An emblem of weatherworn and salt-burnt coastlines, endemic kānuka rise above lowland scrub and lean away from exposed bluffs, their architectural forms bearing the signature of their environment.
Inhabiting both islands, the most common species Kunzea robusta further occupies regenerating forest, montane forest, and ultramafic scrubland (magnesium and iron-rich substrate). It is able to grow upwards of 30m and its trunk can grow 1m in diameter. Initially growing erect, the trunk develops an undulating quality. A distinct branching pattern characterises Kunzea's dynamic form as numerous fine limbs appear to tangle around each other, awkwardly holding a mass of foliage that forms an almost horizontal canopy.
Bowed over the land, their asymmetrical growth and diminutive foliage tell the history of the wind and salt that have shaped them. More than scrub, these trees are sculpture exhibiting the Krummholz effect, an environmental reshaping of trees. The German word describes deformed or stunted vegetation found in subantarctic-subalpine- and coastal- landscapes, shaped by continual exposure to fierce wind and cold conditions. Often it expresses the unilateral habit and almost horizontal postures exhibited by these trees.
A similar species, though smaller and more graceful in appearance, Kunzea ericoides is common in like habitats in the South Island only- north of the Buller and Wairau Rivers in north-west Nelson. Despite this, nursery stock in outer regions may bear this name, but misidentify the species. This is because all plants but a low-growing Great Barrier Island form (K. sinclairii) were recognized as K. ericoides until 2014, when Auckland botanist Peter de Lange identified 10 separate species in the genus Kunzea. It was not until 1983 that Kunzea was even declassified as Leptospermum, a misguided legacy that endured taxonomic revisions since French botanist Achille Richard collected and formally described the species (as L. ericoides) in 1832. Following additional misclassification in the 1980s, Kunzea was maligned as an Australian weed and removed in offensive swaths through the 90s. Population decline persists perhaps not due to miseducation but rather growing pastoralism.
Indigenous Māori had, however, recognized some of this diversity in their different growth forms and wood properties. Importantly, iwi refer to Kunzea as mānuka and Leptospermum as kahikatoa. More distinctly, Kunzea robusta may be referred to as mānuka, manuka rauriki, rawirinui, or kopuka and Kunzea ericoides as manuoea, atitire, titire, and mānuka.
Still of concern is the casual identification of these morphologically similar genera- most easily done by discerning their leaves and flowers. Kunzea's small lanceolate leaves of bright olive-green emit a scent similar to Eucalyptus when crushed and are thought to have a soft texture. Mānuka leaves, though a similar shape, are darker, duller and have a bite when stroked. Their flowers are similarly formed with white petals and a red centre from which sprightly stamens protrude, but are notably borne in corymbiform clusters at the terminal ends of Kunzea's characteristic branches. They are also significantly smaller, no more than 1cm wide, and bloom from late spring-early summer. The subsequent small dry capsules generally open in the autumn, and frequently drop within a few months rather than persisting.
The abundant nectar and pollen feeds native bees and endemic arboreal geckos. Its bark, long, flaky, and tessellated, provides habitat and refuge for native spiders and other invertebrates. Resourceful tūī and kākā fossick through the long grey strips for these insects and splinter dead branches for burrowed grubs. Extensive stands of kānuka are also home to large numbers of forest birds, including threatened whitehead (pōpokatea) and fern bird (mātātā).
The leaves feed most of New Zealand's native stick insects and are the almost exclusive host plant for the two Clitarchus species. At night while sexually seeking, C. hookeri females emit volatile emissions abundant in terpenoids to signal mates (males are searchers). These are also the main compound found in their host plant, suggesting that the females sequester the scent of the kānuka and emit it at night when the plant cannot synthesize the compounds, thereby using it as concealment and a sexual signal.
Associations also exist with the endemic orchids Corybas cheesemanii (helmet orchid, Cheeseman's spider orchid) and Corybas cryptanthus (hidden spider orchid, icky) which grow in the moist semi-decomposed leaf litter at the base of the tree.
A pioneer species, kānuka (and mānuka) colonises impoverished, eroded, and exposed environments following ecological disturbance or the cessation of agricultural land use. Smaller than Kunzea robusta, Leptospermum grow below the kānuka's dominant canopy. This protective cover shelters a nurse crop of podocarps and broadleaved species like kauri, rimu and other species, increasing biodiversity. A productive microbiome of fungi and other soil micro-organisms benefits the emerging plants as well. Recent research even suggests kānuka compounds have a negative effect on kauri dieback spores. The dominant kānuka forest ultimately diversifies and cedes to a mixed forest in natural vegetative succession, typically over one hundred or more years.
Although Kunzea robusta, K. ericoides, and K. serotina (a montane species) are not formally threatened, the other species in this valuable genus are at risk- and all can be vulnerable to habitat loss, disturbing the fragile ecological processes that interact in regenerating forest.
