Laboratory screening of the juvenile responses of grassland species to warm temperature pulses and water deficits to predict invasiveness

1, Two laboratory screening experiments tested the juvenile-phase responses of 14 C-3 and 12 C-4 grassland species to pulses of warm temperature and w ater deficits. The first experiment determined germination response in rela tion to duration of warm temperature exposure (30/20 degreesC day/night). T he second experiment determined the desiccation tolerance of seedlings imme diately following germination. 2, The C-4 species were more dependent on warm temperatures for germination than the C-3 species. However, there was considerable variation within C-3 and C-4 types. In particular, Panicum dichotomiflorum was identified as th e C-4 species least dependent on warm temperatures, exhibiting > 50% of max imum germination in continuous cold (7 degreesC). 3. The C-3 species were generally more desiccation-tolerant than the C-4 sp ecies, but there were several exceptions. Trifolium repens (C-3) was ranked as the least desiccation-tolerant whereas Setaria geniculata (C-4) was the eighth most tolerant species. Large-seeded species were more desiccation-t olerant than small-seeded species. 4. We suggest that poor desiccation tolerance contributes to the observed r estriction of C-4 grass invasion into productive grasslands of the wetter r egions of New Zealand. On the basis of juvenile-phase attributes, we ranked the C-4 species as more invasive of these grasslands than the C-4 species, and annuals as more invasive than perennials. Having invasive juvenile-pha se attributes is an advantageous adaptation for species that rely solely on regeneration from seed.