Diselma archeri ( dwarf pine or Cheshunt pine)
Appearance and ecology
Diselma archeri is a compact, prostrate
shrub which commonly reaches 1–4 m in height but has been recorded to reach greater heights in subalpine rainforest zones. The foliage has a grey-green appearance with branchlets curving downward at their tips. Branches are short, ridged and very numerous. Branchlet foliage appears square in cross-section and scale-like
leaf (2–3 mm) are overlapping and arranged in opposite decussate pairs which are pressed close to the stem.
[Kirkpatrick, J. 1997. Alpine Tasmania. An illustrated guide to the flora and vegetation. Oxford University Press" Melbourne. P. 18-19.] The square leaf arrangement is similar to that of
Microcachrys tetragona (
Podocarpaceae) (creeping pine) and the two species can easily be confused. However,
M. tetragona lives up to its name and grows low to the ground, spreading out with only occasional erect branches. Another species which can be confused with
Diselma is
Phaerosphaera hookeriana (previously known as
Microstrobos niphophilus ) which is in the family
Podocarpaceae. Both these species have a similar growth habit and distribution, however, the opposite pairs of leaves on
D. archeri again make it distinguishable from the other species.
Bark is rough and scaly and often weathered revealing a reddish-brown inner bark.
Being a
gymnosperm no flowers are produced, instead seed development occurs on the surface of the scale-like leaves which are modified to form cones (see image).
[Raven, P.H., Evert, R.F., Eichorn, S.E. 2003. Biology of Plants. 6th Edition. W.H Freeman and Company]
Diselma archeri is a dioecious shrub, where male and female cones are located on separate individuals. Both types of conifer cone are very small (3–4 mm) and occur at the branch tips. The female cone is composed of two pairs of opposite cone scales and only the upper pair of scales is fertile.[Armin Jagel, Veit Dörken: Morphology and morphogenesis of the seed cones of the Cupressaceae - part III. Callitroideae. Bulletin of the Cupressus Conservation Project, Bd. 4(3), 2015, S. 91–103 ( PDF)] At maturity up to four small winged seeds are produced which are wind dispersed.
Taxonomy
Diselma :
dis (meaning double) and
selma (meaning upper) is a reference to either the two fertile scales in the female cone or the arrangement of the overlapping leaves in the opposite alternating pairs.
archeri is named after botanical collector William Archer (1820-1874) who was also a Fellow of the Linnaean Society, an architect and Member of Parliament for Deloraine, Tasmania.
This species is commonly known as
dwarf pine in reference to its prostrate growth pattern or
Cheshunt pine which is in reference to a property belonging to William Archer, although the species would not have occurred there.
[Wapstra, M., Wapstra, A., Wapstra, H. 2010. Tasmanian plant names unravelled. Fullers Bookshop Pty Ltd]
Distribution
Diselma archeri is endemic to Tasmania and is only found in high rainfall alpine and subalpine areas of the South West and Central Plateau of the state. Its altitudinal range varies from approximately 580-1400m above sea level. Like many Tasmanian conifers
D. archeri is very fire sensitive and will only occur in fire free areas of alpine
coniferous heath and
montane rainforest.
This species can form a small tree (3-4m) in closed rainforest at high altitude and some ecologists consider
Diselma to be one of the seven genera which can be used as rainforest indicators in Tasmania.
[Jarmen. S.J., Brown, M.J. 1983. A definition of cool temperate rainforest in Tasmania. Search14. P. 81-87] In more open coniferous heathland the Dwarf Pine grows more prostrate and only reaches approximately 1-2m in height.
Phylogeny
The closest relatives to
Diselma appear to be the South American genus
Fitzroya, another monotypic genus in the subfamily
Callitroideae, the Southern Hemisphere clade of
Cupressaceae.
Fossils of both
Diselma and
Fitzroya have been recorded in Tasmania.
[Jordan, G.J., Barnes, R. and Hill, R.S. 1995. An early to Middle Pleistocene Flora of subalpine affinities in lowland Western Tasmania. Australian Journal of Botany. 43. 231-242] This evidence indicates that
Diselma archeri is most likely a
paleoendemic and is the last remaining species in a genus that was once more extensive and has refuged to specific alpine zones due to changing climate.
[Jordan, G.J., Barnes, R. and Hill, R.S. 1995. An early to Middle Pleistocene Flora of subalpine affinities in lowland Western Tasmania. Australian Journal of Botany. 43. 231-242]
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