Vegetation Components of Riparian Health

Vegetation Components of Riparian Health:


Riparian Vegetation Age-Class and its Role in Health

The Role of Age Class in Riparian Health. For a riparian-wetland area to recover or maintain itself, it has to have more than one age class of riparian-wetland plants. Most riparian areas will recover or maintain themselves with two age classes, as long as one of the age classes is young (for recruitment), and the other is middle-aged (for replacement). Older, mature age classes usually take care of themselves, as they are well-attached to existing water tables. Older age classes can usually persist even with degraded conditions, so they are not a good indicator of a healthy riparian area.

Age Class Distribution Relationships. Expanding populations generally have a pyramid shape of age distribution, with many young plants forming a wide base, fewer middle-aged, and very few old at the top. Stable populations are more bullet shaped, with somewhat equal young and middle-aged groups forming the base and middle, and then gradually diminishing to the oldest individuals. Diminishing populations are more urn shaped, with a narrow base of young, widening toward the older age classes, and then sharply narrowing with the oldest individuals. An indicator of diminishing populations is low proportions or missing classes of young and/or middle aged individuals.

Plant age can be difficult to establish, especially in the Southwest. The USDA Forest Service uses age class in reference to woody species only and establishes a procedure based on number of stems and proportion of live stems for determination of age.

For herbaceous species such as sedges, the term age-class distribution can be somewhat misleading, but the intent is to identify indicators of expanding, stable, and diminishing populations through recruitment and reproduction. While often hard to observe, seedlings of perennial plants indicate recruitment and young sedges colonizing a fresh sediment deposit may be seen as a line of plants tapering in size toward the youngest emerging plants.

Warning Signs. Some warning signs that the age class distribution is out of balance, which may be indicative of declining health or unraveling of riparian areas include:

  • A lack of seedlings and saplings of woody species. Riparian areas that require woody vegetation should have evidence of new and young plants, especially in places where they normally establish such as point bars.
  • Woody plants that browsing keeps from escaping. When willows or other woody plants cannot gain sufficient leader growth to get tall enough to be beyond reach of browing animals, they may not recruit new reproducing individuals into the population.
  • Scattered, individual stems of herbaceous species such as sedges and rushes. Herbaceous species help protect the riparian area by forming dense stands with even denser root masses. In order to be effective, these species should be forming dense mats.

In those few channels where vegetation doesn’t contribute to bank stability, the diversity of age-class of riparian species doesn’t need to be evaluated.

References


Species Diversity

Why Species Diversity is Important. For a riparian area to maintain itself, or to recover, a diverse composition of vegetative species is required. Although all potential plants won’t always be present in a riparian area, it is important that at least two species of the required functional groups are present. The presence of only one species makes a site vulnerable to disease or extreme climate events, which may result in degradation of an area. Composition needs to also be diverse enough to accommodate substantial shifts in the water table or zone of saturation that may occur with drought cycles.

Warning Sign. The presence of only one species. A lack of diversity is a sign that riparian health may be declining. There are some areas that canonly support one species, but they are uncommon and usually limited as a result of a unique soil property, vegetative characteristics, or water regime.

References


The Relationship Between Vegetation and Soil Moisture Characteristics

Vegetation can be an indicator of a water table. The presence of riparian-wetland vegetation can be one way of looking for evidence that the water table level is being maintained or is moving toward its potential extent. The maintenance or recovery of an existing water table is vital to the maintenance or recover of a riparian wetland area.

Categories of Riparian Plants. Riparian-wetland species are divided into categories relative to the likelihood of their occurrence in wetlands or non-wetlands. These categories are:

  • Obligate Wetland. Occurs almost always (estimated probability 99%) under natural conditions in wetlands.
  • Facultative Wetland (FACW) -- Usually occurs in wetlands (estimated probability 67%-99%), but occasionally found in non-wetlands.
  • Facultative (FAC)-- Equally likely to occur in wetlands or non-wetlands (estimated probability 34%-66%).
  • Obligate Upland (UPL)-- Occurs in wetlands in another region, but occurs almost always (estimated probability 99%) under natural conditions in non-wetlands in the regions specified. If a species does not occur in wetlands in any region, it is not on the National List.
  • Facultative Upland (FACU)-- Usually occurs in non-wetlands (estimated probability 67%-99%), but occasionally found on wetlands (estimated probability 1%-33%).

Definitions from USDA Plants Database

Plants that occur in wetlands are hydrophytes, and they have to be in contact with the water table, which is why they can be used as indicators of soil moisture characteristics.

Warning Sign. The site is dominated by facultative or obligate upland plants which could indicate declining health or “unraveling” of the riparian area. These types of plants don’t have the root mass needed to hold streambanks and dissipate energy, as well as maintain the water table.

Some intermittent systems, depending on duration of flow, could be somewhat different, as their potential may be facultative plants.

References


Root Masses and Bank Stability

Having the Right Roots. Vegetation is important in slowing flow velocity, stabilizing streambanks, and reducing erosion. Streambanks dominated by vegetation without extensive root masses are undercut during high flow events and collapse. This collapse results in a change in the active channel’s width/depth ratio, gradient, and sinuosity, which reduces a riparian-wetland area’s ability to dissipate energy. The best soil stabilizers and streambank holders are woody species such as willows and cottonwoods, and herbaceous species such as sedges, and rushes. The extensive root systems of these species are especially effective in the development of overhanging banks, which provide habitat for fish and other aquatic organisms. These types of plants are known as Obligate Wetland of Facultative Wetland plants.

Annual herbaceous species and those species that indicate uplands generally lack sufficiently dense, deep root systems to provide much protection. These species are known as Facultative Upland or Upland plants.

When Vegetation Doesn't Play a Role in Stability. There are exceptions where riparian vegetation with root masses capable of withstanding high flow events is not required. These include high gradient, bedrock, or boulder/cobble stream types. In these systems, vegetation contributes little, if any, to bank stability.

Warning Signs: When vegetation with shallow root systems dominates an area, such as the Kentucky bluegrass dominated system above, the riparian area is incapable of withstanding high flow events and maintaining the stability of the banks. Some warning signs that the vegetation present lacks root masses capable of holding banks, which may be indicative of declining health or “unraveling” of riparian areas include:

  • Undercut BanksStreambanks that are continually undercutting and shearing off indicate the plants present don’t have the root masses needed.
  • Presence of upland plants in the riparian areaSpecies such as Kentucky bluegrass, redtop, blue grama, (most grasses) and sagebrush, do not have the root masses capable of withstanding high flow events. If these plants dominate plant communities along the streambank, it’s an indicator that the stream is in need of better vegetation.

References


Plant Vigor

The Role of Vigor in Riparian Health. Looking at the vigor of riparian plant species will help to ascertain if plants are healthy and robust or are weakened or stressed and leaving the area. What is happening aboveground is a reflection of the condition belowground and the ability for riparian-wetland species to hold an area together. As riparian vegetation weakens or leaves an area, the area is subject to degradation.

Determining Vigor, or the Lack of Vigor. Vigor is difficult to quantify, possibly because the relative health of plants within a community is expressed in many morphological and physiological forms. It is helpful when evaluating vigor to separate woody plants and herbaceous plants. The reproductive indicators explained in Riparian Health - Understanding Riparian Vegetation Age-Class and its Role in Health, as well as plant size, leaf area and size, seed production, and root growth are all associated with relative plant health or vigor. Reduced height or reduced leaf area (production) and signs of stress, such as chlorosis, have traditionally been used as indicators of reduced vigor in herbaceous species. Growth form, leader length, and the amount of dead or dying limbs are indicators of vigor for shrubs and trees.

Warning Signs. Some warning signs that plant vigor is low or in decline, which may indicate declining health or “unraveling” of riparian areas include:

  • High amounts of dead limbs in woody species. A loss of vigor and the subsequent loss of a plant altogether, is a sign that vigor in the plant is declining. This could be a sign of old age.
  • Willow leaves turning yellow during the growing season. This decrease in vigor usually occurs as a result of water being removed or added to a system, which stresses the plants. This can also indicate a disease or soil nutrient problem or climate factors.
  • Herbaceous species such as sedges occurring as isolated plants or broken clumps. Sedges exhibiting high vigor in their systems usually form dense communities which form mats along riparian areas. Low vigor may be exhibited by an inability to form these communities.
  • Narrow leaves of sedges. Some sedges, such as Nebraska sedge tend to have wide leaves (~ thumb width) just up from the base when vigorous. Narrow leaves (~ pencil width) indicate stress from lack of moisture, excess grazing pressure or some other cause.

References


Is Adequate Vegetation Present?

The Connection Between Stream Energies and Vegetation. Streambank erosion is a physical process that occurs along virtually all natural channels. Not only is it a normal part of channel evolution and meander migration, but it is also essential for creating and maintaining a variety of aquatic and riparian habitats. When bank erosion becomes excessive, it can destroy significant channel and floodplain habitats. This occurs as these areas are excavated and sometimes buried under massive amounts of sediment. The best protection against excessive erosion is the preservation of adequate, desirable vegetative cover to dissipate the erosive forces acting upon the channel banks during periods of high stream flow.

Bank erosion occurs when the eroding force (shear stress) of water moving along the bank exceeds those forces in the bank that are resisting the shear force. Shear force on the bank is directly proportional to the rate at which velocity increases when moving away from the bank. Due to friction, the highest water velocity in a stream is about 1/3 of the way down from the water’s surface. This puts more of the energy against the streambank at about 1/3 of the depth. Thus, if velocity increases very rapidly in the near-bank region, the velocity gradient is steep and shear stress is high. Conversely, if velocity increases slowly or not at all in the near bank region, shear stress of the bank will be minimal or negligible. With either situation, the energies associated with the stream’s velocity are being exerted under the surface against the streambanks. Adequate vegetation will ensure that the riparian vegetation root masses are present and capable of withstanding the energies. Root masses are a key factor due to the fact that they increase the tensile strength of the bank. Particularly in noncohesive soils and sediments, the presence of vegetation may greatly increase binding forces in bank material. Tensile strength provided by root masses of riparian vegetation may be the primary source of resistance in the alluvial sediments of many Western streams. Tensile strength will be dependent upon both the kind of vegetation present and the extent and density of root masses in the sediments. Determination of root-mass adequacy is site specific, as less cohesive sediments will require greater root mass to achieve the same level of stability as more cohesive sediments elsewhere.

Vegetation also has the potential to influence the balance of energy by the occurrence of living or dead vegetation (or any other cover, for that matter) that extends into the flow of the stream/river. It has the potential to reduce near-bank velocities, thus reducing erosive shear forces acting upon the bank. In an ideal situation, vegetation along the bank is sufficient to produce a zone of near-zero velocities near the bank, effectively moving the velocity profile away from the bank so that shear stress is dissipated in turbulent eddies in the flow. A similar process occurs in the over-bank region when density of vegetation is sufficient to produce near-zero velocities in overbank flow during flood events.

Looking at the Vegetation. The following characteristics should be assessed regarding riparian species to determine if the vegetation in a channel is adequate:

  • Age-class distribution
  • Diversity
  • Species
  • Root masses, and
  • Plant vigor

If each of these items is evaluated and found to be healthy and doing well, then the vegetation present is adequate. If any of these items are absent or minimal, then there is some question as to whether or not the vegetation is adequate.

Warning Signs. Some warning signs to look for that may be indicative of inadequate vegetation include:

Bareground. Soil that is not covered by vegetation, litter or duff, downed woody material, or rocks--is highly susceptible to erosion. It may contribute both to overland sediment flow and to the erosion of streambanks. In both cases, it can affect water quality as well as contribute to the loss of valuable soil and acreage. Bare stream banks are also prime areas for invasion of noxious weeds or other undesirable plant species. Bare ground increases the possibility of compaction or bank shearing by hoofed animals, vehicles, or people. This reduces the water-holding capacity of the soil.

Riparian species present, but inadequate. If riparian plant species appear to be low in vigor, losing diversity and species, or not producing the root masses needed, there might not be adequate vegetation to sustain the riparian area and protect it during high flows.

Upland plants in the riparian area. Upland plants in a riparian area are indicative of an unhealthy riparian area. Upland species do not have the root masses capable of holding a streambank together and resisting the energies within the stream. 

References


The Role of Large Woody Material in Riparian Areas

Large Woody Material - A Specific Attribute in Specific Places. Some riparian wetland systems, mainly those in the Pacific Northwest, require large woody material (LWM) that falls into the stream to capture bedload, aid floodplain development, and dissipate energy. In these systems, it is important to determine if the streamside and upland plant communities are producing the size of woody material over time that can fulfill this need. Without coarse and/or large wood to dissipate energy, these systems cannot handle the normal high-flow events that occur. The LWM in the system needs to be large enough to stay for a period of time that allows it to operate as a hydrologic modifier.

Large Woody Material Plays a Specific Part in Very Complex Systems. The complexity of forest riparian environments has led researchers to study the hydrology, sediment delivery, vegetation, and biology of these systems to determine how each component, including LWM affect specific products, such as water quality and fish. To visualize forest riparian/stream processes, it is necessary to consider each point of interest as interrelated to the whole stream continuum. The location of interest may be anywhere from the headwaters to the ocean. The way each part of the system functions changes as the streams merge and grow larger, and the enormous variety of stream slope, geology, hydrologies, vegetation types, etc., adds to the difficulty of describing how the whole system functions.

The Role of Large Woody Material (LWM). In order to understand the role of LWM, a knowledge of stream hydrology, forest ecology, fisheries, and climate is also required. To cover all of these aspects would be too lengthy and in depth to write here. The following are some key points that are important to remember regarding the role of LWM in a stream:

  • LWM and living trees are essential to the development and maintenance of some forested riparian stream ecosystems from their headwaters to the downstream end of the forest stream continuum.
  • The riparian/stream continuum is in a state of dynamic stability when it is functioning properly and the movement of LWM down the stream system is normal and necessary. The function of LWM in the stream and on the floodplain changes from the headwater to the wider downstream valleys.
  • Floods, fires, windthrow, torrents, landslides, and normal tree mortality are essential delivery mechanisms needed to maintain and restore the riparian stream system’s functionality.
  • The temporal processes of the forest riparian /stream system must be measured in decades and centuries.
  • The spatial location of LWM is continually shifting during annual and episodic events. This spatial movement replenishes materials that are broken down or flushed out of the system.

Warning Signs. Two warning signs that not enough LWM is present for the stream and may be indicative of declining health or “unraveling” of riparian areas are: 1) There is an absence of large wood near the stream. Without living mature trees present that will access the stream in the future the stream is without a source of LWM. 2) Only isolated pockets of trees exist near the stream. The stream needs to have adequate trees as a source of LWM. Isolated pockets of trees does not fulfill this need.

Reference

  • USDI Bureau of Land Management. 1998. Riparian Area Management: A User Guide to Assessing Proper Functioning Condition and the Supporting Science for Lotic Areas. Technical Reference TR 1737-15. 124 pp. More Information available at: http://www.blm.gov/or/programs/nrst/index.php