ROADSIDE REVEGETATION

An Integrated Approach to Establishing Native Plants and Pollinator Habitat

3.12 Select Site Improvement Treatments

In this stage of planning, the treatments that will improve the site for plant growth (e.g. topsoil, compost) are selected. The selection of treatments begins with identifying the factors that limit plant growth. As the designer identifies limitations specific to the project site, a list of possible treatments that will mitigate or reduce the effects of the limiting factor is developed. Specific treatments to mitigate each limiting factor are presented in Section 3.8. The case study presented in Inset 3-6 shows how a list of mitigating measures can be developed for specific limiting factors.

In narrowing down the possible treatments that will encourage plant success, the designer considers all possible resources on the project site that can be used in lieu of purchasing and transporting materials from offsite sources (Section 3.10). For example, in reviewing the plans of a road reconstruction project, it is found that a portion of the road is being realigned through a wooded area. For this section, the designer lists the potential site resources to include topsoil, large wood, tree branches and foliage, duff and litter. These resources can be used in a variety of ways to improve growing conditions and pollinator habitat.

At this point, the vegetation management strategy is revisited to ensure that the mitigating measures being considered in this process are compatible with road maintenance objectives (Section 3.11.2) or if they could present problems for long-term maintenance and if so, what modifications to the treatments can be made. The final selection of treatments is based on many factors, including project funding, project objectives, experience of the designer or contractor, and availability of resources and equipment. A Limiting Factor and Site Resource tables are available in this Planning workbook.

Inset 3-6 | Case Study—Defining limiting factors and selecting mitigating measures

Site Inventory—During the planning stage of a revegetation project in central Oregon, the soils assessment (Section 3.6.2) highlighted several limiting factors that would negatively affect plant growth.

Limiting Factors—From the list of limiting factors outlined in Figure 3-11, factors affecting plant growth at this site were narrowed down to low precipitation, low water storage, high water loss, and low nutrients.

Mitigating Measures—For each of these factors, a list of possible mitigating measures (described for each limiting factor in Section 3.8) was developed.

Site Resources—A review of the site resources was made at this point to determine if there were any resources on the site that could be used in developing mitigating measures (Section 3.10). It was found that there were several areas where weed-free, high quality topsoil could be salvaged and reapplied. In addition, the project would produce a large amount of slash from cleared shrubs and trees that could be processed into shredded wood and used as a mulch. Other resources available were a local municipal waste treatment plant had Class A biosolids available for application in lieu of bringing in fertilizer, loam borrow from a source of pumice deposit, and an excavator that would be available for subsoiling.

Maintenance—The potential effects of these treatments on long-term maintenance was then considered (Section 3.11). It was determined that deep tillage was not compatible with safety in Zones 1 and 2 but would be done in Zone 3, otherwise there were no foreseen maintenance problems.

Treatment Selection—It was decided to salvage topsoil; however, the amount of topsoil would not cover all the project needs. It was determined that a manufactured topsoil would be created by using shredded wood processed the slash from road right-of-way clearing, loam borrow from excavated pumice, and biosolids from the local municipal waste treatment plant. To increase infiltration and rooting depth, it was decided that once the topsoil and manufactured topsoil was applied that it would be subsoiled only in subsoil Zone 3 but not in Zones 1 and 2. At planting, lupine would be included in the seed mix for additional nitrogen and inoculum would be applied to the seed mix. Because there would be additional shredded wood, it would be blown over the seed as a mulch.

Critical plant factors

Parameters

Limiting

Possible mitigating measures

Water input

Precipitation



Irrigate, water harvesting

Interception

Infiltration



Tillage, organic matter, mulch, avoid compaction

Water storage and accessibility

Soil texture



Organic matter

Rock fragments

Soil structure



Tillage, organic matter, avoid compaction

Rooting depth



Tillage, topsoil, planting islands/pockets

Mycorrhizal fungi



Topsoil, inoculums

Water loss

Wind

Aspect

Competing vegetation

Soil cover



Mulch

Critical plant factors

Parameters

Limiting

Possible mitigating measures

Nutrient cycling

Topsoil



Topsoil, planting islands

Site organic matter



Shredded wood, compost, litter, wood

Nitrogen and carbon



Topsoil, compost, N-fixing plants, fertilizers

Nutrients



Topsoil, compost, fertilizers, biosolids

pH and salts

Surface stability

Rainfall

Wind

Freeze/Thaw



Tillage, mulch

Soil cover

Surface strength

Infiltration

Slope gradient

Surface roughness

Slope length

Slope stability

Permeability

Restrictive layer

Water input

Slope length

Slope gradient

Soil strength