Site Evaluation for Habitat Restoration Plant Selection

by Julie Whitacre, Fourth Corner Nurseries

While making decisions about which plant species are appropriate for restoration projects, several physical characteristics of the project site should be considered. Each plant species has preferences for physical conditions; some plants are very widely adaptable to moisture regimes, soil conditions, and light levels, and others are fairly demanding in their habitat requirements. Charts detailing the habitat preferences of northwest native plants are available in the “Resources” section of our website. This article will focus on riparian projects, with examples of species common in western Washington, Oregon and British Columbia; similar steps, however, can be adapted to other habitat types.

Most riparian projects focus on trees and shrubs, but I hope people remember that native grasses, sedges, bulrushes, and all kinds of herbaceous plants are also part of a functional system. They cover bare soil, potentially slowing germination of non-native grass and other weed seeds, they provide food and cover for wildlife, and they control surface erosion. Technical difficulties exist with planting forbs into weedy situations, (especially pasture grass or clover), confusing the follow-up maintenance needs, but projects starting with bare ground would definitely benefit from the addition of native herbaceous plants. To restore structure and function of riparian areas, I hope people will consider herbaceous plants as part of the habitat we’re trying to recreate, especially in projects with bare ground.

THINGS TO CONSIDER WHILE CHOOSING NATIVE TREES AND SHRUBS FOR RESTORATION PROJECTS:

Choose a reference site. This is a local area, similar habitat (i.e. riparian, wetland, upland forest, meadow, shoreline, estuary, etc.) with native vegetation. Historical documentation may be helpful in areas without surviving native plant communities. The species growing in your immediate area are the ones most likely to be adapted to your local climate, hydrology, and soil conditions. Realize that local plant communities have evolved over time and may be difficult to establish in a new location. Shade-tolerant trees and shrubs present in upstream communities may have a hard time on your site unless you have great soil, supplemental watering, and no weed competition. If you choose a reference site with climax vegetation, consider whether you’ve planned for enough maintenance to establish climax species; otherwise choose vigorous pioneer species.

Document existing vegetation on the project site. The plants you choose to plant will have to compete with the existing vegetation for water, nutrients, space, and sunlight. Some plants fare better than others with root competition in weedy situations. Decide how to prepare the site for planting and maintain after planting. The less site prep and maintenance planned, the hardier the species you’ll have to plant to favor survival. If you can only afford the bare minimum of maintenance, choose vigorous pioneer species like willows, cottonwood, and alders. Many shrubs can also compete reasonably well with grass, such as snowberry, spiraea, and wild roses. Growth will definitely be slower in weedy situations than on bare ground – there is a reason people rototill and weed their vegetable gardens. All plants need space to grow optimally, but the aforementioned species tend to compete reasonably well. Western red cedar, hemlock, maples, crabapples, and filbert need more attention to become established.

If native plants are already present on the site, determine whether any are on the sensitive species list, and, if so, plan site preparation activities so as not to impact sensitive populations.

Look at hydrology in all seasons. Soil moisture is affected both by water levels and soil type. Some of the project site may be upland, in well-drained soils that do not experience standing water at any time. Some species for this habitat are Douglas fir, western hemlock, scouler willow, ocean spray, red flowering currant, snowberry, nootka rose, and Oregon grape.

Some sites will be saturated or even flooded to various degrees in the winter and spring, but quite dry in late summer. Plants for this regime include black cottonwood, red alder, crabapple, hawthorn, willows, red twig dogwood, ninebark, twinberry, Indian plum, peafruit rose, cascara, gooseberries, and red elderberry. Western red cedar and Sitka spruce tolerate winter saturation, but not a great deal of standing water.

Sites that are saturated all year might support willows, red-twig dogwood, twinberry, cascara, cedar, spruce, hardhack spiraea, Oregon ash, ninebark, and crabapple.

Determine sun or shade levels. Nearly any species will grow well in full sun with proper soil conditions and maintenance. Some, like western red cedar and vine maple, may need extra care such as watering and weeding in full sun, although sunlight itself does not inhibit their growth. In fact, almost all plants grow more optimally in full sun, given proper water and soil conditions without competing vegetation like grasses.

Many species are tolerant of partial shade, but may grow slowly or not flower or fruit optimally. Western red cedar, western hemlock, grand fir, salmonberry, vine maple, red twig dogwood, twinberry, beaked hazelnut, Indian plum, gooseberries, and snowberries are tolerant of partial shade. Interplanting among shrubs such as salmonberry and thimbleberry will require some brush removal so that small plants get partial sunlight while they are becoming established. Himalayan blackberries should be removed or killed entirely before planting, as native seedlings will not out-grow them.

Determine soil type(s). Soil type is also very important in determining which species will thrive, and also the condition of the soil, compacted, crusted, or loose. Maps provided by the county provide information of soil series, which is then correlated to all kinds of information on drainage, erosion potential, slope stability, and more. Check with your local county Conservation District for soil survey information. It’s also good to walk around the site with a shovel and dig a few holes. Sandy, rocky soil will support different plant species than peat-based soil. Those are the extremes, gravel or peat, where you should use the most care in plant choice. Many species will thrive between these extremes, from silt to rich loam, with plant survival here more a function of compaction or friability. It will be difficult to get plants established in any soil that’s compacted.

Sandy/gravelly soils are very fast-draining and droughty. Even sites that are flooded in the winter can be bone dry in the summer. Sandy soils often support alders, willows, spruce, shore pine, cottonwood, cascara, ocean spray, roses, twinberry, hawthorn, and dogwood. A silty clay soil has much finer particles, which hold water and nutrients longer. Depending on site preparation, a silty soil can be really dense and hard-packed, or very rich and nourishing if you plow or subsoil. In hard-packed silty-clay choose vigorous growing species, such as willows and alders. Almost any plant will love soil that’s been worked and aerated.

Organic soils can be either rich and loamy forest duff, or peat-based, which will affect both water and nutrient retention and pH. Indian plum, filbert, cascara, birch, maples, crabapples, and cedars love to grow in rich organic soil, along with most other plant species. Peat soils tend to be wet all year, even when the water table drops. On peat soils you can find hardhack spiraea, twinberry, cascara, red twig dogwood, and Oregon ash. Shore pine and spruce will live on peat but be stunted in growth.

Establish height requirements or restrictions. Another aspect to consider is the mature height of the plants. Of course you’ll want plants large enough to provide shade over the water, and that depends on the width of the river or stream. We often envision grand cottonwoods, alders, big leaf maples, and huge cedar and spruce along our rivers, which is excellent in most cases, but sometimes not appropriate. Many of the streams we work on are narrow enough to be completely shaded by shrubby willows and smaller stature trees like crabapple, cascara, sitka alder, bitter cherry or hawthorn. If you’re in a power line right-of-way, in a view corridor, or if there’s an agricultural field next to the project and the farmer is worried about shade reducing the crop production, use smaller plants. These are more appropriate in some locations than huge trees.

Consider wildlife value of plants. For salmon, you’re looking for plants that shade the water, diversify stream flow with in-bank root structures or large woody debris potential, and support bugs (willows tend to have lots of aphids, which fall into the water for salmon food, also feeding many bird species). You can also choose plants that provide a diversity of food sources (nectar, berries, seeds and nuts, browse) for upland animals during all different seasons. Amphibians, many species of waterfowl, young salmon and other fishes use many species of herbaceous plants along stream and lake margins for food, sanctuary from predators, and nesting/egg laying habitat.

Consider invasiveness of plants. Plants with invasive root systems or seeding habits may not be welcome by neighboring landowners. Wild roses, snowberry, salmonberry, thimbleberry, and hardhack spiraea tend to run rampant once established.

Consider disturbance regime. Winter flooding, beaver activity, deer and elk browsing, dredging, weed-eater accidents and vandalism are common phenomena on restoration sites. Plant appropriately tolerant or thorny species and plan for protection devices. In the case of dredging, leave access for equipment, or plant snowberries, spiraea, roses or salmonberry, which, once established, sprout back after being driven over.

Consider the availability of plant material and timing of availability. Call us at Fourth Corner Nurseries for your native plant needs (Julie Whitacre, 360-592-2250). We have a standing inventory of over 3 million native plants of 300 species as well as contract growing capabilities. Bare root trees and shrubs are available from December through mid-April, and wetland emergents are available during most months of the year. Container plants are available all year.

The genetic provenance of the plant material often impacts survival, with local genetic stock being better adapted to local pathogens and weather patterns.

It is also a good idea to know where your plants are coming from, i.e. whether they’re nursery-propagated, ethically salvaged, or wild-collected and thus negatively impacting our remaining wild plant communities. This is not just a plug for us; there are other great nurseries around, and many who conscientiously salvage before development activities, but collecting of native plants from wild habitats can be extremely damaging if done carelessly.