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Seasonality

Our picture of the Cumberland Plain Woodland flora at the Australian Botanic Garden is of species with characteristics that enable persistence in the face of fluctuating seasonal rainfall and temperature conditions. Most species are perennial and able to survive severe dry periods at ground level as rootstocks or soil stored seed. These characteristics also allow them to survive fire events. Growth cycles are aligned with long-term annual rainfall patterns, with most species being autumn-winter growing and flowering. Under good rainfall conditions growing and flowering will extend into spring with a slight mid-winter hiatus in very cold years. This is different to the Sandstone flora where spring flowering and growth is the main response.

Seasonality: annual and short-term changes

 

Any patch of ground in Cumberland Plain Woodland is likely to change the way it looks during the year. The herbs and grasses grow during favourable weather and die back when it’s hot and dry, or when it’s too cold. In recording species presence in our plots, the number of species has varied from as low as 2 per square metre during a very dry time to 27 after rain has stimulated growth. The number of species one can find varies over space as well as time. In our 5 m x 5 m plots, the total number of species recorded as present at some time has varied from 34 to 53.

Changes over a two year period 2001-2002, based on individual species abundance (frequency) from the monthly recording of Plots 4 and 9, show the following features:

  • Most species fluctuate in abundance showing a peak in frequency between March and August (autumn-winter) with a decrease to a low frequency in December - January. This follows the mean annual rainfall trend which peaks in January with a trough in August - September. This is followed by a period of high maximum temperatures in November - March which slows recovery.

  • Rainfall in 2001-2002 was below average in both years (actual rainfall in Camden was 530 mm in 2001 and 459 mm in 2002, compared to Campbelltown's mean annual rainfall 747 mm over 78 years ) though most rain did fall within the March to August period.

  • Many species disappeared completely during the dry period, to reappear in March, not just the annuals e.g. *Anagallis arvensis, but perennials including Caesia parviflora, Geranium homeanum, Hypoxis hygrometrica, Wahlenbergia gracilis, Tricoryne elatior, Sporobolus creber. Even more species disappeared in the extreme 2002-2003 summer.

  • A few species did not show a seasonal fluctuation during 2001-2002 e.g. Brunoniella australis, Dichondra repens, Sida corrugata (though they showed in 2003), Themeda triandra, Scleria mackaviensis (fluctuating but in different way).

  • Spring favouring species are few, Lomandra filiformis shows a later growth period June to November (spring) than others such the similar-sized Cyperus gracilis February-July (autumn-winter).

  • Some species are opportunists - The single high rainfall event in February 2002 (250 mm), triggered an immediate clearly marked growth/flowering response in Tricoryne elatior, and also Bothriochloa macra, but was absorbed as part of the autumn seasonal growth by other species.

  • Some species grow similarly in the two years, Chloris ventricosa, Cyperus gracilis, Oxalis perennans, Sporobolus creber. Some species grew better in 2001 e.g. Veronica plebeia, Hypericum gramineum, while others grew better in 2002 e.g. *Senecio madagascariensis, Geranium homeanum, Bothriochloa macra. 2002 had a lower rainfall but a lot in February.

  • Winter cold did not have a noticeable affect on growth patterns. Sida corrugata showed a decrease in August 2001 but not in 2002. *Sida rhombifolia showed a decrease in abundance in July of both years but decreases mainly in dry summers.

  • In terms of weed native interactions some weeds deteriorated over the period e.g. *Paspalum dilatatum, *Verbena bonariensis but others are expanding e.g. *Hypericum perforatum.

  • Species tended to show similar patterns at the different sites even though abundance differed e.g. Chloris ventricosa, Brunoniella australis. For some species there were some differences between the two sites, e.g. Bursaria spinosa fluctuated seasonally at Plot 4 but remained constant at Plot 9. For Wahlenbergia gracilis abundance was higher in 2001 than in 2002 at Plot 9, but higher in 2002 compared with 2001 at Plot 4.

Some difficulties in monitoring Cumberland Plain Woodland

  • Dry conditions seasonally and even daily
  • Tiny species make location and identification difficult, particularly if they are also short lived
  • Similar closely-related species


Asterisk * indicates exotic species naturalised at the Australian Botanic Garden.
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