Variable seed bed microsite conditions and light influence germination in Australian winter annuals

Environmentally cued germination may play an important role in promoting
coexistence in Mediterranean annual plant systems if it causes niche
differentiation across heterogeneous microsite conditions. In this study,
we tested how microsite conditions experienced by seeds in the field and
light conditions in the laboratory influenced germination in 12 common
annual plant species occurring in the understorey of the York
gum-jam woodlands in southwest Western Australia. Specifically, we
hypothesized that if germination promotes spatial niche differentiation,
then we should observe species-specific germination responses to light. In
addition, we hypothesized that species’ laboratory germination response
may depend on the microsite conditions experienced by seeds while buried.
We tested the laboratory germination response of seeds under diurnally
fluctuating light and complete darkness, which were collected from
microsites spanning local-scale environmental gradients known to influence
community structure in this system. We found that seeds of 6 out of the 12
focal species exhibited significant positive germination responses to
light, but that the magnitude of these responses varied greatly with the
relative light requirement for germination ranging from 0.51 to 0.86 for
these species. In addition, germination increased significantly across a
gradient of canopy cover for two species, but we found little evidence to
suggest that species’ relative light requirement for germination varied
depending on seed bank microsite conditions. Our results suggest that
variability in light availability may promote coexistence in this system
and that the microsite conditions seeds experience in the intra-growing
season period can further nuance species germination behaviour.