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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.
Problem | Compliance
| Health & Safety | Sampling
| Analytical | Data
Quality
Site Assessment | Prediction
| Construction | GIS
| Monitoring & Assessment
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