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

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

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.

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.

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.

Ecological History, Idaho

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

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

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.

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

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.

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.

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

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?

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

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

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.

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.

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