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Drought and rainfall

While we were well aware that rainfall was important for the successful growth of woodland species, we had not truly realised its importance in the initiation of growth and particularly seedling growth events. We had always assumed that rainfall was distributed more or less evenly throughout the year and was not generally a limiting factor, rather, that triggering factors like fire or increasing temperatures were most important in regulating growth and recruitment.

Drought

The drought in 2002 occurred at just the right time in our recording sequence. Our monthly recording had begun in 2001 and had given us a year of average year effects. This was followed by the drought year - there was virtually no rain from the end of March to December 2002. Some rain came in late Summer and Autumn 2003, with really good rain in May. (Rainfall 2001: 631 mm, 2002: 524 mm, 2003: 644 mm, longterm average 828 mm).

  • Native species we had classified as annuals - and considered lost as we had not seen them since 1988 - reappeared! This was really exciting! These species were behaving, not as conventional annuals, that go through their life cycle regularly every year, but like ephemerals, species in the semi-arid areas of Australia that only appear every few years after good rainfall.

  • Daucus glochidiatus, a native species in the carrot family, reappeared patchily, but was abundant in those patches. Most plants emerged in areas that hadn’t been burnt. We couldn’t have missed it if it had been present in other years. It is a herb that grows to 50 cm tall in our woodland, and, like a carrot, produces small fruits covered in adhesive hairs.

  • Ranunculus sessiliflorus, an even smaller herb up to 25 cm high, reappeared widely throughout the woodland, in both burnt and unburnt areas. It is in the buttercup family, and, though its flowers are tiny, it flowered profusely.

  • Both are small plants that don’t need a lot of resources to grow to maturity and produce seed for the next generation.

  • Also interestingly, these two species appeared at different times after the breaking of the drought. The Ranunculus sessiliflorus appeared in May, while the Daucus glochidiatus was not seen until October, so the two species have different dormancy characteristics in their seeds. Even though they both reappeared after drought and rain, they did it differently in relation to the seasons. This is an example of how plants behave in a similar fashion - but in slightly different ways. We believe this is a factor that enables coexistence of many species together, as they are not competing directly with each other.

  • A third species behaving as an ephemeral appeared for the first time, a tiny herb in the chenopod family, barely 5 cm tall - Chenopodium carinatum. We found only a few of these tiny plants, so they could easily have been missed before. Being tiny, they completed their life cycle very quickly after the rain, flowering in May and setting seed in June.

  • Ironically, although it was the drought that seemed to prepare these species for germination, ephemerals need periodic episodes of high rainfall, like that in May, to enable production of good seed crops to replenish their soil seedbanks.

Rainfall and its importance

Rainfall in the late 1980s and early 1990s was generally average or above. However since 2000 there have been a series of dry and sometimes hot years. 2002 was a particularly dry and hot year, with record high temperatures in December.

In the area burnt in our 2002 burn there was little regeneration because of the lack of rainfall, so in summer 2002-2003 we took soil samples and watered them in the glasshouse to see what might be in the soil seedbank. What came up in the soil samples was an impressive array of seedlings, many of which we had never seen in the field, though we were familiar with the adult plants there. We also got seedlings of one species Ranunculus sessiliflorus, which we had recorded once back in 1990, and, but for the fact that we had kept a small specimen of it, we would have regarded as an error of identification. As it was, we regarded the species as likely to be extinct in the woodland.

Subsequent rain in March 2003 (drought-breaking) resulted in the germination of many seedlings in the woodland, including Ranunculus sessiliflorus and Daucus glochidiatus which has also been recorded only once before. Seedlings of both species, and many others, were recorded in both burnt and unburnt areas, indicating that it was particular site conditions not the effect of fire that allowed seedling establishment, though the open conditions in some burnt areas may have helped by providing locally higher soil temperatures or reduced competition from established plants.

Short-lived ephemeral species

We have been able to compile a list of species, almost all short-lived ephemerals, that recruited well in drought-breaking conditions, ie substantial rain following a long hot dry period. The list includes Ranunculus sessiliflorusDaucus glochidiatus, *Conyza sumatrensis. While some of these species will occur at other times, generally in smaller numbers, we suggest that factors relating to drought conditions such as temperature may play a part in their dormancy breaking mechanisms. Further work is needed.

As a result of these observations we are convinced that the main force driving processes in the woodland is rainfall rather than fire or temperature. The decreased rainfall events expected as climate change takes hold may have a significant impact on the long term floristic composition of the woodland.

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

Number of woodland species with seedlings recorded in burnt (September 2001) or unburnt sites before and/or after good rain (February-March 2002), the Australian Botanic Garden, Mount Annan.

Seedlings observed October 2001-August 2002

SpeciesBurnt Burnt Unburnt
 before rain after rain after rain
NATIVES      3 26 18
EXOTICS      4-5 16-19 14-17
Unidentified dicots  3 
Unidentified monocots  3+ 
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