Regrowth of Herbaceous Riparian Vegetation
following Defoliation
Chad S. Boyd and Tony J. Svejcar
Introduction
End-of-growing-season herbaceous stubble height is an important consideration for managers of
riparian areas grazed by livestock.
Residual vegetation can influence
riparian ecosystem structure and
function by filtering and stabilizing
water-borne sediments. This helps
encourage streambank development
and improves soil water storage
capacity and site conditions for
riparian-associated plant species.
End-of-growing-season stubble
height requirements are often used
to ensure adequate residual vegetation in systems grazed by livestock.
In such cases, managers need to
know how much regrowth can be
expected by the end of the growing
season. Previous research indicates
that both timing and intensity of
defoliation can impact regrowth
of herbaceous riparian vegetation.
However, only a limited number of
studies have addressed the simultaneous effects of timing and intensity
of defoliation. Our objective was to
determine the impact of timing and
intensity of defoliation on regrowth
of herbaceous riparian species.
treatment bands that were located
adjacent to the stream (CLOSE)
and 12 ft distant (FAR). In all cases,
the FAR plots were still within the
stream’s zone of influence (i.e.,
influenced by its water status).
Within a treatment band, we located
15- by 20-inch treated (clipped)
plots. Plots were clipped to 2, 4,
or 6 inches, or left unclipped, in
either June or July. End-of-growing-season stubble height was then
measured in October of each year.
Within a clipping treatment, values
for end-of-season stubble height
were averaged across years.
Results and Management
Implications
Regrowth varied with clipping
height, time, and distance from
stream channel (Fig. 2). End-of-season height increased with increasing clipping height and generally
was higher for plots close to the
stream channel and those clipped
in June. Values for end-of-growingseason height ranged from a low
of 3.5 inches for the 2-inch, FAR,
July clipping treatment, to a height
of 7.5 inches for the 6-inch, FAR,
June clipping treatment (Fig. 2). We
selected study sites to be representative of the variability of stream
systems in our area. Despite the
resultant variability in plant community types between and within
creeks, clipped stubble height and
time of clipping were strongly associated with end-of-season regrowth performance. The regrowth
response of plants to timing of
clipping supported the management
concept that later clipping (July)
produces less regrowth than early
clipping (June). This trend probably
would have been accentuated with
clipping even later in the growing
season.
From a management perspective, height regrowth response at
the clipping heights and times used
Experimental Protocol
We used three small (generally
less than 8 ft wide) streams in Harney County, Oregon. On each creek,
four research sites were selected
and fenced with electric fencing in
April of 2000 (Fig. 1). Data were
collected during the growing seasons of 2000–2002. Plant community types varied across and within
streams and included sedge, rush,
and grass-dominated stands. Treatments were applied within 3-ft-wide
Figure 1. In this study we used a total of 12 research sites located along three
different streams in Harney County, Oregon.
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in this study generally provided
sufficient regrowth to meet endof-growing-season stubble height
requirements on federal lands (approximately 4–6 inches). A 4-inch
stubble height requirement was met
by all but the 2-inch, FAR, July
clipping. Conversely about one-half
of the clipping treatments (mainly
those clipped in June) met a 6-inch
requirement. Managers should
also consider that stubble height is
only one of many tools available
to gauge management impacts on
resource integrity. Use of stubble
height guidelines as a grazing
management tool should be within
the context of a more comprehensive management plan that includes
consideration of upland forage
conditions, habitat use patterns of
livestock, and variability associated
with site and climate factors. Further research is needed to improve
our understanding of stubble height
impacts on below-ground (root)
production dynamics, and the role
of residual vegetation in influencing bank building processes and site
development.
Figure 2. The influence of clipping height and time, and distance from active stream
channel on end-of-growing-season (October) height for herbaceous riparian plants
in plots located along small streams in Harney County, Oregon. Lines have been
superimposed to indicate a 4- and 6-inch stubble height requirement.
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