About Carmichaelia australis R.Br.
Carmichaelia australis R.Br., commonly known as New Zealand broom, is a rapidly spreading, ascending shrub that typically reaches 2 to 8 meters in height and 2 to 5 meters in width. This species has distinct morphological features that make it easy to identify, based on its stem structure, flowers, and coloration. It has flattened photosynthetic stems called cladodes that absorb sunlight, so it does not need true leaves; however, 3 to 5 foliates can sometimes be found on seedlings and young plants. When leaves are present, they are bright green compound leaflets that sometimes have dark spots. Both leaves and stems have smooth surfaces with very few hairs, and each leaflet has a distinct notch at the apex, called an emarginate tip. The flowers of C. australis are arranged in racemes along the main stem, with 1 to 3 short stalks per node, and 5 to 8 flowers on each stalk. The outer green floral bracts of each flower are typically bell-shaped and triangular, and pressed against the inner flower petals. Flowers are very small, white with purple veins. At the center of the flower are the reproductive structures: the male stamens measure 3–4 mm long, and the female pistil measures 4 mm long. The seed pods of New Zealand broom are oblong, with an outer surface that is grey, brown, or black. When mature, these pods split open to release their seeds, and each pod holds 1 to 3 seeds. The seeds are oblong or bean-shaped, and their color ranges from orange, red, to green, sometimes with black mottling. This species was first formally described by Robert Brown in 1825, and its specific epithet australis means "southern". The earliest collected specimen, gathered by Joseph Banks and Daniel Solander in 1769 (catalog number AK102896), is held at Auckland Museum. All species in the genus Carmichaelia are derived from a single common ancestor that is thought to have dispersed from Australia approximately five million years ago. Modern distribution of Carmichaelia species suggests this common ancestor was present on the landmass that eventually separated to form New Zealand. The entire genus has diversified only in this region: 23 out of 24 known Carmichaelia species are endemic to New Zealand, and the single exception, Carmichaelia exsul, is endemic to Australia's Lord Howe Island. Carmichaelia australis itself is a New Zealand endemic, widely naturally distributed across the North Island and the northern part of the South Island, and does not grow in southern South Island. It forms many distinct localised populations across both islands, a pattern caused by its seed dispersal method: when C. australis seed pods burst, seeds only travel short distances to sprout in nearby soil, unlike wind or bird-dispersed seeds that form less localised populations. Carmichaelia australis occupies a wide range of habitats across New Zealand, with a broad realised niche. Its optimal habitats include coastlines, low-lying areas, montane river terraces, riverbanks, colluvium inclines, tussock grasslands, thick bush edges, swamp margins, and areas around rock outcrops. It occurs in most New Zealand habitats, with the exceptions of cliff faces and poor pioneer land. Information about the phenology of Carmichaelia australis is limited, and more research is needed. Seed germination occurs within one to three weeks after seed dispersal, and the species can take up to one year after germination to reach adult reproductive maturity. Adult stems typically emerge leafless from above the cotyledons (the first leaves produced by the plant) at the base of juvenile stems. Flowering typically occurs annually between October and February, while annual fruiting occurs between November and May. The duration of seed viability before dormancy has not been determined, and further research is needed to understand this species' seed banks. The timing of these life cycle events varies based on local environmental and geological conditions. As an endemic New Zealand shrub, Carmichaelia australis has several roles and limitations in local natural ecosystems. It is a nitrogen-fixing species, cannot easily outcompete invasive weeds, and holds a specific cultural association with Māori. As a member of the Fabaceae (legume) family, the Carmichaelia genus has nitrogen-fixing traits. The roots of C. australis form a symbiotic relationship with Rhizobia bacteria. This relationship lets the shrub capture nitrogen from the atmosphere, which is stored in root nodules; the bacteria make this nitrogen available to the surrounding soil, increasing overall soil fertility. Competition with weeds is a major limiting factor for Carmichaelia australis. This species has not adapted to outcompete introduced weeds, so it is often suppressed when weeds grow in large abundances. Māori, the indigenous tangata whenua of New Zealand, view people as integrated with rather than separate from ecosystems, a perspective reflected in their language describing native plants and animals. In Māori culture, Carmichaelia australis is compared to a human, and associated with the saying E rito koe ki te taunoka, which translates to comparing a bad man to this species and notes it has no heart. This association indicates a historical negative perception of the species by tangata whenua, and highlights that relationships between tangata whenua and native flora are not always positive.
