ECOLOGY

Adaptations | Water Stress | Dispersal | Recruitment | Fire | Communities

Recruitment and lifespan

The hardness of the seeds in a batch of Acacia seed varies and this determines their dormancy or readiness to germinate. Some seed will be stimulated to germinate readily while the remainder of the seed will germinate over varying lengths of time as the impervious testa (seed coat) deteriorates and adequate moisture and oxygen penetrate the seed. Some Acacia seed testa may also contain an inhibitor (as observed in Acacia farnesiana) which needs to be removed by adequate soaking to break the dormancy. This variation in hardseedness and dormancy is a survival strategy to ensure that germination occurs under the most favourable conditions. If adequate growing conditions following the first germination event do not eventuate, then the seedlings will fail. But other seed will be available in the soil seed store for future germination. However, if mass germination of all seed occurs, such as after a fire and no soil seed store remains, the result can be catastrophic if all the seedlings fail due to subsequent drought conditions. Such fire induced crashs may lead to the local extinction of a species and the fragmentation of the population.

Fire induced germination is also dependent upon the fire intensity and its speed. Slow hot fires may stimulate germination of seed ( for example, of Acacia silvestris)(Floyd 1966) buried at depths of 20 cm but kill seed buried to about 5 cm, while moderate and 'flash' fires may stimulate germination at depths of 5 cm and less respectively. Temperature greater than about 110^oC may be lethal to Acacia seed.

In forests and woodlands recruitment of seedlings is low because of the need for high light levels for the growth of acacias. Never-the-less it does occur more or less continuously where sufficient light reachers the floor. Large scale canopy openings, such as may occur due to tree fall or canopy leaf loss due to drought, leaf predation or fire or other disturbance allows massed regeneration to take place leading to the periodic establishment of localised even aged stands.

In arid areas recruitment is frequently by root suckering and some species (e.g. A. carneorum) rarely set seed. When seed is set, regeneration occurs only infrequently when a rare sequence of suitable weather events occurs. In Mulga (A. aneura) communities, for example, summer rains are required to stimulate late summer flowering and this needs to be followed by winter rains to ensure seed set, followed by late spring rains with temperatures around 27^oC to initiate germination and seedling establishment²⁵. Further rain in the following winter is then probably required to maintain seedling growth for survival during the following summer. Such sequences of events is unusual so that the occasional regeneration from seed results in 'puncuated' even aged stands.

Many acacias are short lived, 7 to 20 years often being their lifespan, while others may live for 50 to 100 or more years. There are also a large number of species which fall between these extremes.

The short lived species are typically found in the moister areas than the loner lived species. They grow quickly and are able to extract nutrients from the soil and return them in the form of litter and detritis which is then readily broken down and made available for a new generation of plants, and for the more slow growing members of the community.

The long lived species occur mostly in either arid communities or in, or near, rainforests. These long lived, slow growing species make minimial demand on the nutrient and moisture supply and probably have a high nutrient-use efficiency. In arid regions, nutrients are scarce and unreliable, and since recruitment and regeneration occurs infrequently, species such as Acacia aneura and its allies may live for 50 or more years. In rainforest environments, competition for available resources is high and species such as A. bakeri, A. aulacocarpa and A. melanoxylon are slow growing and probably have have lifespans of over 100 years.

Adaptations | Water Stress | Dispersal | Recruitment | Fire | Communities |