GREENLINE MONITORING AND RANDOM SAMPLING
ON RIPARIAN PASTURES
Prepared by Pat Larson
PHOTO 1: This stream is shown
in early spring at or just below the highest flow for the year. It is an
intermittent stream, is grazed each year, and is generally surrounded by a
Ponderosa pine dominated forest with some larch, Douglas-fir, and grand-fir.
Scattered junipers can be found in the riparian zone as well as a few alders.
PHOTO 2: Impacts to fish habitat are monitored at the greenline on
stream banks. The greenline is located where the riparian vegetation has
water available throughout the growing season. Some people say it is
important to leave a specific height of grass stubble after grazing. Some
believe a 4 or 6 inch grass and sedge stubble should be left next to the
stream each year after grazing to prevent sediments from being deposited in
the stream.
Science research has shown that the stubble heights are not major factors
influencing the amount of sediment carried in runoff. It has been shown that
soil texture is more important than stubble height when estimating the amount
of sediment deposited in streams from runoff.
Sediment did not increase or decrease in a shallow flow riparian zone in plots
where the vegetation heights increased (Pearce et al. 1998). The organic
layer on the soil surface of plots tended to repel water and reduce
infiltration rates (Frasier et al 1998), and plots with Kentucky bluegrass,
tufted hairgrass, water and beaked sedges had no consistent differences among
specific vegetation heights in the removal of Nitrogen and phosphorous
nutrients (Corely et al. 1998).
In 2001 runoff and sediment yields were compared
on 100 plots in a sagebrush-grass community (Mergen et al, 2001).
The study grouped the plot measurements by plant height, soil density,
and organic matter in the soil. Soil moisture, soil organic matter,
and silt content were found to be “somewhat” important with surface
cover explaining 5 to 28% of the differences while soil characteristics
accounted for 18-34% of the variation in sediment yields.
Generally, when plant heights are quantified and compared to other riparian characteristics, the research tells us that sedimentation of streams from runoff is dependent on the type of soil found at the site rather than the stubble height of the riparian vegetation. However, federal grazing allotments are assessed using greenline monitoring with a focus on the stubble heights of plants.
PHOTO 3: This photo is the same stream shown in previous pictures but
is in late summer when the intermittent stream is nearly dry. A few pools
remain as well as a few wet areas where the water table is near the surface.
Sedges and thin bentgrass thrive where the areas are saturated, but give way
to tufted hairgrass and other grass species on the terraces above the pools.
Greenline monitoring (displayed in the photo by a blue line) is used by the
federal agencies at the end of the growing season to determine if sites meet
the objectives of the local forest or land resource plans.
·
Annual operating instructions for
livestock grazing identify the required stubble heights that must remain on
streams after livestock are removed from a pasture.
·
Monitoring is conducted on perennial and
intermittent streams at designated, permanent sites located through use of GPS
units to insure the same sites are measured year to year.
·
Stubble heights are measured at the
greenline for approximately 100 meters.
In some areas greenline monitoring is the only measurement used to assess all
grazing activities on Forest Service and BLM allotments. On many allotments,
compliance with the permit and planning objectives are assessed by
“eyeballing” stubble heights during routine checks by agency personnel.
PHOTO 4: This is a perennial stream where water flows through the
channel throughout the year. The plant community is the same as shown on the
intermittent stream in Photos 1-3. In the Blue Mountains of Oregon thin
bentgrass and species of sedges generally are found on banks where water is
readily available throughout the growing season. Their frequency decreases on
terraces and floodplains where soil moisture decreases and are replaced by
species of grasses and forbs that require less moisture. Soil depth and soil
moisture are factors that influence the types and amount of grasses found in
riparian areas.
Stubble height measurements taken on the greenline are not a direct
measurement of impacts to fish habitat caused by grazing activities. For
example, in Photo 4 above, greenline monitoring might result in data that
describes the amount of grazing on the narrow area next to the stream, but the
monitoring results would not be representative of grazing throughout the
riparian pasture. When the monitoring is conducted at the end of the grazing
season (1 time each year) on a transect placed at the greenline
(non-randomized sampling), the 1 sample is of little value to assess the
impacts of grazing in the riparian pasture. Also, the greenline data cannot
be associated with the streambed or stream water unless the monitoring also
includes streambed or water quality samples that are taken at the same time.
Therefore, no conclusions can be made about grazing activities that impact
fish habitat.
Following are 2 diagrams to further illustrate how greenline monitoring is
conducted and how random sampling is conducted.
GREENLINE MONITORING
Figure 1: Stubble height measurements are taken using a greenline (shown in
green). Due to the selection of a starting point and ending point when the
agencies conduct greenline monitoring, the defined line and distance creates 1
plot and becomes 1 sample.
RANDOM SAMPLING
Figure 2: In this diagram, random plots (red areas) are independent of a
beginning and ending point, the plots are scattered across the area to
capture information about grazed and ungrazed areas along the stream. The
sampling area is defined at each point shown in red which represents a plot
frame that covers an area that is 1 square foot or 0.1 square meter. Stubble
heights are measured and averaged within each plot frame. 30 or more plots
could be needed to obtain and adequate number of samples to fully represent
the heights of grazed and ungrazed plants after grazing activities. The
amount of variation between grazed and ungrazed plants along the greenline
determines how many total plots are needed.
Greenline sampling is designed to answer questions about livestock use at the
edge of a stream and provides no information about the use throughout the rest
of the pasture.
Figure 3. Random sampling conducted in a riparian pasture as demonstrated
above, captures information about the grazing conducted throughout the
pasture. Rather than focusing on a narrow strip that runs parallel to the
stream, this sampling design addresses the grazing activity throughout the
pasture.
References:
Corley, Carin J., Gary W. Frasier, M.J. Trlica, Freeman M. Smith, and E.M.
Taylor, Jr. 1998. Technical Note: Nitrogen and phosphorus in runoff from 2
montane riparian communities. Journal Range Management. 52:600-605.
Frasier, Gary W., M.J. Trlica, W.C. Leininger, R.A. Pearce, and A. Fernald.
1998. Runoff from simulated rainfall in 2 montane riparian communities.
Journal Range Management. 51:315-322.
Pearce, Robert A., Gary W. Frasier, M.J. Trlica, Wayne C.Leininger, John D.
Stednick,, and James L. Smith . 1998. Sediment filtration in a montane
riparian zone under simulated rainfall. Journal Range Management. 51:309-304.
Mergen, D.E., M.J. Trlica, J.L. Smith, and W.H. Blackburn. 2001.
Stratification of variability in runoff and sediment yield based on
vegetation characteristics. Journal of Am. Water Res. Assoc.
37:3:617-628.