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Solids Sampling

TECH GUIDE

Vegetation

Plant cover and aboveground biomass production (production) are two methods commonly used to assess plant growth. Production is a more exact measure of how much growth has occurred on a given site or area. Nonetheless, plant cover is often used as a surrogate for production because production measurements require the destructive harvest of the measured plants. Plant production is also prone to large yearly variation, especially in arid climates, reflecting the amount of precipitation available to the plant. Frequency and density are two additional vegetative measurements. Frequency describes the abundance and distribution of species and is useful to detect changes in a plant community over time. Density is basically the number of individuals per unit area. The term refers to the closeness of individual plants to one another. Density is rarely used as a measurement by itself when describing plant communities because comparisons can only be based on similar life-form and size.

Production is measured as the mass of dried plant material covering a given amount of ground surface area. In order to determine aboveground biomass production, frames of a specified area are placed either at random locations or along a transect line within the area to be measured. All aboveground plant parts are clipped just above the ground surface, dried until they reach a constant weight, then the final weight is recorded. Plant production gives a more quantitative measure of plant growth, especially if plant growth is thick or comprises multiple layers over the ground surface. Nevertheless, plant cover is often a more convenient assessment method than production because a cover measurement or estimate does not require a destructive harvest of the sampled plants.

Cover is an important vegetation and hydrologic characteristic. Cover is generally referred to as the percentage of ground surface covered by vegetation. However, numerous definitions exist. It can be expressed in absolute terms (square meters/hectares) but is most often expressed as a percentage. The objective being measured will determine the definition and type of cover measures. Examples of cover: i) vegetation cover is the total cover of vegetation on a site, ii) foliar cover is the area of ground covered by the vertical projection of the aerial portions of the plants, iii) canopy cover is the area of ground covered by the vertical projection of the outermost perimeter of the natural spread of foliage of plants, it may exceed 100%, iv) basal cover is the area of ground surface occupied by the basal portion of the plants, v) ground cover is the cover of plants, litter, rocks, and gravel on a site. Ground cover is most often used to determine the watershed stability of the site (NARSC, 1996).

The publication, Interpreting Indicators of Rangeland Health, Version 3 (2000) is a result of interagency coordination between the Bureau of Land Management (BLM), the Natural Resources Conservation Service (NRCS), the Agricultural Research Service (ARS), and the USGS Forest and Rangeland Ecosystem Science Center. In this publication, ground cover represents the proportion of the soil that is protected from being hit directly by a raindrop. Ground cover is the percentage of material (e.g., litter, standing dead vegetation, gravel/rocks, vascular plants, and biological crust), covering the land surface. Over-lapping cover classes are not estimated. Ground cover is estimated by recording cover estimates of the first contact (i.e., highest contact above soil surface) with live vascular plants, standing dead vegetation, litter, biological crust, rock/gravel, and bare ground. The sum of these six cover categories should roughly total 100 percent.

Litter refers to both persistent and non-persistent litter and includes all dead organic matter in contact with the soil surface. Standing dead vegetation includes all plants that have been dead more than one growing season that are not in direct contact with the soil surface. Standing dead vegetation is important in protecting the site from raindrop contact, while litter provides the same site protection and is an important source of organic matter via decomposition in many areas. Rock/gravel includes all material with a diameter greater than 0.2 inch. Any gravel less than this diameter is recorded as bare ground.

Biological crust includes lichens, mosses, cyanobacteria, and algae that grow on the soil surface. It is sometimes difficult to differentiate biological crust from bare soil or dead organic matter during the dry portion of summer. Spraying questionable areas with water and waiting a minute will often give live lichens or mosses a greenish tinge indicating live tissue. Conversely, cyanobacteria crusts are often very difficult to identify, especially when weakly developed, without a careful examination of the internal structure of the crust. Cyanobacteria crusts are generally not included when estimating cover.

Cover can be measured or estimated in several different ways, all of which have advantages and disadvantages. The four main ways to measure cover are the point intercept, the line intercept, the subplot frequency, and the visual estimate methods.

The point intercept method utilizes a frame interlaced with a grid of equally spaced points. Sometimes the points have vertical or slightly inclined pins sticking out from the grid intersection points in one direction (Wilson, 1960). The pins can stick through the vegetation and mark clearly the plant that the grid point overlays. The frame is placed over the vegetation and the type of plant at each point of grid intersection (or pin prick) is recorded. For a less detailed survey, the observer can simply record the number of points that hit live vegetation, litter, rock, or bare ground. The total number of points available on the grid then divides the number of grid points that hit live vegetation. This ratio is converted into a percent for the percent live cover estimate. For a more detailed discussion of the point intercept method, NARSC (1996), Jonasson (1988), Floyd and Anderson (1987), and Wilson (1960). More recently, ocular scopes mounted on a tripod have been used to project a crosshair at the ground surface to record the number of vegetative point hits.

The line intercept method uses a theory similar to the point intercept method. To perform a line intercept cover measurement, a transect of known length is set on the ground. The length of the line that intercepts each encountered species is divided by the total length of the line to give a proportion of the line intercepted by that species. Again, the proportion is multiplied by 100 to give a percent cover estimate for each species intercepted. According to Floyd and Anderson (1987), the point intercept method gave a measurement that rivals the line intercept method in precision, in approximately one-third of the sampling time.

To utilize the subplot frequency method, one divides a frame, which will be placed on the ground, into a grid. The grid is placed over vegetation in random sampling locations, and the number of the 'subplots' (grid spaces) that have vegetation in them are counted. The number of subplots that contain vegetation is divided by the total number of subplots in the grid, then multiplied by 100 to obtain an estimate of plant cover.

Visual estimates of plant cover are often preferred in applied contexts for their rapidity (Sykes et al., 1983). Two of the most common methods of visual cover estimates were created by Braun-Blanquet (1932) and Daubenmire (1959). Each of these methods estimates the percent of the ground area covered by different classes of vegetation within a specified area or frame quadrat. The percent of cover attributed to the different vegetation classes can then be summed by the observer for a total estimate of plant canopy cover.

Additional information that may help identify the appropriate sampling technique to meet a specific objective can be found in the Interagency Technical Reference, Sampling Vegetation Attributes (NARSC, 1996).

All of these methods of vegetation sampling require that the estimate be made on an area smaller than the entire area that needs to be assessed. Care must therefore be taken during selection of the sampling area. Sometimes permanent vegetation monitoring plots or line transects can be installed during site reclamation, and vegetation can be monitored in the same location year after year. If such forethought or design is not possible at a site, random sample locations can be chosen for site analysis, and statistical probability theorist can be used to analyze the sampling results and extrapolate them across the site.

Because of topographic and edaphic factors (such as slope angle, aspect, soil water holding capacity, salinity, pH, levels of contamination, etc.), vegetation establishment and growth often vary across a site. In the case of environmental variability and limited sample size, sampling design might include some sort of stratification. Stratified designs divide the population to be measured into subpopulations based on some discernable characteristic. The stratification serves to limit population variability in a predictable way before measurement, so that trends or significant differences in the collected data are not obfuscated by too large a variability (caused by an identifiable factor) in the results. Additionally, if a site is divided spatially because of heterogeneity in the environment, specific management decisions can be made based on the different land units.

As an example, in the 10,000-acre Anaconda Superfund Regional Water and Waste site, reclamation scientists were tasked with the goal of evaluating the necessity of remediation across a large, heterogenous area. In order to evaluate the ecological integrity of the site, they divided the site into distinct 'polygons' based on similarity of vegetative characteristics within a polygon. Site assessment and vegetation sampling was then conducted once at a specific site within each polygon. By delineating polygons with similar vegetative and ecological characteristics before environmental assessments were made, in-depth sampling conducted on a limited area could be applied to the larger polygon area. Management decisions, such as remedial design options, could then be made on a polygon-by-polygon basis.

See the revegetation section to learn more about the steps involved with the revegetation portion of the reclamation process.

 

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