Recruitment limitation in dry sclerophyll forests: Regeneration
requirements and potential density-dependent effects in Eucalyptus tricarpa
(L.A.S. Johnson) L.A.S. Johnson & K.D. Hill (Myrtaceae)
C. K. Orscheg1, N. J. Enright1, F. Coates2 and I. Thomas1
1Department of Resource Management and Geography, Melbourne School of Land and Environment,
The University of Melbourne, Parkville, and 2Department of Sustainability and Environment, Arthur
Rylah Institute for Environmental Research, Heidelberg,Victoria, Australia
Austral Ecology (2011) 36: 936–943
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.
Key words: competition, Eucalyptus, germination, recruitment limitation, regeneration, seed, stem density.