Brachychiton populneus -- Arizona

Brachychiton populneus is native to eastern Australia, from southern inland Queensland and the western slopes of New South Wales, through to eastern Victoria (Australian Native Plants Society, 2024). GBIF (2024) lists Brachychiton populneus as introduced in the USA, Cyprus, Ecuador, France, India, Italy, Libya, South Africa, and Turkey. GBIF (2024) also presents a distribution map which shows multiple occurrences of Brachychiton populneus in additonal sites in Western Australia, New Zealand, northwest Africa, and south-central parts of South America. The species has a history of repeated introductions outside its natural range (HEAR.org, 2012).

Brachychiton populneus occurrs in regions with a climate match to Arizona. These include southern California, eastern South Africa, northwest Africa, southern Europe, and Western Australia (GBIF, 2024).

GBIF (2024) lists Brachychiton populneus as introduced in the USA, Cyprus, Ecuador, France, India, Italy, Libya, South Africa, and Turkey. However, GBIF (2024) does not note the species as showing evidence of impact in any of these countries.

Brachychiton populneus occurrs in regions with a climate match to Arizona. These include southern California, eastern South Africa, northwest Africa, southern Europe, and Western Australia (GBIF, 2024). However, GBIF (2024) does not note the species as showing evidence of impact in any of these regions.

Brachychiton is a genus of 30 or more species, most of which occur in tropical parts of Australia in dry areas or in rainforest. They are large shrubs or trees. One of the most commonly cultivated is the Illawarra flame tree (Brachychiton acerifolius) which is popular due to its spectacular crimson flowers. It is native along the east coast of Australia (Australian Native Plants Society (2024). GBIF (2024) indicates Brachychiton acerifolius has evidence of impact where this plant has naturalized in Western Australia (GBIF, 2024), which is also a climate match to Arizona. GBIF (2024) tracks a large number of species of Brachychiton. The others listed include some with naturalization in other parts of the world, but apparently without impact.

Brachychiton populneus appears to be found predominately in regions with a climate match to Arizona where it is introduced in North America, though elsewhere around the world its association with climate matched areas is less correlated, or uncertain due to its frequency in cultivation. Occurrences mapped by GBIF (2024) which are not climate matches (Kenya, New Zealand, Uruguay) are otherwise not well documented on GBIF or elsewhere.

The HEAR (2012) report on Brachychiton populneus rates the species as "no" in the category of climbing or smothering growth habit. It is a small to medium-sized tree which may reach 20 metres in height, although it is often much smaller. It has a compact and densely foliaged habit. The trunk is stout and grey. (Native Plants, 2024).

The HEAR (2012) report on Brachychiton populneus indicates the species creates a fire hazard in natural ecosystems. The Kings Park bushland (invaded by B. populneus) has been frequently burnt, and the results of this study suggest that B. populneus can survive at least one fire by resprouting from basal dormant buds (Buist et al, 2000).

Brachychiton populneus has been noted as causing pollen allergies. It was included in a study of highly-allergenic species in cultivation by Velasco-Jimenez et al, (2020). HEAR.org (2012) compiles accounts which suggest foliage of Brachychiton populneus is palatable, susceptible to browsing, and one of the best fodder trees for cattle and sheep. But also, animals have been poisoned if their diet is almost exclusively of this tree. Seeds may potentially be toxic to livestock. Experimentally the seeds have proved to be toxic. Animals have been poisoned only when their diet consisted almost totally of this tree." (HEAR.org, 2012).

Buist et al (2000) notes that in Kings Park, B. populneus (invasive in the area) was most commonly observed beneath the canopy or close to the trunks of another tree, and in many cases there was clumping of many B. populneus seedlings and saplings around a tree. By inference, the potential for forming thickets may exist. Issues caused by thicket formation are not noted in the literature (HEAR, 2012). Hamad et al, (2023) observed B. populneus germination is highest in full sun and is suppressed by shading. This counts against the formation of thickets by seed reproduction. Brachychiton populneus does not produce spines, thorns, or burrs nor has a climbing or smothering growth habit (HEAR, 2012) which could contribute to impenetribility.

The easiest way to propagate a Kurrajong tree is usually from seed. You can choose cuttings from the smaller branches of an established Kurrajong tree in the dry season. Brachychiton populneus is sensitive to the cutting process. Dip the cuttings in rooting hormone and then plant. It takes around 90 days for roots to appear (Clarke, 2024). By inference, it seems unlikely to reproduce and spread vegetatively.

In horticulture, Brachychiton populneus is usually propagated by seed, and it is sensitive to the cutting process. Cuttings are advised to be treated with a rooting hormone. It takes around 90 days for roots to appear (Clarke, 2024). By inference, it seems unlikely to reproduce from detatched fragments.

The HEAR (2012) report on Brachychiton populneus rates the species as producing viable seed. Buist et al, (2000) report an average-sized reproductive tree produces up to 7000 seeds in one flowering event, and observations of flowering in the years before and after the study suggest that large amounts of seed are produced annually. Consequently, although there are only a small number of reproductive trees present to act as foci, prolific annual seed production in these individuals is enough
to support an invasion.

With an estimate of 8–12 cubic meter of foliage per average-sized reproductive tree, approximately 4736–7104 seeds are produced per tree (Buist et al, 2000).

A study of germination pre-treatments by Kheloufi et al, (2018) found a 0% germination of untreated Brachychiton populneus seeds under laboratory conditions. Treatments with sulfuric acid for 60 minutes produced 100% germination. Germination success was also described for immersion in hot followed by soaking for 12 hours in cooled water. Seeds buried in bags by Buist et al, (2000) saw germination coincided with the onset of the winter rains and continued into winter so that by August when the seeds had been buried for 6 months 98% had germinated. Seeds that had not germinated in the shorter burial periods were found to have high rates of viability. In this study 98% of the seed that was buried in late summer (March) had germinated within 6 months; germination began with the onset of winter rains and continued throughout the wet winter months. While these two studies were contradictory, the relevant evidence is from the field studies by Buist et al, (2000).

The HEAR (2012) report on Brachychiton populneus cites a report that time to first flowering is possibly 8+ years. Since this is a large-stature tree with large fruits, by inference this supports the likelihood of over five years required.

Flowering time: April - June (in Malta, where introduced) (Mifsud, 2002).

Australian ravens were observed on several occasions in flight carrying B. populneus fruit or perched in trees with B. populneus fruit in their beaks. It is possible that ravens, perched in trees feeding on transported fruit, do not consume all the seed, and some seed falls to the ground. The foraging behaviour of black rats may also be responsible for establishment patterns, with caches of eaten and uneaten fruit and seeds observed near the base of trees. The foraging behaviour of the vertebrates may facilitate the dispersal of seeds for relatively long distances away from parent plants (Buist et al, 2000).

The HEAR (2012) report on Brachychiton populneus cites "Dispersal: Birds, rats and possibly other mammals." [no evidence of water dispersal]. The tree is not reported to occurr preferentially along waterways.

The HEAR (2012) report on Brachychiton populneus categorizes the propagules as unlilely to disperse as a produce contaminant. The fruits and seeds of B. populneus are quite large and unlikly to be disperesed inadvertantly.