Colonization Success of a "Biennial" Plant Species: Experimental Field Studies of Species Cohabitation and Replacement

In order to gain understanding of the relationships of life history characteristics to colonization success, species cohabitation, and species replacement, a known number of seeds (fruits, achenes) of teasel, a biennial' plant species, Dipsacus sylvestris Huds. were experimentally introduced into 52 permanent plots within each of 8 contiguous abandoned fields of various vegetation types in Michigan, USA. The nearest naturally occurring teasel population was 11.2 km from the study site; no teasel seeds or plants were present in the fields except where controlled introductions were made for this study. Permanently marked teasel and control plots were monitored over a 5—yr period for colonization success of teasel, species compositions, and species net annual primary production. Results fall into three categories, the first of which has implications for studies of plants strategies and the last 2 have implications for studies of community structure and succession. 1) Life history characteristics of teasel and initial colonization success: A cohort of teasel seeds spreads its germination over more than one growing season; the length of time in the rosette stage may extend to 4 yr or more. Teasel did not successfully colonize the field that had thick quackgrass litter due to low germination of seed and high mortality of seedlings. In other fields, teasesl rosettes grew ate various mean rates, depending upon the presence of grass, other herbaceous dicots, and shading shrubs. Critical size for flowering was reached in either 2, 3, or 4 yr in six of the eight fields and these populations produced °7x more seeds than was originally introduced. 2) A mechanism for recolonizaton on a site: A teasel rosette creates an opening in vegetation that is colonized by teasel seedlings, or if teasel seeds are unavailable, by short—lived winter annuals, specially Barbarea vulgaris L. The number of species in a field (excluding teasel) increased with teasel colonization, probably due to the creation of these openings by teasel rosettes. 3) Cohabitation with teasel: Grass growing with teasel was not reduced in productivity (compared to that in Control plots), despite the significant production by teasel. However, the herbaceous dicotyledonous plants were slowly reduced over 4 yr as teasel increased in biomass. Roots of flowering teasel plants do not occupy the same soil layer as roots of the shallow grasses in this study, but probably do compete with those of the morphologically similar herbaceous dicots. Studies of communinty succession have often recorded changes in the number of species and in site productivity; such changes can here be directly attributed to colonization of the sites by a new species.