Investigating the potential of using R* theory to manage nonindigenous plant invasions

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*.