ECOLOGY

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

Water stress avoidance

Unlike the bipinnate and usually dry-season deciduous acacias of the African drylands, the Australian species are mostly phyllodinous and evergreen, including those from the arid regions.

Australian acacias survive water stress by the development of scleromorphic features and a number of xeromorphic mechanisms. These include sunken stomata where the stomates (the phyllode pores throught which gasses and water vapour pass in and out of the plant) are sheltered from drying winds and direct sunlight by being in grooves below the surface; by stomatal closure when moisture stress becomes critical; by phyllode pubescence or pruinosity where the hairy covering or the whitish waxy coating - a covering of microscopically small waxy filbrils (Conn & Tame 1997) reflect heat and protect the underlying surface and stomatal openings; and by physiological inactivity slowing growth through a reduction in photosynthesis. Only when the water stress becomes extreme are phyllodes sometimes shed. Species, such as Acacia aneura and A. tetragonophylla survive prolonged drought by shedding their phyllodes and becoming physologically inactive.

The bipinnate acacias (section Botrycephalae) of eastern Australia fold their leaves when under moisture stress, and some species, such as Acacia fulva, also fold their leaves at night. These bipinnate acacias are generally associated with the forests and woodlands of the more moist areas and tend to hold their leaves horizontally, thereby receiving a greater proportion of light than the phyllodinous species. It is interesting to note, however, that those species associated with rainforests are mostly phyllodinous.

In Acacia phyllodes the central mesophyll tissue contains water storage cells. In a study of the phyllode structure as it relates to the various sections of the phyllodinous acacias it was found (Broughton 1986) that the acacias of sections Juliflorae and Plurinerves which usually have several to many primary longitudinal veins in the phyllodes are more xeromorphic in structure than the acacias of section Uninerves, which have only one main longitudional vein. The phyllodes of the former two Sections have a greater ratio of central mesophyll thickness to outer mesophyll thickness, indicating their greater ability to store moisture and hence to cope with low moisture regimes.

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