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

Summary 1 Two laboratory screening experiments tested the juvenile-phase responses of 14 C3 and 12 C4 grassland species to pulses of warm temperature and water deficits. The first experiment determined germination response in relation to duration of warm temperature exposure (30/20 °C day/night). The second experiment determined the desiccation tolerance of seedlings immediately following germination. 2 The C4 species were more dependent on warm temperatures for germination than the C3 species. However, there was considerable variation within C3 and C4 types. In particular, Panicum dichotomiflorum was identified as the C4 species least dependent on warm temperatures, exhibiting > 50% of maximum germination in continuous cold (7 °C). 3 The C3 species were generally more desiccation-tolerant than the C4 species, but there were several exceptions. Trifolium repens (C3) was ranked as the least desiccation-tolerant whereas Setaria geniculata (C4) was the eighth most tolerant species. Large-seeded species were more desiccation-tolerant than small-seeded species. 4 We suggest that poor desiccation tolerance contributes to the observed restriction of C4 grass invasion into productive grasslands of the wetter regions of New Zealand. On the basis of juvenile-phase attributes, we ranked the C3 species as more invasive of these grasslands than the C4 species, and annuals as more invasive than perennials. Having invasive juvenile-phase attributes is an advantageous adaptation for species that rely solely on regeneration from seed.