Eucalyptus coccifera

''Eucalyptus coccifera'', commonly known as the Tasmanian snow gum, is a small to medium-sized tree endemic to Tasmania. It has smooth, grey and cream-coloured bark, elliptic to lance-shaped adult leaves, flower buds in groups of between three and nine, usually white flowers and conical, hemispherical or cup-shaped fruit.

Description

''Eucalyptus coccifera'' is a tree that typically grows to a height of but is sometimes a mallee to . The bark is smooth and light grey to white, with streaks of tan. Young plants and coppice regrowth have sessile, blue-green, elliptic to heart-shaped leaves long and wide. Adult leaves are arranged alternately, elliptic to lance-shaped, the same glossy green to bluish on both sides, long and wide on a petiole long. The flowers are borne in groups of three, seven or nine in leaf axils on a peduncle long, the individual buds on a pedicel long. Mature buds are oval, glaucous, long and wide with a warty, hemispherical to more less flattened operculum. Flowering occurs between December and February and the flower are white, or rarely, pink. The fruit is a woody conical, hemispherical or cup-shaped capsule long and wide.

Taxonomy and naming

''Eucalyptus coccifera'' was first formally described in 1847 by Joseph Dalton Hooker and the description was published in William Jackson Hooker's ''London Journal of Botany''. The specific epithet (''coccifera'') is derived from the New Latin ''coccus'', from ancient Greek ''kokkos'' (κόκκος), "seed of tree fruit" and Latin ''ferre'', "to carry",Backer, C.A. (1936). ''Verklarend woordenboek der wetenschappelijke namen van de in Nederland en Nederlandsch-Indië in het wild groeiende en in tuinen en parken gekweekte varens en hoogere planten'' (Edition Nicoline van der Sijs). referring to the leaves having a ''Coccus'' infection.

''Eucalyptus coccifera'' is a member of the peppermint group (series ''Piperitae'') within subgenus ''Eucalyptus'' (formerly ''Monocalyptus''). It is endemic to central and southern Tasmania, where it generally occupies sub-alpine habitats above in altitude.

Distribution and habitat

''Eucalyptus coccifera'' is endemic to Tasmania, and is the dominant species of many exposed, rocky, dolerite rich sub-alpine regions in Tasmania's south and Central Plateau.

Ecology

Reproduction and propagation

''Eucalyptus'' flower buds generally take around a year to fully develop, however development takes longer in ''E. coccifera'' and other high altitude species. Insects and birds pollinate the emergent flowers, and fertilization can take up to 20 days after pollination. ''E. coccifera'' flowering typically occurs between November and February. After fertilization, the hard woody capsule develops. Seeds are released through the capsule valves, which open when the capsule dries. This drying out is either caused by extreme heat and drought, or by fire. Dispersal of seed relies partially on wind, but mostly on gravity: the seeds will usually fall within a couple of metres of the parent tree. ''E. coccifera'' is also known to regenerate from underground lignotubers after fire.

Clinal variation

The distribution of ''E. coccifera'' is limited by altitude: rarely will it be found lower than , or higher than above sea level.
Around the tree line is often unclear, and ''E. coccifera'' can be found in small stands or even as solitary trees. At these altitudes, ''E. coccifera'' generally grows to heights below , often blending into the alpine shrubbery at the highest reaches of its distribution. A strong correlation has been found between increasing altitude and declining stem growth rate in ''E. coccifera''.

At the lower range of its altitudinal cline, ''E. coccifera'' has been observed growing to in height, but it is more commonly measured at around , especially in sheltered sites on the Central Plateau and in Mt. Field National Park.

Whilst individual and population tree height is the primary change in ''Eucalyptus coccifera'' at higher altitudes, observational studies have shown that with increasing altitude, the degree of leaf and bud glaucousness intensifies. This is most likely linked to the increased frost resistance of ''E. coccifera'' with increasing altitude. The juvenile foliage of ''E. coccifera'' is also more likely to persist for longer at high altitude.

Geological associations

The dominant bedrock underlying both the Mt. Field and Mt. Wellington sub-alpine habitats is Jurassic dolerite. Dolerite is also a feature of the geology of the Central Plateau. These dolerite-rich substrates provide sufficient drainage for the establishment of ''E. coccifera populations'', which prefers well-drained soils. They also offer a potentially rich nutrient source to the sub-alpine plant communities, but the slow rate at which dolerite weathers results in soil of only moderate fertility.

Associations with rainfall and water availability

It is generally accepted that in Tasmania annual rainfall increases with altitude, and thus the most exposed ''E. coccifera'' populations must contend with high annual rainfall (greater than on Mt. Wellington, and around at Mt. Field). Despite this, water availability can often be an issue for ''E. coccifera'' and other subalpine species. This is due to the sporadic nature of Tasmanian rainfall, and a number of other factors including soil type, slope and wind action.

The rocky substrates of sub-alpine Mt. Field and Mt. Wellington drain very effectively, therefore waterlogging isn’t generally a problem. However, it has been shown that waterlogging is a limiting factor on the growth rate of ''E. coccifera'' and can, as a consequence, affect the success of the species in susceptible areas.

Other environmental associations

''Eucalyptus coccifera'' has a high tolerance to frost and to drought, allowing it to occupy very exposed substrates at high altitude. ''E. coccifera'' must contend with hot and dry temperatures during the peak of summer, as well as very cold, frosty conditions in winter.

= Temperature and light

=
At above sea level on Mt. Wellington, the mean maximum temperature in July is , and the mean minimum is . In January, the mean maximum is and the mean minimum is . There is a stronger maritime effect (oceanic climate) on Mt. Wellington, due to its closer proximity to the ocean, than at Mt. Field. This results in milder temperatures, especially in summer. Despite the increased exposure of sites at very high altitudes, solar radiation is not thought to markedly affect temperatures, nor does it have much impact on the growth rate of ''E. coccifera''.

= Frost, snow and mist

=
Whilst ''E. coccifera'' is considered fairly frost-hardy, rare extreme frost events are thought to be one of the principle determinants of subalpine species distribution. Similarly, the duration of snow lie dictates, to a large extent, the species that can persist in alpine/sub-alpine habitats. As snow lie in Tasmania is not permanent, woody species such as ''E. coccifera'' and its underlying shrub population can persist at higher altitudes.

Another factor that can impact on the growth of ''E. coccifera'' is mist. Mist is generally more prevalent with increasing altitude, particularly in the colder months, and can affect the ability of eucalypts (and other plants) to photosynthesize by decreasing light penetration and increasing condensation on leaves.

Co-occurring species

Table 1. Showing changes in eucalypt species with increasing altitude in Mt. Field National Park.

The understorey of subalpine habitats is typically dominated by woody shrubs of the families Ericaceae (genera ''Richea'', ''Epacris'' and '' Cyathodes'') and Proteaceae ('' Orites acicularis'', '' Orites revoluta''). Low-lying conifers of the Cupressaceae ('' Diselma archerii'') and Podocarpaceae ('' Microachrys tetragona'', '' Phaerosphaera hookeriana'') are also common in these sclerophyllous heaths.

At lower altitudes, other eucalypt species such as ''E. subcrenulata'' (Mt. Field), ''E. urnigera'' (Mt. Field and Mt. Wellington) and ''E. delegatensis'' (Mt. Field and Mt. Wellington) become increasingly common in ''E. coccifera'' woodland.

Further reading

Reid, J.B., Robert, S.H., Brown, M.J., & Hovenden, M.J. (editors) (1999). ‘Vegetation of Tasmania’, Australian Biological Resources Study.

References

{{Taxonbar, from=Q159782

coccifera
Myrtales of Australia
Flora of Tasmania
Endemic flora of Tasmania
Plants described in 1847
Taxa named by Joseph Dalton Hooker