Tradeoffs between growth rate and water-use efficiency in seedlings of native perennials but not invasive annuals.
Tradeoffs among species' traits play an important role in shaping communities. These relationships may also mediate community response to environmental change. In plants of water-limited ecosystems, tradeoffs between growth and water use may impose an important physiological constraint within and across species. I investigated how functional traits associated with this tradeoff differ between seedlings of native perennials in California and the invasive annuals displacing them. I created plant community mesocosms of native and invasive species grown under altered N and water availability, measuring multiple plant functional traits. Natives responded positively to N and water when grown separately, but grew best under low resources when in competition with invasives. Invasives grew much larger than natives and exhibited traits associated with rapid growth. Native species also suffered a tradeoff between relative growth rate (RGR) and water-use efficiency (WUE), while nonnatives exhibited both high RGR and high WUE, especially under high resource availability. The ability to grow rapidly and use limiting resources efficiently undoubtedly contributes to the dominance of these invasive species over native seedlings. Such superior trait combinations and differences in physiological tradeoffs could explain reduced native seedling establishment and restoration success in the presence of these invasives.