Identification of the Limiting Resource within a Semi-Arid
Plant Association
Jane Krueger-Mangold, Roger Sheley, and Tony Svejcar
Introduction
Competition for essential resources, such as water, nutrients, and
light, is a key force structuring plant
communities. The Idaho fescue/
bluebunch wheatgrass plant association is widespread throughout
the Pacific Northwest and is typical
of rangeland in western Montana.
In other plant associations typical
of western grasslands, research has
indicated that water is the major
limiting resource, with nitrogen (N),
and phosphorus (P) having lesser
influence. The Idaho fescue/bluebunch wheatgrass appears to be susceptible to invasion from nonnative
species; therefore, it is important to
identify the major limiting resource
within this plant association. The
objective of this research was to
determine the resource most limiting to plant growth within an Idaho
fescue/bluebunch wheatgrass plant
association.
Experimental Protocol
The study was conducted on two
sites in western Montana. Site 1
was located at Redbluff Research
Ranch about 1 mile east of Norris,
Montana. Site 2 was located in the
Story Hills about 3 miles northeast
of Bozeman, Montana. In the spring
(site 1) and fall (site 2), six essential
plant resources were applied to 8.2by 16.4-ft plots. Treatments included 1) water added at a rate of the
50-year average for May, June, and
July (plus ambient precipitation); 2)
light reduction of 50 percent; 3) 100
lb N/acre; 4) 54 lb P/acre; 5) 100 lb
potassium (K)/acre; and 6) 100 lb
sulfur (S)/acre. In addition to these
main treatments, potential interactions were addressed by applying water in combination with all
resources except for the untreated
control. Above-ground biomass
was sampled during peak standing
crop in 2000 at site 1 and in 2001 at
site 2. Below-ground biomass was
collected at site 2, but not at site 1
because the soils were too rocky for
such sampling.
Results and Discussion
The results indicate that N is the
most limiting resource for the dominant functional group in the Idaho
fescue/bluebunch wheatgrass plant
association at the two test sites. Addition of N yielded the highest total
above-ground biomass (1,647 lb/
acre), which was statistically higher
than any other resource treatment,
except the control (1,239 lb/acre).
Nitrogen produced the highest
above-ground grass biomass, which
was statistically different from the
control (1.8 times greater than that
of the control) (Fig. 1). No other
treatment was statistically different
than the control, including the water
treatment. Addition of N increased
Idaho fescue biomass over that of
any other treatment (Fig. 1).
Although the sites were characterized as Idaho fescue/bluebunch
wheatgrass plant associations, the
majority of the biomass in our study
was that of Idaho fescue. The effects of N addition on biomass were
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most pronounced in native grasses,
especially Idaho fescue. This suggests that competition models based
on limiting resources may be most
accurate and predictive when constructed specifically on the dominant species’ response.
Management Implications
It is often assumed that water, not
N, is the limiting resource in semiarid systems. Our results suggest
that competition for N, especially
among grasses, may be a critical
factor even in semi-arid grasslands.
The information obtained from this
research is a crucial initial step in
developing competition models that
may be useful for understanding
plant community dynamics, including invasions by nonnative species.
Figure 1. The mean total above-ground grass biomass and Idaho fescue biomass
as affected by resource treatment for the two sites. Significant differences within
plant groups, across treatments, are shown with lower-case letters. Error bars
show mean +/- 1 SE.
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