Ecological dynamics

Vegetation Dynamics:
Riparian complex ecological site produces about 2000 pounds per acre of biomass annually up to 4.5 feet Understory production is dependent on canopy cover.
Low canopy = high production. High canopy = low production

Ecological Dynamics Overview:

Riparian areas are heavily influenced by fluvial processes. This provisional ecological site concept is composed of a variety of different riverine systems and will require more detailed field investigations to refine the site concepts and likely develop several new sites that are correlated to similar geologic structure and processes, hydrologic regimes, and vegetation characteristics. This ecological site concept captures variety of typical riparian vegetation expressions. The band of riparian vegetation may be broader if part of a larger river system, or narrower if part of a small stream system.

Basic Understanding of Riparian Systems – Abiotic Factors/Primary Disturbance
(from Kendra Moseley, NRCS):

Riparian forests are a complex interaction of many various physical and biologic factors, including function of valley morphology, physical processes, vegetative legacies, and life history strategies. The watershed geomorphology and physical processes form the basis for understanding the spatial extent of the riparian forests, which includes the valley shape, hillslope processes, fluvial processes, soil processes, and hydrologic processes. Soil development within alluvial environments is highly variable. Frequent erosional and depositional disturbances from flooding create a complex mosaic of soil conditions in the active floodplain that fundamentally influences vegetation colonization and establishment. Well-drained soil or recently deposited mineral alluvium may be found adjacent to very poorly drained organic soils in abandoned high-flow channels. This variability in soil conditions is a major factor in maintaining the high plant diversity typical of riparian ecological sites.

The disturbances that drive this ecological site concept are dependent on the type, frequency, predictability, extent, magnitude, and timing of the disturbance. The fluvial processes that are dominant in this riparian ecological site concept include stream power, basal shear stress, channel migration, and sediment deposition. The characteristic vegetation pattern of these low-gradient valleys is maintained by fluvial disturbances and geomorphology. The amount of force exerted on the channel bed and vegetation growing in the active channel and floodplain during a flood is a product of fluid density, gravitational acceleration, flow depth, and water surface slope.

The diagram below outlines the complexity of abiotic factors that play a role in these mostly alluvial river systems and the riparian vegetation expression and dynamics that are associated with them.

Basic understanding of Channel Evolution
(From Stream Visual Assessment, Version 2, December 2009:

Some understanding of stream geomorphology helps our understanding of the ecological dynamics of these fluvial systems.

Channel slope is directly related to topography, geology, sinuosity, bed material and watershed size. Straight stream channels are indicative of strong geologic structure (bedrock) or human control. Braided streams have multiple interwoven channels. Meandering channels are highly variable and sinuous.

The shape of a stream channel changes constantly, im¬perceptibly, or dramatically, depending on the condi¬tion of the stream corridor (channel, riparian area, and flood plain) and how it transports water and materials. Channel condition is a description of the geomorphic stage of the channel as it adjusts its shape relative to its flood plain. Channel adjustments resulting in a dra¬matic drop in streambed elevation (incision or degra¬dation) or excessive deposition of bedload that raises the bed elevation (aggradation) affect the degree of bank shear and often decrease stream channel stabil¬ity. Such channel adjustments can have substantial effects on the condition of streams, adjacent riparian areas, associated habitats, and their biota. For ex¬ample, the greater the incision in a channel, the more it is separated from its flood plain, both physically and ecologically. Conversely, the greater the aggradation, the wider and shallower a stream becomes, which can affect riparian vegetation, surface water temperatures, and stream and riparian habitat features.

Conceptual models of how a channel evolves or ad¬justs over time illustrate the sequence of geomorphic changes in a stream that result from disturbances in the watershed. Such sequences are useful for evalu¬ating trends in channel condition. The stages of the Schumm Channel Evolution Model (CEM), as shown in figure 3, provide a visual orientation of the pattern of streambed adjustment in an incising stream, its gradual detachment from the existing flood plain, and eventual formation of a new flood plain at a lower elevation. A similar model by Simon (1989) is also de¬scribed in the Stream Corridor Restoration Handbook (FISRWG 1998) available in most NRCS field offices.

Evolution of a Stream:

Stage I channels are generally stable and have fre¬quent interaction with their flood plains. The relative stability of the streambed and banks is because the stream and its flood plain are connected, and flooding occurs at regular intervals (Q2). Consequently, the stream’s banks and flood plain are well vegetated. The Stage I channel undergoes initial incision. In Stage II the bed degrades, and banks are stable. During Stage III the bed aggrades, banks are unstable, and the channel goes through the widening phase. Stage IV is the stabilizing phase where the bed continues aggrading, but the banks are stable. Then in Stage V the slow aggrading continues but now banks are stable, and the new floodplain is forming. The new floodplain is lower and narrower than the original floodplain.
 
Especially on larger streams, variations on riparian areas (clumps and strips) are common. Variations are controlled by valley width, sediment type and overbank flooding. Clumps and strips of dominant trees species found on riparian areas can include – black cottonwood, aspen, water birch, white alder and several different willow species. Smaller streams are less diverse than larger streams. Overbank flooding and gravel bars are required for tree regeneration for many riparian trees, especially for black cottonwood.

Ecological Dynamics (from Frank G.):

The riparian woodland sites in MLRAs 007X, 008X, and 009X will vary based on their locations in various watersheds of the region and available moisture and soil texture/depth of the sites. Sites that have sufficient soil texture and depth will have a tall tree component like black cottonwood and/or quaking aspen. Sites that are drier will have a short tree or shrub component like hawthorn, chokecherry or mockorange. Also, the size of the watershed that drains an area and adjacent land cover will influence the amount of water entering the riparian area.
Riparian woodlands are essential to keep streambanks stable, stream temperatures cool, and buffer potential impacts from adjacent land uses. Riparian woodlands provide valuable wildlife habitat for a variety of species. Native riparian woodlands can recover from low intensity fires through tree/shrub root sprouting or seed regeneration. Serious impacts can occur when these areas are continually overgrazed or removed/reduced for another land use.

Fire Ecology:
Forested riparian areas (Riparian Woodlands) are unique ecosystems which provide critical habitat for many species of terrestrial and aquatic species. Forested riparian zones occur adjacent to two broadly associated upland ecosystems. In dryer areas, the adjacent upland type was historically steppe or shrub/steppe, and in areas of increasing rainfall, the adjacent upland was dominated by coniferous forests. In large scale watersheds with appreciable climatic gradients, the riparian zone passes through and connects these two upland types.
Riparian areas are subject to a number of ecologic disturbance influences, including wildfire. Wildfire frequency (the historic intervals between various types of fires), severity (the impacts of any given fire episode), and the type (surface, crown, mixed) of fire occurring within these riparian zones was highly variable.
Riparian ecosystems that were adjacent to prairie vegetation experienced a relatively frequent surface fire return interval. Fire typically entered into the riparian zone from fires originating in the upland grass dominated vegetation. Black cottonwood hardwoods were likely the only true tree species, growing along with adapted understory shrubs and grass species.
Riparian areas adjacent to upland conifer forests have a wider expression of tree and understory species. These areas would typically burn in the same upland fire event, and experience the impacts similar to the surrounding upland forests, especially when upland the fire episode was more severe.
Fire impacts, and natural adaptation to common riparian tree, shrub and grass species:
Black cottonwood is easily killed by fire, but coppice sprouting is common. Fire can improve seedling establishment by increasing light penetration and exposing mineral soil to allow seedling establishment if moisture is available. It can endure on site as well as invade following fire.
Ponderosa pine is resistant to fire by the development of thick, platy bark which begins to form at a young age. It naturally prunes the lower bole, decreasing the likelihood of crown ignition, and the open nature of the needles resist damage when upper crow fires do occur. It is also long lived, and will regenerate on open, sunny areas which are often produced by surface fire.
Quaking aspen regenerates by clonal regeneration following light to moderately severe fire.
Various species of willows, sedges and grasses regrow following all but severe fires, from root crowns or underground rhizomes.
Lupine, manzanita and Ceanothus produce hard shelled seeds that require fire scarification to germinate and become established.
Many other plant species produce light weight seeds which easily travel on wind currents to disturbed areas.
In the past century, in what is referred to as the period of post-European settlement, riparian forests have undergone many human caused changes. Logging up to the edge of the stream, conversion to agriculture, fire exclusion, forest fragmentation and especially unregulated livestock grazing have all contributed to the degradation and proper function of riparian woodlands. These factors have all contributed to changes from the historic fire regimes and impacts of native riparian woodland forests.