CHOOSING THE RIGHT SPECIES
Choosing the Right Species
There are many possible species to plant in streamside areas, but not every species is well suited to every site. For clues about species that will do well on your site, look at what is currently growing there, or just upstream or downstream your location. This may provide a partial list of suitable species, though other ecologically suitable species may not be present. For example, many lowland riparian zones in Jackson and Josephine County currently lack conifers, but conifer species such as ponderosa pine may be appropriate for planting there. Undisturbed sites where the full range of riparian species can be found (i.e., “reference sites”) are lacking, but some older patches of streamside forest may provide clues about the types of species that grow locally.
Local practitioners with watershed councils and public agencies may have experience planting trees in riparian areas and suggestions about species selection learned from trial and error.
Another good source of information is the county soils survey. Soils surveys provide information about soil drainage, an important factor for some species. Also, the surveys list typical tree and shrub species growing on a particular soil type. These lists don’t include all the species that could potentially grow on that soil type, but they’re a starting point. For more information about soils, visit your local NRCS (Natural Resources Conservation Service) office or the web at: http://www.or.nrcs.gov/pnw_soil/or_data.html.
In selecting species to plant, consider their tolerance to key environmental factors such as drought, shade, flooding, and poor soil drainage:
Drought. Moisture stress is often the limiting factor in seedling survival in southwest Oregon due to the hot, dry climate. Even sites close to streams may have sandy or rocky soils with low moisture-holding capacity that dry out in the summer. If you are not planting where the seedling roots can access the mid-summer moisture level, choose a species that is drought tolerant.
Shade. Many riparian tree species are intolerant of shade, and are thus not suitable for planting in the understory of other trees. Examples are willows, cottonwood, and alder.
Flooding and poor soil drainage. Some species are tolerant of seasonal flooding, while others cannot survive even short periods of inundation. How close are you planting to the stream? Is it likely the site will be flooded in an “average” winter? If so, make sure to plant flood tolerant species. Also, be aware of areas with poor soil drainage, which species such as Douglas-fir and incense cedar cannot tolerate. Poorly drained soils are a feature of some soil types. Consult your soil survey for more information.
Other factors to consider in species selection
- Consider your objectives. Which is most important – shade, erosion control, habitat, etc? Some species are better than others at providing these benefits.
- Coniferous species are often a priority for riparian plantings due to their high value for functions of shade and large woody debris. However, most hardwood tree species also will contribute to important riparian and may be better suited to the site. Generally, plant flood-tolerant hardwoods closer to the stream and drought-tolerant conifers further away.
- Trees should be emphasized in most riparian plantings because they are the key to enhancing important riparian functions such as shade, bank stability, and inputs of large woody debris. Once trees are established, favorable conditions for other vegetation are often present. However, planting shrubs along with trees can improve habitat values, diversity, and aesthetics.
- Consider the varying growth rates and shade tolerances of the species planted. Don’t plant a fast growing species next to a slower growing species that is less tolerant of shade, for example, such as alder next to pine. Species mixtures can be complex to manage and there is little research or practical experience to guide us. Planting single-species clusters is probably safest.