Botanic Gardens Trust, Sydney, Australia

Life cycle stages of a woodland plant

A good way to look at the ecology of individual species is in terms of cycles, processes and impacting events. These are the stages in the life cycle of the plant.

life cycle

Flower | Fruit | Seed | Seedling | Juvenile | Adult

Ecology of Cumberland Plain Woodland

 

 

 

 

 



 

 

 

Flower

When we first visit the woodland we want to see flowers. Flowers however are not produced to decorate the woodland for us humans, but are the sexual organs of the plant and the initial stage in the reproduction of the plant. The flower’s purpose is to enable the pollination process to take place. Following pollination the flower develops into the fruit and seeds.

Flower types

Most of our woodland plants have male organs (stamens and anthers with pollen) and female organs (stigma and ovaries) in the same flower (bisexual) on the same plant (monoecious). A few species produce separate male and female flowers (unisexual) on different plants (dioecious), e.g. Asperula conferta, Clematis glycinoides, Lomandra multiflora, Lomandra filiformis. In small populations of dioecious species where there are few or even no plants of one sex, there may be inadequate pollination and failure to set seed. For example in the woodland at the Australian Botanic Garden there is only a very small population of Lomandra multiflora.

Many plant families have very distinctive flower types - the family Fabaceae has pea-shaped flowers e.g. Lotus australis, while the grass and sedge families have specialised flowers that you might hardly recognise as flowers e.g. Scleria mackaviensis.

Flowering seasons

For most native woodland species flowering is not tied to particular seasons but is dependent on adequate rainfall and may happen at any time. This is in contrast to the plants in the Sandstone woodlands of coastal Sydney which have a distinctive peak of flowering in spring (September-October).

Flower colours

Unlike the colourful Banksias and Waratahs of the sandstone areas, most of the woodland flowers are small and not colourful. Flower colours are predominantly white, with some pink, purple or yellow. Colour and shape relate to pollination modes.

In contrast to the native species, many of the exotic weed species that are naturalised in the woodland have conspicuous and colourful flowers e.g. *Heliotropium amplexicaule, *Verbena rigida, *Hypericum perforatum, as many of these were introduced to Australia as garden plants, and have subsequently escaped from cultivation.

Flower opening times

While we are used to seeing the flowers of most cultivated garden ornamental plants remaining open all day, there are different daily flower opening times for some of the woodland species. The flowers of some species remain open all day and under all conditions e.g. Bursaria spinosa, Pimelea spicata, Indigofera australis. Other species have flowers that are only open when it is sunny, and even then, for only part of the day e.g. Caesia parviflora and Wahlenbergia gracilis are open in the afternoons, Hypoxis hygrometrica and Tricoryne elatior are often open in the morning. On very hot days most flowers tend to wilt e.g. Ajuga australis.

Flower longevity

Flowers of some species may last a few days, but others are only open for a day e.g. Caesia parviflora, although the plant may produce a succession of flowers. Flower availability is likely to be important in pollination, and presumably the period and time of opening relates to the presence of pollinators. However we know very little about the native pollinators of the woodland species.

Non-flowering plants

Some plants do not produce flowers. Ferns produce spores which grow into prothalli on which the next generation embryo grows. Of the four fern species in the woodland, two are confined to moist habitats but the other two Cheilanthes sieberi and Cheilanthes distans grow in the dry woodland similarly to the other ground species.

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Flowering Geranium homeanum
Geranium homeanum 

Flowering Dianella longifolia
Dianella longifolia

Flowering Lotus australis

Lotus australis

Flowering Calotis lappulacea
Calotis lappulacea

Flower opening
Caesia parviflora

Fruit

Fruiting is the development of the fertilised embryo within a protective cover. Fruit sizes for species in the woodland range from the small fruit of Mentha satureioides (4 mm) to the large capsules (10 cm long) of Parsonsia straminea.

Fruiting times

Fruiting time for many species, particularly grasses, is mostly in autumn - but there are some spring/summer fruiting species, depending on adequate rainfall.

Time to maturity

Fruit maturing time from pollination to maturity may vary from 30 days for daisies such as Rhodanthe anthemoides, to 8 months for the vine Parsonsia straminea. Bursaria spinosa seed may take up to 3 months to mature.

Rainfall necessary for successful fruiting

Good rainfall is necessary for successful fruiting in most species, and even where flowering has been good, there will be few or no viable fruit set in dry conditions.

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Fruit of Parsonsia straminea
Parsonsia straminea

Fruit of Einadia nutans subsp linifolia
Einadia nutans subsp. linifolia

Seed

Seed size

Seeds of woodland species range in size from tiny microscopic orchid seeds e.g. Pterostylis curta to the seed of Hardenbergia violacea which is 9 mm long, and seed of the exotic *Olea europaea subsp. cuspidata which is up to 12 mm. Fern spores of Cheilanthes are also microscopic (40-80 um). Size and shape varies and is important for dispersal of the seed.

The soil seedbank

Unlike the other life stages of plants, which occur mainly above ground, the seed is dispersed from the parent plant with adaptions to assist it to enter the below-ground soil seedbank. The woodland at the Australian Botanic Garden has no species that have plant-stored seedbanks (bradysporous) like many of the sclerophyllous Banksia and Hakea species in the sandstone woodlands.

The enclosed protective nature of the seed, and its limited metabolic requirements, allows species to survive in the soil seedbank, avoiding unsuitable conditions such as drought, until moisture and temperature conditions are suitable for germination. How long seeds remain in the soil seedbank depends on moisture and temperature conditions, and the capacity of the seed to remain dormant. Persistence in the soil seedbank varies for different species.

Seed longevity

Seed longevity ranges from about a year for some annual species to to more than 20 years for some Acacia species. Some seed may be kept alive for longer periods in artificial seed storage in the the Australian Botanic Garden seedbank, though such periods are unlikely to occur in natural conditions, where moisture and temperature conditions vary and soil seed predators are present.

In most woodland species the adult stage is probably longer-lived than the seed in the soil, but populations of short-lived ephemerals such as Ranunculus sessiliflorus and Daucus glochidiatus may appear as adults for 2-4 months followed by 1-3 years in the seed bank.

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Seed of Desmodium brachypodum
Desmodium brachypodum

Seeds of Dianella longifolia
Dianella longifolia

Seedling

For most species the seedling stage is less than a month, and in many probably less than 2 weeks. Observation of seedlings in grassy woodland vegetation is hindered by their small size and the difficulty of species identification. Young seedlings of dicotyledonous species are generally recognisable by their characteristic pair of cotyledons (seed leaves). The single cotyledons of monocots however are very difficult to recognise.

The seedling stage is the most vulnerable part of the life cycle for most woodland species; seedlings are very susceptible to drying conditions and vulnerable to predators, and continuing moist conditions are necessary for rapid growth.

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Ranunculus sessiliflorus seedling
Ranunculus sessiliflorus

Juvenile

Juvenile growth is the period from the seedling to first flowering as an adult. This may be as short as 3-4 weeks for some quick-growing ephemerals such as Ranunculus sessiliflorus or at least 10-15 years for trees like Eucalyptus crebra, Eucalyptus moluccana and Eucalyptus tereticornis. For many of the perennial groundcover species it is probably of the order of 3-6 months.

During the juvenile period many of the herbaceous species develop thickened rootstocks that allow them to survive in drier conditions. The long-lived shrub Bursaria spinosa develops a similar thickened rootstock during its early juvenile period. Once this is developed, the juveniles may persist for years in suboptimal conditions (e.g. from competition or grazing) before being able to mature.

Successful juvenile growth needs the right conditions, primarily moisture, and space. Our observations are that most juvenile growth takes place after periods of extended good rainfall, in autumn and spring, often in relatively open sites, often bared by previous surface disturbance, drought or fire. Mortality of juveniles in hot dry periods is high.

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Juvenile Eucalypts
Juvenile Eucalyptus tereticornis on the left and Eucalyptus moluccana on the right

Adult

The majority of individual plants in the woodland at most times are adults. This is because most of the plant species are perennial, relatively long-lived and able to survive the hot summers and periodic droughts by regrowing from thickened rootstocks or tubers.

Adult plant growth is mostly in response to rain periods, and mostly during the cooler autumn and spring seasons. At other times plants may stay in a non-growing or dormant state, or in severe drought may lose all above ground tissue. Most species will resprout from underground rootstocks when conditions improve.

Life spans

Populations are most vulnerable to change when adult plants die, particularly if a large number of individuals die at the same time. One of our particular interests has been to try to determine the potential lifespans of individual species. Our longevity estimates range from about 2 months for shortlived ephemeral species (e.g. Ranunculus sessiliflorus, Daucus glochidiatus, *Anagallis arvensis) to several hundred years for some long-lived trees and shrubs. General indications are that eucalypts may live up to 200 years, and judging by the size of some of our Narrow-leaved Ironbarks Eucalyptus crebra and Forest Red Gums Eucalyptus tereticornis, that some of these trees could pre-date European settlement.

There is also a group of species that have indeterminate life spans, that is they keep growing indefinitely, assuming that conditions remain favourable. These plants include species that reproduce by bulbs and tubers such as the lilies e.g. Tricoryne elatior or spread by rhizomes e.g. Scutellaria humilis and root suckers e.g.Clerodendrum tomentosum. There are also some woody plants, in particular Bursaria spinosa, where the leaves and stems may be killed by fire but resprout from the base again and again. In the absence of death from debilitating environmental conditions, such as severe drought or disease, these plants appear to have the ability to live indefinitely.

About half of the native species are relatively short-lived perennial species, that is living less than five years, though few of the native species (about 10%) are annuals with lifespans of less than one year. Indeed very few of these are true annuals in the sense that they come up reliably every year, annually. Rather they appear episodically from soil-stored seedbanks following specifically favourable conditions such as wet periods following drought, or in particular wet years, with gaps of perhaps 5-10 years when they are not evident.

In contrast to the native species most of the exotic weed species are short-lived perennial species, with nearly half as annuals. However it is the few long-term perennial species, particularly *Olea europaea subsp. cuspidata, and indefinite perennial species *Hypericum perforatum, *Heliotropium amplexicaule and *Nassella neesiana, that cause the main ongoing weed problems in the Australian Botanic Gardens woodlands.

Table

Lifespan classes for native and exotic species in Cumberland Plain Woodland at the Australian Botanic Garden

Annual
1 year
Biennial
2 years
Short perennial
1-5 years
Long perennial
5-50
Very long perennial
50-200 years
Indefinite perennial
Indeterminate years

Native species
104

8 44 26 5 20
Exotic species
59
26 7 14 6 1 5

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