TESTING LIFE HISTORY CORRELATES OF INVASIVENESS USING CONGENERIC PLANT SPECIES

Abstract

We used three congeneric annual thistles, which vary in their ability to invade California (USA) annual grasslands, to test whether invasiveness is related to differences in life history traits. We hypothesized that populations of these summer-flowering Centaurea species must pass through a demographic gauntlet of survival and reproduction in order to persist and that the most invasive species (C. solstitialis) might possess unique life history characteristics. Using the idea of a demographic gauntlet as a conceptual framework, we compared each congener in terms of (1) seed germination and seedling establishment, (2) survival of rosettes subjected to competition from annual grasses, (3) subsequent growth and flowering in adult plants, and (4) variation in breeding system. Grazing and soil disturbance is thought to affect Centaurea establishment, growth, and reproduction, so we also explored differences among congeners in their response to clipping and to different sizes of soil disturbance. We found minimal differences among congeners in either seed germination responses or seedling establishment and survival. In contrast, differential growth responses of congeners to different sizes of canopy gaps led to large differences in adult size and fecundity. Canopy-gap size and clipping affected the fecundity of each species, but the most invasive species (C. solstitialis) was unique in its strong positive response to combinations of clipping and canopy gaps. In addition, the phenology of C. solstitialis allows this species to extend its growing season into the summer—a time when competition from winter annual vegetation for soil water is minimal. Surprisingly, C. solstitialis was highly self-incompatible while the less invasive species were highly self-compatible. Our results suggest that the invasiveness of C. solstitialis arises, in part, from its combined ability to persist in competition with annual grasses and its plastic growth and reproductive responses to open, disturbed habitat patches. Corresponding Editor: D. P. C. Peters.