorelines were logged off by the turn of the century. Old-growth trees were often eight feet or more in diameter and they were usually two hundred or more years old when they were cut. Thus, an old-growth stump found today indicates that a site has probably experienced no appreciable mass movement for at least three hundred years. This, of course, is not an inflexible rule and does not always mean the site is currently or permanently stable. All indicators should be used in context with other available information.
Partially buried old-growth stumps can indicate soil movement from up slope in the form of debris avalanches.Downed Trees
The presence of downed trees may indicate several things. In sites where rooting is shallow, wind may cause trees to blow down. Shallow rooting can be the result of wet soils like those found in wetlands, or can be caused by shallow soils underlain by impervious layers that resist penetration of roots.
Fallen trees may also result from adjacent clearing or excessive tree removal within the stand, which often exposes previously stable trees to unusual wind stresses. In some cases, diseases such as root rot may cause substantial windthrow on a site. Another potential and common cause of downed trees is a slope disturbance such as excavation of the toe, or previous thinning, which leads to local erosion undermining downslope portions of the rootmass. This condition becomes obvious when bare roots and "caves" are observed under trees.
Whatever the cause of fallen trees, the results are similar: accelerated erosion, de-stabilization of the slope, and substantial disturbance to the area. These sites should be examined carefully to determine the cause, impact and severity of a disturbance. Any remedial actions deemed necessary should be accomplished quickly.Single Dominant Species and Even-aged Stand
Occurrence of a predominantly single-species, even-aged stand of Red alder or willow accompanied by understory vegetation such as stinging nettle or bracken fern, can indicate a fairly recent, large-scale, mass soil movement. A plant community similar to that described above, though apparently indicating a stable site, hints at the presence of recurrent large scale disturbances. Linear down-slope "stripes" of such vegetation commonly mark the paths of debris avalanches.
These vegetation types are sometimes associated with high water tables, shallow soils, and marginally stable slopes. They are often adjacent to wetlands and underlain by impervious soils. They are extremely difficult to manage successfully for most residential development. It is often impossible to attain shoreline amenities such as a view on these sites because they are predominantly deciduous and even when fully vegetated are barely stable. In many cases attempts at forest thinning can cause blowdown and subsequent erosion.
Single-age stands can also indicate past clearing or tree removal. Look for old stumps and note size and condition to estimate how long ago the trees were removed. Tree rings can tell you how old the trees were when cut.Recently Cleared Areas
Partial clearing of uplands and slopes to allow access for prospective buyers and reveal views can cause modifications that could precipitate erosion. Seldom has the clearing been planned and executed with long-range slope stability in mind. Since the impacts of clearing may take several years to become evident, an unwitting buyer may purchase a potentially unstable site. Though this is not always the case, previous clearing will reduce your options for site development.Dead/Dying Trees
Properties with large numbers of dead or dying trees indicate that there is cause for concern. Look for insect or disease incidence, signs of past wildfire, changes in local hydrology, or other probable causes. Healthy vegetation is important to your property's long-term stability.Multi-species, Multi-age Vegetation
A site that has a wide variety of vegetation of various ages, is usually stable. A variety of vegetation (groundcovers, shrubs, and trees of deciduous and evergreen species) often indicates the site has not been recently disturbed and that local soil movements are likely to be stabilized naturally by the surrounding vegetation.
Each plant, from the smallest herb to the largest tree, contributes a stabilizing influence to the soil through its rootmass. Some plants have shallow, fibrous roots; others have deep roots. Together they form a strong mat that resists erosional stresses.
As a result of the inherently stable nature of a diverse vegetative community, your management options are increased.Low-growing Brush May Hide Problems
Because many brush species grow fast and luxuriantly, a slope face that appears fully vegetated may be actively or potentially unstable. Many brush species found on logged slopes in the Puget Sound area can hide signs of old slides or the clues that would indicate an inherently unstable site. It is sometimes necessary to investigate beneath this vegetation to inspect for signs of seepage, soil movement, or surface erosion. Sites with extensive cover of Himalayan blackberry or salmonberry should be carefully inspected.
If you explore Puget Sound by boat or walk the beaches you will notice a wide variety of trees, shrubs, and other plants growing along the shores and bluffs. In some places the slopes are densely wooded with evergreens and broad-leaved trees while other places support mostly brush or herbaceous plants such as ferns and foxglove. There are places where madrone and salal line the shores and others where barely anything grows. What causes this variety and variability? What are the implications for site development and slope stability? Property owners need to be familiar with the interactions between what grows on their land and the environmental conditions that influence that growth.
In previous sections of the guide we have discussed the geologic origins and natural processes shaping much of Puget Sound. We have described some of the clues that help explain the recent geologic history of shore properties and how to recognize unstable situations. Now we will explore some of the general factors that influence the shoreline vegetation. Keep in mind that invariably more than one factor will influence the growth and variety of vegetation on any given site. Refer to the tables in the Appendix, Plants Commonly Found on Puget Sound Shorelands.Steepness
The steepness of a slope is often a controlling factor influencing its stability. On steep slopes prone to mass soil movements plants may never become established and large mature trees are scarce. The effect of slope gradient on vegetation establishment is strongly related to soil type, stratigraphy, and hydrology. Many steep slopes remain stable and well-vegetated until some critical factor is altered.Examples:
Stable sites offering good rooting conditions will support densely wooded slopes with great vegetative diversity.
Unstable sites will show obvious slide paths and have a high proportion of species such as alder, willow and wild cherry which are relatively short-lived but readily colonize disturbed areas.Soil types
Soil type and development influence plant growth and vigor, rooting depth, and available moisture.Examples:
Deep, porous soils that have a high humus content are more productive and hold water better than soils that are mostly mineral.
Poor or recently disturbed soils will often be colonized by species such as Scot's broom and Himalaya blackberry, which thrive in poor soils.
Deep, productive soils will support mature, diverse plant communities comprised of conifers, broad-leaved trees, various shrubs, and herbaceous growth.
Shallow or saturated soils may support a wide range of brush species such as salmonberry, gooseberry, thimbleberry, and elderberry, but trees requiring solid rooting such as Douglas-fir may be absent.Hydrology
Hydrology is always a factor to consider. Plants are sensitive to both saturated and droughty soil conditions. Some plants can tolerate wide extremes of soil moisture while others cannot.Examples:
Shore pine can be found on both wet and dry sites, butterfly bush is common on dry sites, and Black cottonwood is an indicator of wet sites.
Aspect, the orientation of a slope face in relation to the sun, influences the vegetation growing on shore sites in several important ways. It determines the amount and duration of sun exposure, temperature, and the severity and type of environmental stresses, especially wind, that plants are subjected to.
A south-facing slope is generally hotter and dryer than a north-facing one. A steep east-facing slope will receive full sun in the morning during summer but be in shade by afternoon. A slope oriented towards the west will be exposed to the sun throughout the afternoon and evening during long summer days. The influence of aspect is complicated by topographic features such as canyons and stream courses, causing complex local microclimates that can support radically different plant communities within a small geographical area.
Microclimate is a word that refers to the existence of localized conditions of shade, wind, air temperature, and humidity that can combine to influence plant occurrence and growth and which can vary from the general conditions existing on a slope. The effects of factors such as steepness, soil type, hydrology, and aspect can be locally modified by microclimate influences such as fog and frost pockets and the movement of cold air down canyons and stream channels.Microhabitat
Microhabitats are created by these microclimate conditions and the presence of localized differences in soil, topography, and hydrology. Microhabitats are places within a larger area that support plants or communities of plants different from those more generally found on a site.
An awareness of these factors will help you to understand and explain the sometimes complex nature of the plant communities seen on Puget Sound shorelands.Environmental stresses
Environmental stresses influence the type of vegetation and its position on a slope. Drought, periods of cold, intense rain, heat, and exposure to wind can reduce plant vigor. Some plants have a broad natural adaptability and can thrive under a wide range of conditions, while others are more limited in the stresses they can withstand. If conditions change slowly over a long period of time, most species can adapt. When natural and human-caused environmental stresses combine to rapidly alter microclimate and habitat characteristics, plant communities change as less-adaptive species weaken and are replaced by plants more able to adjust to new conditions.
Listed below are common conditions to which species found around Puget Sound have adapted.
Many of our common plants are adapted so well to various conditions that they can be found almost anywhere. Pacific madrone, Red alder, willows, oceanspray, and Himalayan blackberry (an invasive, non-native) are a few of these.Site Disturbance
Site disturbance, whether caused by natural processes or human impacts, affects the nature of plant communities and how long they have had to develop and mature. Below, we discuss the causes of site disturbance.
Natural processes contributing to site disturbance include erosion (both surface and mass soil movements), fire, extreme episodes of wind, rain or cold, seismic activity, and unusual tidal/storm events that de-stabilize the toe of slopes.
Human impacts that can cause severe site disturbance include logging, clearing, road building, and grading of shore areas.
The impact of removing mature trees from a site, while not as disruptive as clearing and grading, can severely alter microclimate conditions. Many smaller native trees and shrub species have adapted to the low-light conditions under forest cover. When large trees such as Douglas-fir, Western hemlock, Western red cedar, Sitka spruce, and Grand fir are removed these understory plants suffer from light increases and may die and be replaced by less desirable brush species.
Salal, Evergreen huckleberry, Oregon grape and Pacific yew are all valuable native species that supply wildlife habitat, erosion control benefits and are easily maintained. They are all, to some extent, adapted to flourish under the shade provided by tree canopies.
Species such as Sword fern, Vine maple, snowberry, and Red huckleberry are also valuable native species. They are more adaptive and able to survive environmental modifications.
Many of the shrub and herbaceous plants that thrive in full sun or increased light conditions are less beneficial than those above because they have inferior erosion control abilities, are extremely invasive, and/or create maintenance problems. They respond to increased light by height increases and by rapid spread. The worst of these for view and access management on bluff crests include Himalaya blackberry, English ivy, salmonberry, Devil's club, nettle, oceanspray, and Scot's broom.
Succession is a term used by ecologists to describe the natural development of plant communities over time. Starting with bare soil, certain highly-adaptive plants such as alder, willow, and fireweed will colonize the disturbed soil if nearby seed sources are present. These pioneer species are often short-lived and contribute organic material to the bare soil, and allow various other species, such as Evergreen huckleberry, Oregon grape, Salal, and Western hemlock to become established under their shade.
Factors such as soil type, hydrology, aspect, and local climate all influence the composition of various plant communities and how well they develop. Natural plant succession can require many years to produce a heavily wooded site. Generally, a plant community that is composed of a wide variety of evergreen and deciduous trees and shrubs is more resistant to environmental stresses and erosional processes than a "younger" plant community.
Often, though, plant species from other parts of the world, such as English ivy, Scot's broom, Himalaya blackberry, and Butterfly bush have been introduced and become well-established here. They are termed "non-native" and "exotic" plants and can compete successfully with the native pioneer species that form the first link in the succession towards a stable plant community.
They are called "invasive" when they colonize sites and spread to surrounding areas, often at the expense of native plants. In the case of Himalaya blackberry and English ivy the erosion control capabilities of these plants are inferior to the natives they dispossess. Himalaya blackberry has a deep root system but does not hold or bind soil well. English ivy creates a dense mat that discourages other species growth and establishment. Both of these invasive exotics grow extremely fast and rob the soil of nutrients. Scot's broom offers good erosion control but reduces the establishment of evergreen and hardwood species. Butterfly bush and foxglove, while exotics, do not displace natives and offer wildlife benefits.
Many exotics spread readily by seed or plant parts. They can be inadvertently introduced to a site in loads of structural fill and topsoil Once established they can be very difficult to control and they compete with landscape plantings.
Off-site influences can impact the plants growing on your property and indirectly increase the potential for erosion in various ways. Adjacent clearing can modify the hydrologic and drainage characteristics on your property. Sudden increases or decreases in surface and sub-surface water can subject the vegetation (especially evergreen trees) to environmental stresses that can weaken them. Madrone, our only broad-leaved evergreen tree, can be rapidly killed by even minimal increases in summer soil moisture.
Off-site clearing can also remove wind protection or change wind patterns. It is difficult to generalize, but frequently windthrow or blow-down of nearby trees results.
In some areas salt-laden wind has affected barrier trees (trees between the wind and an inland stand of trees) over many years and they have adapted to the prevailing conditions. They protect the trees and shrubs to leeward (behind them). These barrier trees are often misshapen, broken, and gnarled, but they have developed root systems that have allowed them to withstand many winter storms. If they are removed, the trees to leeward are exposed to stresses they are not adapted to. Windthrow and damage from salt often result.
We have discussed the value of vegetation in minimizing and reducing erosion and described the vegetative clues for diagnosing slope stability. Some of the factors that influence why certain plants grow where they do have been examined and the concept of a constantly changing plant community has been introduced. See if you can use this information to answer the questions posed in the next section.
Copyright © Washington State Department of Ecology. See http://www.ecy.wa.gov/copyright.html.