The structural and compositional diversity of riparian forests is an outcome of fluvial inundation, wind-throw, variable light conditions, irregular substrates and other influences. Important structural features include large diameter trees, standing dead wood (snags), coarse woody debris, multilayered canopies and diverse understory vegetation. The structural complexity offers important benefits to in-stream habitat. Riparian forests regulate water temperature and surface runoff and provide a source of large woody debris, organic material, and nutrients for the aquatic environment. Riparian areas also provide a variety of habitat niches for aquatic and terrestrial species. Since European arrival in North America, old-growth riparian forests have been significantly reduced, due to logging and settlement. Logging has simplified complex habitats, altering riparian vegetation and disrupting the natural processes that sustain aquatic and terrestrial ecosystems.
Riparian restoration techniques modify riparian areas with intent of improving the aquatic and terrestrial habitat conditions. Restoration is not intended to return a system to a pre-altered state or fixed condition but to help restore the structure, function, and ecological processes of a system. Habitat restoration is at the foundation of many land management strategies and endangered species recovery initiatives.
Riparian polygons are classified in a five-class system, called Riparian Vegetation Types (RVT). This system is based on the four most common stand conditions that would benefit from riparian restoration, while the fifth category is for stands that do not require treatment. The classification system is used by riparian restoration specialists to define a restoration prescription based on the observed site level characteristics and the desired future site conditions.
Riparian Vegetation Types (RVTs), 1 - 5
- RVT 1: Brush dominated, where conifer stocking is low due to competition from brush, insects, frost or disease.
- RVT 2: Conifer-dominated site where high stocking densities have significantly reduced conifer diameter and crown development. They are usually pure conifer, but can be mixed stands of conifer and deciduous.
- RVT 3: Deciduous-dominated site with an understory of conifers. The overstory usually consists of alder, but may have other deciduous species such as cottonwood or bigleaf maple. In most situations, the deciduous trees form a pronounced overstory that has the effect of suppressing the growth and survival of conifers.
- RVT 4: Similar stands to RVT 3, but contain sparse (<100 stems per hectare) to nil conifers in the understory.
- RVT 5: Stands are functional riparian sites or stands that contain all of the characteristics necessary to achieve a desired future condition for the site. They may be (i) mature forests or old-growth forests that are already in the desired condition, or (ii) young forests—even pole-saplings—that are on their way to the desired future condition. Evaluation of RVT 5’s is an important step in riparian assessment. These stands provide ecological rationale for treatment recommendations and help to determine the “desired future condition or target stand” for an impaired site.