Shrinking opportunities for establishment of native annual forbs in fragmented grassy woodlands

Questions Repeat vegetation surveys in fragmented woodlands in southeast Australia show native and exotic annual plant species have different trajectories of abundance; native annuals are declining while exotic annuals are increasing. We ask: is the divergent species origin response due to fundamental differences in germination cues and/or “safe‐sites” for establishment? Location Temperate grassy woodlands, southern Victoria, Australia. Methods We quantified patch type frequency in the field. Diurnal temperature effects on four native annual forbs and three exotic annual grasses were assessed in a laboratory germination trial. We used a glasshouse experiment to assess the effects of patch type (bare ground, tree litter, grass litter, moss mat) on the emergence and growth of two native and two exotic annuals. Results Exotic annuals typically had higher and more rapid germination across a wider range of diurnal temperatures than native annuals. Emergence of native and exotic annuals was highest on bare ground patches but was suppressed by leaf litter and moss mat. Native annuals did not emerge from under a moss mat and germinated much less than exotics in all patch types. Seedling growth of native annuals was suppressed by tree litter whereas growth of exotic species was not suppressed by litter or the moss mat. Conclusions The regeneration niche of the native annual forb species we examined was typically narrower than that of exotic annual grasses. Many of the differences in response between native and exotic species may be due to seed traits such as seed mass; native annuals have much smaller seeds than exotic annual species. This likely affects the persistence, germination cues, co‐existence and restoration potential of native annuals in fragmented grassy woodlands that have lost their endogenous disturbance agents.