This month’s plant is a rare, ‘relict’ (meaning a remnant of a formerly widely distributed group) species that was once common in the Auckland and upper North Island regions. Ewekuri, known as the large-leaved milk tree or Streblus banksii, is a smaller broadleaf tree species that grows to around 15m tall. They are known as ‘milk’ trees due to their milky sap that is rumoured to have once been used as a milk replacement by early colonists. We’re not sure it would make the nicest addition to a cup of tea, though! Female ewekuri produce an abundance of bright red fruit, which are beloved by many native birds including kererū, tūī, tauhou, korimako, and kōkako. Ewekuri are similar-looking (from a distance) to a variety of other small species that often inhabit the same niche, such as some Coprosmas, putaputawētā, and māpou. This means they often blend into the background of the forest and are disregarded, and thus seldom learnt about or studied. The natural distribution of ewekuri is from Cape Reinga to the Marlborough Sounds, but today they are very rare on the mainland due to rat and possum browsing, and instead pockets of them are mostly found on offshore, pest-free islands such as in the Hauraki Gulf.
Ewekuri are dioecious, meaning that individuals are either female (fruit-producing) or male (pollen-producing). This can however cause some issues when the population is scarce - as they rely on the wind to carry their pollen, having skewed sex ratios or having males and females separated by long distances can greatly reduce their reproductive capacity and thus the numbers of seedlings in the forest as the next generation. There have been some observations of apomixis (asexual reproduction without fertilisation in the female trees) with viable fruits produced, but the resulting plants are essentially clones of the mother tree, so future populations will have low genetic diversity. High amounts of genetic diversity is important in plant and animal populations to allow a variety of characteristics to be available in the face of new threats and changing conditions, and thus security for the survival of the species in the long term.
Ewekuri are a member of the Moraceae family, also known as the ‘fig’ family, and are one of only three native Aotearoa species in that family, along with tūrepo/the small-leaved milk tree (Streblus heterophyllus) and the Three Kings Milk Tree (Streblus smithii). Tūrepo is the closest relative of ewekuri and can be found in lowland and sometimes coastal areas of the North and South Islands. Tūrepo is known to hybridise with ewekuri and many examples of these hybrids exist particularly around disturbed habitats (i.e. those that have been modified by humans). It’s possible that a process known as genetic introgression may be occurring between the species, which is the movement of genes from one species into the gene pool of another. Introgression can threaten plant species by ‘merging’ species together and reducing the genetic integrity of one over the other. There is no evidence yet that this is occurring between ewekuri and tūrepo, or that it may be harmful. Hybrids can be spotted in local reserves, such as Smith’s Bush, Stancich Reserve, and Shakespear Regional Park. Tūrepo can be distinguished from ewekuri as juveniles tend to have leaf lobes (indentations) and much smaller leaves overall. Hybrids appear as a middle-ground of features of the two, sometimes with many leaf lobes or middle-sized leaves.
The main threats to ewekuri are deforestation, invasive pests, and skewed sex ratios/lack of gene flow between populations that can both cause limited reproduction and inbreeding depression. Deforestation and lack of gene flow threatens most rare plant species, and can be limited by (responsibly) planting more of these rare species in reserves and backyards that they would normally inhabit, as well as lobbying local councils and government for greater protection of trees overall. Ewekuri are also very vulnerable to rat and possum browse; their fruit, seedlings and even adult trees’ bark is highly palatable, so in untrapped sites they are often entirely wiped out.
At Pest Free Kaipātiki, we focus on reforestation in reserves, lobbying of local government bodies for greater tree protection, and invasive pest control fiercely to ensure that our ngahere are diverse and protected for future generations. Join us to help out nature (including tree species like ewekuri) by trapping in your backyard or by getting involved in plantings and working bees at reserves near you!