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Ecological Functions of Riparian Vegetation

Removal of the natural riparian vegetation through past forest harvest activities, in particular by clearcutting to the stream bank, can impair the functional role of the riparian zone in providing stable and diverse physical and biological conditions within the adjoining terrestrial and stream ecosystems. Ecological functions of the riparian vegetation include (Figure 1):

  • regulation of the physical structure of the stream channel by determining the input and characteristics of large woody debris (LWD) which partly controls sediment storage and transport; local flow characteristics; and the creation of fish habitat
  • maintenance of bank and channel stability by provision of solid root mass and ground cover
  • regulation of stream temperature by providing shade

Figure 1. Ecological functions of riparian vegetation.
  • regulation of instream biological production by determining the inputs of small organic debris (SOD) (leaves, detritus, terrestial insects, large woody debris, dissolved organic carbon) to the channel
  • regulation of instream algal production by controlling the amount of sunlight (for photosynthesis) reaching the stream
  • buffering the stream from fine sediments by intercepting surface flow
  • provision of wildlife habitat features, including coarse woody debris (CWD), wildlife trees, nest and perch sites, and summer and winter dennings
  • provision of summer and winter forage for terrestrial fauna.

The characteristics of the riparian vegetation thus can strongly influence the diversity and productivity of both the aquatic and terrestrial biota, and the physical stability of the streambank and channel (Additional details regarding riparian functions are provided in Appendix 1.).

Overview of the Riparian Assessment and Prescription Procedures

The WRP riparian assessment and prescription procedures (RAPP) attempts to identify opportunities and appropriate techniques to restore the aquatic and terrestrial functions of the riparian zone. The assessment provides a standard methodology for reviewing existing information, conducting field surveys, and interpreting the results.

The assessment and prescriptions procedure consists of the following steps:

  1. Identification of harvested riparian areas within the project watershed (harvested before implementation of the provincial Forest Practices Code [Code]).
  2. Prioritization of harvested sites for field visitation.
  3. Collection of field data in priority riparian areas.
  4. Evaluation of impaired riparian functions (dysfunctional stands) within the field-surveyed areas.
  5. Identification of opportunities for riparian rehabilitation.
  6. Development of riparian restoration prescriptions (alternatively called restoration plans).

There are three distinct stages in the procedure:

  1. Office-based "overview" assessment of existing information from, for example, maps, air photos, forest data files.
  2. Reconnaissance "Level 1" field-based assessment.
  3. Detailed "Level 2" field-based assessment, where required, and prescription development stage.

The overview stage has the broadest geographical focus, with Level 1 likely reduced to a subset of the area assessed at the overview (priority sites only), and Level 2 likely a subset of the Level 1 sites.

The first step is to identify and delineate riparian areas that have been harvested in the past. It is assumed that these areas are most likely to contain impaired functions. Field visits are then carried out to these sites to determine their level of functionality and whether there is sufficient regeneration occurring on site. In most cases, this will require collecting data related to the overstorey vegetation (tree sizes, densities and dominant species), understorey vegetation (per cent cover and height of dominant shrubs, herbs and mosses) and a brief description of soil conditions. Those sites which are not providing sufficient aquatic and terrestrial functions, and have insufficient regeneration, are then recommended for further assessment (if required) and prescription development.

The riparian assessment procedures occur sequentially and include: identification of harvested riparian areas; field assessment and evaluation of level of impairment; identifying opportunities for restoration (e.g., tree thinning, planting, fencing); prioritizing sites for restoration, developing restoration plans; implementation of restoration works; followed by maintenance and monitoring.

Assumptions and Limitations

The procedure reported here for assessing riparian habitats and developing prescriptions is limited in scope and application. First, the procedure is limited to assessing impacts from past logging practices. Second, it is assumed that impacts are most prevalent in previously harvested areas and therefore assessments are limited to these areas. Third, the assessments will focus primarily on previously harvested riparian areas adjacent to fish bearing streams more than 1.5 m wide. Fourth, the focus will be on the riparian "reserve zone" of these fish bearing streams (see below for definitions and exceptions).

In the Code, fish bearing streams are divided into S1 to S4 streams (Table 1). An S1 stream has a channel width of > 20 m; S2 from 5-20 m; S3 from 1.5-5 m and S4 < 1.5 m. There are also S5 (>3 m wide) and S6 (<3 m) streams within the Code. S5 and S6 streams are not fish bearing.

Table 1. Key to stream riparian classification (from Riparian Management Area Guidebook, 1995)
Is stream a fish stream or in a community watershed?a
Yes
No
Avg channel width
Riparian Class
Stream Width
Riparian Class
>20 m
S1
>3m
S5
>5 - 20 m
S2
<3 m
S6
1.5 - 5 m
S3
 
 
<1.5 m
S4
 
 

aTo determine if a stream is within a community watershed and to locate watershed intakes, consult the Community Watershed Guidebook and contact the local BC Ministry of Environment, Lands and Parks, Regional Water Manager.

The riparian management area (RMA) is a designated term contained in the series of legal documents (Table 2). It is made up of the riparian management zone (RMZ), and, immediately adjacent to fish bearing streams of >1.5 m channel width (Wb), the riparian reserve zone (RRZ). Focusing assessments and restoration activities within the RRZ limits the degree of overlap with other forest management programs and thus increases efficiencies at implementing and completing restoration works.

Table 2. Stream classification
 
S1a
S1
S2
S3
S4
S5b
S6b
Stream width (metres)
>100
20-100
5-20
1.5-5
<1.5
>1.5
<1.5
Riparian reserve zone width (RRZ)
0
50
30
20
0
0
0
Riparian management zone width (RMZ)
100
20
20
20
30
30
20
Riparian management area width (RMA)
100
70
50
40
30
30
20

The primary focus of WRP riparian assessments are streams with RRZs (i.e., S1-S3 streams).
aS1 large rivers may have a RRZ if designated by regional ministry staff.
bS5 and S6 streams are non-fish bearing.

The following exceptions apply to the limitations listed above: riparian assessments outside of the above stated RRZ, or at other than S1-S3 streams, and in riparian areas adjacent to wetlands or lakes can be of equal or more biological value. Issues such as the need to stabilize upstream channels, general biodiversity protection, and protection of endangered wildlife species may result in exceptions to our recommended focus, but these should only be undertaken upon the specific direction of the contracting agency. Where the fish-bearing status of a stream is unknown and there is no Code S1-S6 classification, assume that all streams with <20% gradient and >1.5 m Wb are acceptable for assessment. One other exception to the above is in collecting field data in old-growth riparian vegetation stands. This can be of much value in assessing the extent of impairment found in adjacent previously harvested stands and for subsequent use as restoration templates.

Finally, it is important to recognize that RAPP is a tool, and is not a replacement for training and experience.

Project Scope

Within the context of the larger WRP program, whole watersheds are the units for which restoration plans should be developed. The assessment procedures in WRP Technical Circulars No. 2-9 emphasize the potential impacts of forest harvest on aquatic resources. To assess the cumulative effects of forest harvest, the most usual first step is to complete the appropriate watershed assessment procedure, such as a Code coastal or interior watershed assessment procedure (CWAP, IWAP), or a similar review. Next, it is necessary to examine the state of roads, hillslopes, gullies, riparian areas, stream channels or fish habitat, as appropriate, to identify specific problems that may be treated through restoration projects (see WRP Technical Circular No. 1). Riparian assessment and prescription development should not be conducted without knowledge of upslope or upstream conditions and risks that may affect the project.

The appropriate spatial scale for applying the assessment and restoration procedures from Technical Circulars No. 2-9 is, third to fourth order basins on 1:50 000 national topographic series (NTS) maps. Watersheds of this size are sufficiently manageable for integrated restoration projects.

Related Assessment Procedures

The channel conditions, and fish habitat assessments (WRP Technical Circulars No. 7 and 8) provide information that is useful to the evaluation of riparian habitats. Where possible, all three assessments should be closely coordinated to ensure consistency in common methods, and the exchange of information to avoid duplication in field surveys. This is especially true for remote sites. Where possible, air photo analyses and field surveys should simultaneously gather data for all assessments that are indicated as necessary by the WRP contracting agency.

There are also many justified instances where the riparian assessment procedure may be exclusively carried out; for example, where fish habitat and channel assessments are already completed, or where the project proponent or project funding is limited to riparian works only.

Who Should Do the Assessments?

The riparian assessment procedure provides a standard methodology for reviewing existing information, conducting field surveys, and interpreting the results consistently to identify opportunities for effective riparian restoration projects. To use the methodology effectively, it is imperative to have a good knowledge of riparian vegetation and soil characteristics, and of fish and wildlife values. It is also essential to be familiar with standard field survey methods outlined in Ministry of Forests (MOF) and Code guides (see References section for sample references).

The riparian overview assessment and the Level 1 field assessment are intended to be done by experienced field technicians with an understanding of riparian vegetation and riparian habitat restoration options. Technical staff should work under the supervision of an experienced professional biologist or silvicultural specialist. Those involved in the overview assessment should also be experienced at air photo interpretation. Detailed Level 2 assessments and prescription development will usually be done by a silvicultural specialist experienced in riparian vegetation prescription development. Exceptions to the Level 2 expertise recommendation may be where the prescription is focused on, for example, streambank works (bioengineering), or some types of wildlife habitat restoration.

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