Grasstrees of southwestern Australia
Main Club, 6 October 2023
Distinguished Prof Emeritus Byron Lamont is well known to our club for his botanical work and previous fascinating talks. This year, his work was recognised with a lifetime achievement award from the International Association of Fire Ecology. He will travel to California in December to receive it.
There are three genera of grasstrees: Xanthorrhoea (28 species), Kingia (single species, Australia) and Dasypogon (single species, D. hookeri). The latest taxonomy of Australian grasstrees groups the endemic Australian genera Kingia, Dasypogon, Calectasia and Baxteria in the family Dasypogonaceae, and isolates Xanthorrhoea (subfamily Xanthorrhoideae) in the family Asphodelaceae.
Xanthorrhoea originated in Australia 60 million years ago. They are usually single-stemmed (caudex) but may dichotomise after flowering. The caudex is made up of persistent leaves and resin. Xanthorrhoeas have unique secondary anomalous growth which engulfs the original stem and thickens the caudex at the base. Leaf production for X. preissii reaches a peak in summer. Approximately 500 leaves/year are produced that die after two years but stay on the plant. Growth never stops, even in the cool winter or dry autumn seasons. Summer rain may be sufficient to stimulate leaf production. Isolateral leaves can photosynthesise from all sides exposed to sunlight. Root growth is confined to the usual wet season and renewed from the edge for easy access to summer water.
Xanthorrhoea leaves are highly flammable; however, plants are fire tolerant as adults: the foliage burns at >1000C, but the apex < 60°C is achieved by the insulating properties of the youngest leaves protecting the apical bud. A flush of leaves emerges immediately (mainly from the growth of leaves that survived intercalary growth) even before nutrient uptake from ash is possible. The resin binds the leaf bases to the caudex between fires, forming an impermeable layer.
Xanthorrhoeas usually only flower after a fire (pyrogenic). The apex is converted to a reproductive bud with the inflorescence comprising the spike and the scape. After flowering, the plant has to start again from a new bud at the side (sympodial growth). The inflorescence replaces ~ 250 leaves. Flowering is more likely the taller the plant. Spike length is independent of caudex length; the longest spike is 5.5 metres. The stimulus for post-fire grasstree flowering is likely smoke, which contains ethylene, as do vehicle exhaust fumes. More flower spikes are noted close to roadway edges!
Grasstrees grow 5-40 mm/y and average 15 mm/y, depending on growing conditions. Grasstrees do not actually “resprout” after fire as they never stop growing. The tallest is 6.5 m, the oldest is 400 years, and most crown heads are 23.
Grasstrees have an important ecological role for animals (and humans), including pollination (wide range of insects; birds and possums), seeds are eaten by parrots (survive two years, germinate readily), bardies/palack/ witchety grubs (jewel and longicorn beetles) in decaying caudex.
Kingia differs from Xanthorrhoea: tallest ~8 metres, oldest 530 years, no secondary growth, sheath of aerial roots descend inside the caudex from the apex, taking 50 or more years to reach the soil. Lateral roots penetrate the leaf bases. Kingia inflorescences emerge as a crown of drumsticks after fire (fit for a king!), with monopodial growth, and flowering within a month of fire as floral primordia already exist. Kingias have a similar growth rate and fire history to Xanthorrhoea but appear more fitful; they are 1-2 m tall before they can flower, but some flower in the absence of fire; caudex pith may survive for 400 years (probably a record among living organisms).
Joan Sharpe
Notes prepared by Distinguished Prof Emeritus Byron Lamont, June 2023 are provided on page 2