1. Modifying Site Potential as a Model for Revegetation Design in Riparian Areas
Tom Parker and Sarah Flynn
Geum Environmental Consulting, Inc.
April 1, 2005

2. Disturbance Cycles and Site Potential
Over the past 50 to 100 years, many riparian areas have been converted from periodic high-intensity disturbance cycles (beaver, floods, fire) to short-duration, moderate-intensity disturbance (grazing, farming)
Changes in disturbance cycles have caused sites to support simpler plant communities with quick establishment periods, short life cycles, high light and nutrient tolerances, and few barriers to prevent mechanical damage and browsing/grazing.
As a result, vegetative site potential has shifted

3. Frequent Disturbance results in a simplified system
Frequent Disturbance results in a simplified system. In many ways, an agricultural floodplain is the plant community equivalent of a continuous riffle in a natural stream system.

4. Site Potential (practitioner’s definition)
Vegetative site potential is the range of possible plant communities that can colonize and occupy a site where that site has a particular combination of plant species, light, nutrients, soil texture, organic matter, soil compaction, microtopography, soil micro-organisms, hydrologic processes and other factors.

5. Ecological History, Idaho Example
The same site can support several plant communities. This site has been an herbaceous wetland, a conifer forest, burned, covered by volcanic ash, and submerged under a reservoir.

6. Ecological History, Idaho

7. Restoration as modifying site potential
If site potential is: the range of possible plant communities that can colonize and occupy a site where that site has a particular combination of plant species, light, nutrients, soil texture, organic matter, soil compaction, microtopography, soil micro-organisms, hydrologic processes and other factors.
Restoration actions should: create conditions to support natural processes that will support a desired range of plant communities

8. Carefully and slowly shift the disturbance cycle
To change site potential from one driven by frequent or undesired disturbance, we must first remove the disturbance.
However, if you remove disturbance, you still have a site that is adapted to disturbance.
In addition, restoration actions are often a form of designed disturbance

9. For example, removing grazing disturbance and tilling to prepare soil for seeding and planting can result in strong expression of the seed bank. Here, the seed bank is an important component of site potential.

10. It may be necessary to aggressively suppress invasive plant species.
Invasive plant species like reed canarygrass can occupy most available niches, thus severely narrowing the range of plant communities a site can support

11. Suppress invasive species
Herbicide can effectively suppress invasive species if timing, rate and frequency of application are carefully designed. However, herbicide application is a disturbance that opens ecological niches. Carefully plan how these niches will be filled.

12. Suppress invasive species
Completely replacing substrate with a cardboard/mulch treatment can modify the nutrient regime; increase organic matter; smother invasive plants, rhizomes and seed; and result in substrate that can support a wider range of plant communities.

13. Manage weeds during a site transition period
Modify site potential so it is less favorable to invasives by making the site more complex
Manage weeds during a site transition period
Broad herbicide use first one or two years
Coarse wood amendments to reduce exposed mineral soil and limit available nitrogen
Modify seed bank by inter-seeding native species
Add microtopography to increase the number of available niches
Restore rooting zone hydrology
Add large wood—promote soil microbial processes

14. Monitor site transition
Once the first phase of site transition is complete, monitor vegetative response for another one or two years. Continue to supplement the seed bank with native seed.
Lightly plant islands of one-gallon containerized native shrubs and trees grown from truly site-adapted seed.
Monitoring may result in no need to plant?

15. If native planting is necessary (note that this is far from the first step)
Proper planting requires skilled labor
Manage competition from grasses and weeds
Eliminate browse pressure for four full growing seasons to allow root establishment
Provide shelter from sun, wind, extreme cold
Deep water for first two growing seasons after planting
Use plants grown from a site-adapted seed source

16. What is site-adapted seed?
Consider seed dispersal mechanisms
Wind or water dispersal
Migratory flyways
Animal dispersal

17. Water
While native plants are adapted to the locale, individual plants need time to adapt to a particular site. Deep watering for two growing seasons (if needed based on soil moisture monitoring) may allow the root systems to develop sufficiently to support the plant.

18. Maintain
Plant communities cannot be constructed as a single-entry project. Changing a site to one that is self-sustaining will take years on many sites. In addition, invasive plant species are a perpetual management challenge. Restoration is neither gardening nor landscaping. Rather, it is a continuous process governed by uncertainty.

19. Conceptual transition model to shift site potential within a riparian restoration site
Frequent, moderate disturbance
Low spp richness, short life cycles
High N availability, fast cycling
Weedy, annual seed bank
Flat, uniform topography
Low or simple organics
Uniform hydrology
High, consistent light
Extreme variation in wind and temperature
Few herbivory barriers
Simple soil biology
Mixed, uniform soil texture
Accelerated erosion
Periodic, intense disturbance
High spp richness, complex structure
Slower nutrient cycling
Diverse seed bank
Complex topography
Organics present in multiple forms
Complex hydrology
Variable light and shade
Moderate wind and temperatures
Many herbivory barriers
Complex soil biology
Intact soil horizons
Erosion present, but infrequent and eposodic