Step 2: Assess your site conditions
Roadside revegetation typically involves working with disturbed systems. The challenge is to understand and address limiting factors and restore the system to an optimal condition for long term sustainability.
The following "site conditions" are representative of key limiting factors. The list is instructional only and not all inclusive:
Topsoil is the surface layer of soil characterized by high organic matter, abundant roots, high nutrients, good infiltration rates, and high water-holding properties. It's the layer, or soil horizon, where the greatest biological activity occurs. The depth and quality of topsoil directly affects site productivity - the deeper the topsoil, the greater plants grow. Any activity, therefore, that reduces the depth or quality of topsoil will reduce the productivity of a plant community.
Road building activities often remove topsoil or mix topsoil into less fertile subsoil. When this occurs, revegetation can be extremely difficult. Minimizing the construction footprint will reduce the amount of topsoil disturbed, but when this is not possible, removing and stockpiling topsoil prior to construction, and placing it back on the site after disturbance, is a common, effective practice. When topsoil is not available, applying compost and tilling it into the soil, can begin the process of topsoil development.
A top soil assessment of less than or greater than 12" is typical.
A healthy soil is composed of interconnecting air spaces (or pores) that allow roots, air, and water to move through unimpeded. Soil compaction is the loss of that air space. When it occurs, soils are less able to transmit water which increases runoff and soil erosion during rainstorms. Roots have greater difficulty penetrating compacted soil which reduces the productivity of plant communities.
Compaction can occur when any type of weight is placed on the soil. On road building projects compaction occurs by heavy equipment driving over the soil, but compaction is not limited to these operations: even animals and humans can cause soil compaction. Reducing the construction footprint or concentrating traffic in specified areas (paths) is the simplest means of preventing compaction of undisturbed soils. If compaction occurs, tillage operations that loosen the soil, as well as the incorporation of organic matter, can help recreate air space.
A soil compaction assessment of compacted or not compacted is typical.
Unlike soil, rock lacks air space (or porosity). This means that rock cannot hold water and nutrients or be a habitat for roots and soil organisms, all factors which are important for plant growth. The quantity of rock present in a soil directly reduces the amount of water and nutrients a soil can hold. Soils with high rock content have low productivity because there is less moisture, nutrients, and biological activity which are critical for the productivity of a plant community. These soils are often very difficult to revegetate.
One method of mitigating rocky soils is to remove large rock with screens. Another method is to cover rocky soils with a layer of topsoil or soil low in rock fragments. In addition, the addition of compost can offset the effects of high rock content by increasing the water holding capacity and nutrient content of the soil.
A rockiness assessment of less than 35%, 35-50%, or greater than 50% is typical.
Rooting depth is the distance from the surface of the soil to the lower reaches roots can penetrate. It is important in revegetation because the greater the volume of soil that roots can access, the greater the amount of water and nutrients available to the plant community. Sites with shallow rooting depth might support only annual grasses, whereas deeper rooting depths will support shrubs and trees. Rooting depth is controlled by restrictive layers, such as bedrock, compacted soils, saturated horizons, or toxic horizons (e.g. extreme high or low pH).
Road building activities often create sites with low rooting depths by removing or compacting the soil. Sites with low rooting depths can be mitigated by importing soil and strategically placing it at the required thicknesses for plant growth. Compacted soils can be tilled to increase rooting depth. Where no options are available, selecting the appropriate species adapted to the rooting depth is important.
Soils are composed of small minerals ranging in sizes and shapes. Most of these particles are too small to observe visually, nevertheless the interaction of these particles control how soils form air spaces (pores), store water, or release nutrients; all vital to the productivity of a plant community. Particle size also determines how easily soils erode and the type of water quality problems that might result.
The composition of particles characterizing a soil is referred to as soil texture. There are several soil classification systems but for general discussion, soil texture can be grouped as: fine (clays), medium (silts and loams) and coarse (sands). Fine textured soils tend to hold high amounts of moisture and nutrients, while coarse textured soils hold less water and dry out quickly. Coarse textured soils are typically low in nutrients. Compost can be added to coarse textured soils to increase nutrient levels and water holding capacity.
When animal species become a problem to plant establishment, they are referred to as pests. Large mammals are often the most obvious type of pest affecting shrubs and trees. Deer, elk, and cattle browse on new foliage or trample newly planted seedlings. Smaller mammals such as gophers and beavers, feed on the roots and bark of young seedling. Less obvious pests are insects and diseases, which can affect the health and survival of all plant species.
The mitigating measures depend on the type of pest encountered. An effective way of controlling large mammals is by protecting plants with fencing or netting. Insects and diseases are best controlled by using healthy planting stock (unhealthy nursery-grown seedlings are more susceptible to insects and diseases) and planting the proper species adapted to the environment of the site (species that are not suited or adapted to the site are more susceptible to insects and disease).
Pests can include deer, gophers, beaver, cattle.
The angle of the slope is referred to as slope gradient. Road construction quantifies slope gradient as the rise (vertical distance) over run (horizontal distance). Slope gradient is an important factor in revegetation because it can affect how stable the slopes are going to be. On steep slopes, the soil surface will move by the forces of wind, water, and gravity making it difficult for seeds to remain in place long enough to germinate and establish into plants. These sites will often take years to revegetation.
Mitigating measures for steep slope gradients include: 1) reducing the steepness of the slope and 2) protecting the soil surface from erosion. Reducing slope steepness can be accomplished by making longer cut and fill slopes or adding retaining walls at the base of the slopes. Protecting the soil surface from erosion can be accomplished with mulches and erosion mats.
A slope gradient assessment of 1:1, 1:2, or 1:5 is typical.
Aspect is the direction a slope is facing. It is measured by facing the fall line (the direction a ball would roll) and taking a compass bearing downslope. Aspect strongly influences the microclimate of a site. It affects the maximum temperatures in the summer and minimum temperatures in the winter. The importance of aspect on species selection is evident in arid and semi-arid environments where it is common to find a different suite of species on southern and northern aspects of the same road segment.
Selecting the appropriate species mix that matches the aspect can improve revegetation. South and west aspects might require a suite of species that are more drought tolerant, while seed mixes on north and east aspects might be composed of species adapted to cooler conditions. Seedlings planted on southern aspects might require protection from hot surface soil conditions occurring during the summer months. Installing shade cards or planting behind obstacles that shade the plants can increase survival.
A slope gradient assessment of south/west or north/east is typical.
