• competition;
  • Eucalyptus;
  • germination;
  • recruitment limitation;
  • regeneration;
  • seed;
  • stem density


Recruitment limitation in canopy trees is receiving increasing attention as restoration of tree cover for connectivity, biodiversity offsets, carbon-trading and improved catchment health becomes more prominent. Recruitment limitation is often addressed by examining seed traits and germination requirements. Distance between trees is also often explored as a factor, particularly in agricultural landscapes where forest structure has been altered, with large distances between fragments generating Allee effects linked to pollen limitation. Fewer studies have examined how short distances between trees (high stem densities), which can characterize regenerated forests, might affect recruitment. This study examined recruitment limitation by exploring germination requirements of Eucalyptus tricarpa (red ironbark) and the effects of stem density on seed characters and germination. Eucalyptus tricarpa is a canopy species in the box-ironbark forests of central Victoria, Australia. During the early European settlement phase in the 1800s these forests were heavily impacted by human disturbances including mining, logging and livestock grazing. In many regenerated stands, now more than 100 years old, current stem density exceeds those documented in 19th century reports and there is little regeneration of key canopy species such as E. tricarpa. In laboratory trials seed germinated at 18°C in darkness, and while viability varied among populations, overall it remained high (91–96%) and did not differ among populations with different stem densities. An in situ germination trial addressing sowing season, ground-cover, soil-ripping, canopy-cover and stem density recorded no germination. High stem density in coppicing box-ironbark forest was hypothesized to limit resources for reproduction; however, no relationship was found between nearest neighbour distance, allocation to selected reproductive structures or seed viability. The relative insensitivity of reproductive allocation to intraspecific competition (high stem density) may be interpreted as a trait linked to unpredictable environments and consistent with long-lived species. In terms of management, recruitment limitation in E. tricarpa appears not primarily attributable to seed viability, tested seedbed treatments or stem density so further factors, including fire, now require exploration.