Limiting similarity, biotic resistance, nutrient supply, or enemies? What accounts for the invasion success of an exotic legume?

Abstract

Models for community invasion can inform grassland management if patterns involving the environment, other species, and invader success are evident. We evaluated unplanned invasion by Lespedeza cuneata into an experiment on a reconstructed tallgrass prairie. Our goal was to determine whether the pattern of invasion by this exotic legume was consistent with either the limiting similarity hypothesis (LSH), which predicts poorer establishment of exotic species in communities with related species, or the biotic resistance hypothesis, which predicts a negative association between species richness and invasion. We also examined how the abiotic environment and hemiparasite abundance affect spread of L. cuneata. From 2006 to 2015 we manipulated fertilizer and presence of the hemiparasitic plant, Pedicularis canadensis, in 1-m2 plots, and manipulated shade for the first 5 years. We found greater support for the LSH. Biomasses of legumes and other forbs in 2008 were negatively associated with change in L. cuneata biomass over the next 7 years but species richness was not a significant effect. Lespedeza cuneata cover in 2015 was lower in fertilized plots relative to unfertilized plots and was negatively associated with a composite variable that described the number and combined cover of legume species and number of exotic species currently in the plots. Shading for five seasons reduced L. cuneata growth, but only significantly in fertilized plots from which the hemiparasite had been removed. Other analysis suggested that L. cuneata increased more as the hemiparasite abundance increased. Prairie management that increases the kinds and abundances of native legume species and increases soil fertility may slow the spread of L. cuneata, but we caution against adding P. canadensis to large areas without greater knowledge of site-specific effects.