Restoring Species Richness and Diversity in a Russian Knapweed-infested
Riparian Plant Community using Herbicides
Roger L. Sheley and Stephen M. Laufenberg
Introduction
Experimental Protocol
Russian knapweed (Acroptilon
repens) was introduced to North
America in the 1900s and is now
found throughout the United States.
It is most commonly found in
the semiarid portions of the West
but also occupies river bottoms
and riparian woodlands. Russian
knapweed spreads aggressively
and has a competitive advantage
over many native species, often
forming monocultures after becoming established. Nonnative plant
species such as Russian knapweed
can displace native vegetation and
decrease plant diversity, thereby
altering the structure and function
of ecological systems. Reduction in
plant diversity is detrimental to the
productivity and stability of ecological systems. Previous research
indicates that a variety of herbicide
formulations can provide short-term
suppression of Russian knapweed.
However, further research on controlling Russian knapweed with herbicides appropriate for use in wet
areas and river bottoms is needed.
Our specific objectives were to
determine the influence of three
herbicides, clopyralid plus 2,4-D
(Curtail®), glyphosate (Roundup®),
and fosamine (Krenite®), at different application rates and timings,
on richness and diversity of total
species, total native species, and
total nonnative species within a
Russian knapweed-infested plant
community.
Two study sites were selected
in north-central Montana on the
Charles M. Russell National
Wildlife Refuge along the Missouri River riparian corridor. In a
randomized complete block design at both sites, 28 treatments (3
herbicides, 3 rates of application,
3 application timings, untreated
control) were applied June through
August 2000. The three herbicides
(clopyralid plus 2,4-D, glyphosate, and fosamine) were applied
in June (spring rosette stage of
Russian knapweed), July (bud to
bloom stage), and August (flowering stage). Low, medium, and high
rates of each herbicide were applied
at each application date. Density of
each species was recorded during
June and August of 2001 and 2002.
In addition, species richness (the
total number of species per experimental plot) and species diversity
(the number of individuals of each
species) also were calculated.
Results and Discussion
In June 2001, there were no
significant differences in species
richness between the control and
herbicide treatments (Fig. 1). By
August 2002, only the glyphosate treatment (5.5 species/yard2)
yielded greater total richness over
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that of the control (4.2 species/
yard2, Fig. 1). Although glyphosate
increased total species richness
compared with untreated controls,
the majority of the species were
nonnative annual forbs. Sampling in
June 2001 indicated no differences
in diversity between any herbicides
and the control (Fig. 2). In August
of 2002, diversity after application
of clopyralid plus 2,4-D remained
similar to that of the control (1.7
species/yard2), but glyphosate (2.8
species/yard2) and fosamine (2.4
species/yard2) increased total species diversity (Fig. 2). Between
June 2001 and August 2002, total
species diversity for glyphosate
treatments increased from 2.3 species/yard2 to 2.8 species/yard2.
Management Implications
In this study, glyphosate increased
total species richness compared
with untreated areas. Although
the majority of these species were
nonnative annual forbs, they play
an important role in recovering the
function of the system. Management strategies aimed at enhancing
ecosystem function, and possibly
niche occupation to prevent reinvasion by Russian knapweed, may
possibly be met with glyphosate application; however, restoring plant
communities with native species using this herbicide seems less likely.
Figure 1. The effect of herbicide by year on total species richness. Control is no
herbicide treatment, Clop. + 2,4-D is clopyralid plus 2,4-D (Curtail®), glyphosate
(Roundup®), and fosamine (Krenite®).
Figure 2. The effect of herbicide by year on total species diversity. Control is no
herbicide treatment, Clop. + 2,4-D is clopyralid plus 2,4-D (Curtail®), glyphosate
(Roundup®), and fosamine (Krenite®).
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