Simultaneous intraspecific facilitation and interspecific competition between native and annual grasses

• Intraspecific facilitation was greater for bottlebrush squirreltail than for medusahead.
• Competitive effects of a perennial grass on an invasive annual grass increased as the life stage of the perennial increased.
• Drought stress reduced the competitive effect of the annual grass on the perennial grass more than vice versa.
• Both species displayed intraspecific facilitation but the net effect of their interspecific interaction was negative.

Invasive annual grasses tend to construct thinner and less dense root and leaf tissue than native perennial grasses. This allows invasive annuals to grow faster and produce more biomass in the arid grasslands of the United States. Based on these differences we tested the hypotheses that: 1) Competitive effects of the native perennial on the invasive annual will increase as plant developmental stage increases and as drought stress increases. 2) Drought stress will reduce the competitive effect of invasive annuals on native perennials proportionately more than drought stress reduces the competitive effect of native perennials on the invasive annual. 3) Facilitation among native perennial grass seedlings will decrease as developmental stage increases. Competitive effects of native squirreltail, on invasive medusahead, increased as the initial developmental stage of squirreltail increased, but not vice versa. Drought stress reduced the competitive effect of medusahead on squirreltail target biomass more than drought stress reduced the competitive effect of squirreltail on medusahead target biomass. While both squirreltail and medusahead displayed intraspecific facilitation, the net effect of their interspecific interaction was negative for both species. Habitat amelioration manifests itself differently depending on species traits, and can create conditions that simultaneously benefit one species while hindering another.