Native perennial grasses show evolutionary response to Bromus tectorum (cheatgrass) invasion

Abstract

Invasive species can change selective pressures on native plants by altering biotic and abiotic conditions in invaded habitats. Although invasions can lead to native species extirpation, they may also induce rapid evolutionary changes in remnant native plants. We investigated whether adult plants of five native perennial grasses exhibited trait shifts consistent with evolution in response to invasion by the introduced annual grass Bromus tectorum L. (cheatgrass), and asked how much variation there was among species and populations in the ability to grow successfully with the invader. Three hundred and twenty adult plants were collected from invaded and uninvaded communities from four locations near Reno, Nevada, USA. Each plant was divided in two and transplanted into the greenhouse. One clone was grown with B. tectorum while the other was grown alone, and we measured tolerance (ability to maintain size) and the ability to reduce size of B. tectorum for each plant. Plants from invaded populations consistently had earlier phenology than those from uninvaded populations, and in two out of four sites, invaded populations were more tolerant of B. tectorum competition than uninvaded populations. Poa secunda and one population of E. multisetus had the strongest suppressive effect on B. tectorum, and these two species were the only ones that flowered in competition with B. tectorum. Our study indicates that response to B. tectorum is a function of both location and species identity, with some, but not all, populations of native grasses showing trait shifts consistent with evolution in response to B. tectorum invasion within the Great Basin.

Figure 1. Community composition in invaded and uninvaded populations.

Percent cover of B. tectorum, other plant functional groups, litter, and bare ground in invaded and uninvaded populations averaged over all four sampling locations. Bromus tectorum cover ranged from 2% in uninvaded areas to 40% in invaded areas. Asterisk indicates significant differences between invaded and uninvaded communities (P<0.05).

Figure 2. Tolerance and competitive effect of five native perennial grasses.

Mean percent decline in biomass of target plants from Bedell Flats, Little Hill, McClellan Peak, and Tule Peak when grown in competition with B. tectorum (a) and biomass of B. tectorum when grown with target species (b). Dotted line indicates B. tectorum biomass when grown in monoculture (not included in analysis but plotted for comparison). Different letters within figures indicate significant differences (P<0.05) using Tukey adjusted least square means for multiple comparisons. POSE  =  Poa secunda, ACHY  =  Achnatherum hymenoides, HECO  =  Hesperostipa comata, ELMU  =  Elymus multisetus, and ACTH  =  Achnatherum thurberianum.

Figure 3. Phenology of invaded and uninvaded populations.

Mean number of days before growth commenced for plants that were not green at the time of collection from invaded and uninvaded communities from Bedell Flats, Little Hill, McClellan Peak, and Tule Peak (a) and the number of days until flowering for each species from invaded and uninvaded communities at each sampling location (b). Asterisk indicates significant differences between invaded and uninvaded communities (P<0.05) based on post-hoc contrasts. POSE  =  Poa secunda, ACHY  =  Achnatherum hymenoides, HECO  =  Hesperostipa comata, ELMU  =  Elymus multisetus, and ACTH  =  Achnatherum thurberianum.

Figure 4. Tolerance and competitive effect of five native perennial grasses from invaded and uninvaded populations.

Mean percent decline in biomass of target species from B. tectorum invaded and uninvaded communities at Bedell Flats, Little Hill, McClellan Peak, and Tule Peak when grown in competition with B. tectorum (a) and biomass of B. tectorum when grown with target species (b). Dotted line indicates B. tectorum biomass when grown in monoculture (not included in analysis but plotted for comparison). Asterisks indicate significant differences between invaded and uninvaded communities (P<0.05) based on post-hoc contrasts.