Vegetation of Girraween National Park - Flora ... - www.rymich.com!

Geology
Explorer and botanist Allan Cunningham first visited the area in June 1827. His diary entry for the
26th of June 1827 states:
“Large detached masses of granite of every shape towering above each other, and in
many instances standing in almost tottering positions, constituted a barrier before
us; beyond these a deep ravine formed a curve from E. to S.W., which was itself
bounded by a rocky ridge at least 250 feet high.”
Girraween National Park and its associated flora and vegetation communities are influenced by the
underlying geology, in this case granite, and its associated soils. The Stanthorpe Granite was intruded as a
molten mass deep below the surface about 240 million years ago in the early Triassic period, where it
cooled and crystallised slowly to a coarse-grained rock. Since then erosion has removed the overlying rocks
and exposed the granite mass. Where this has been closely fractured, erosion has created valleys, but
where less fractured, great domes and slabs of bare rock have remained. Granite breaks down
(decomposes), by environmental and chemical processes, into coarse sandy soils that provide limited
fertility and water holding capacity (Wilmott 2004, pp. 49, 51). Fine clays resulting from the decomposition
of the feldspar in the granite tend to wash away easily, leaving only the more coarse quartz grains. The
depth of soils is shallow except in riparian areas where the components of decomposed granite can
accumulate. Plant nutrients such as calcium, magnesium and iron are limited due to the lack of these
minerals in the parent rocks (Wilmott 2004). This underlying geology and associated soils influence the
plant species and vegetation communities that populate the area.
Climate
During summer, days are a warm 28-32°C, with nights averaging 15-18°C. Winters are usually dry
and cold with overnight temperature falling to an average minimum of -4°C and sunny days reaching 15-
22°C. Most years in winter there are heavy frosts, sleet and even light snowfalls. Heavy snow falls have
been recorded. Officially recorded temperature extremes vary from -16°C to 40°C. Most rain falls between
November and March with an average annual rainfall of 850 mm per year; the eastern areas of the park
receive the highest rainfall totals (McDonald, et. al 1995). This gradient of rainfall, between the western and
eastern areas of the park, and in addition to the geological and topography of the park mentioned briefly
above, influences the vegetation communities and individual species that occupy various sections of the
park (McDonald, et. al 1995).
Vegetation of Girraween National Park, July 2011
Page 9