Investigating the potential of using R* theory to manage nonindigenous plant invasions
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Date
2004
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Montana State University - Bozeman, College of Agriculture
Abstract
R* theory has been proposed as a mechanism for plant community dynamics, but the theory is poorly
tested and has not been developed into a principle for management. This theory states that the outcome
of succession is based on the ability of a plant to sequester a limiting resource when it is below the
uptake level of neighboring species. ' R* is the resource level a species requires to persist in an
environment, and the species with the lowest R* dominates over time. Knowledge of the R*s for
species within a plant community could lead to effective weed management with predictive
capabilities.
The overall objective of this research was to explore R* theory’s potential for managing nonindigenous
plant invasions on western rangeland. Specific objectives included: 1) determining the most influential
limiting resource for a western rangeland plant community, 2) determining R*s for two native species
and a nonindigenous invader, 3) predicting the outcome of succession based upon species’ R*, and 4)
altering resource availability to favor desirable species based upon their R*.
Three studies were conducted beginning in 2000. For Study 1, availability of essential plant resources
were altered and the plant community was sampled for a biomass increase. Results of Study 1
suggested nitrogen was the major limiting resource for the dominant functional group. Study 2 was a
greenhouse study that attempted to quantify the R* for nitrogen for three species. The R* for annual
sunflower was 0.6 ppm NO3-N; the R* for bluebunch wheatgrass was less than 0.6 ppm NO3-N; and
spotted knapweed’s R* was between 0.4 and 0.6 ppm NO3-N. Two- and three-species mixtures
suggested estimated R*s accurately predicted the outcome of competition between natives, but not with
natives and spotted knapweed. Study 3 tested succession dynamics under natural and
nitrogen-manipulated conditions. Soil nitrogen was altered by nitrogen and sucrose additions. Annual
sunflower did not persist. Spotted knapweed biomass increased with nitrogen addition but did not
decrease with carbon addition, even though soil NO3-N was below its R*. Bluebunch wheatgrass
appeared unaffected by nitrogen manipulation, but soil NO3-N concentration was not decreased below
its R*.