Environment – Tiri a treasure still in the making

Visitors’ first impressions after landing on Tiritiri Matangi Island are likely to be of lush bush of mainly native trees forming a closed canopy over the terrain, with access via well-formed tracks and boardwalks. 

They will also notice that there is lots of birdsong that sounds quite different from the mainland. Those who take a guided walk will hear about the history of the island, how the once farmed land was replanted with trees grown from locally collected seed, how the kiore were killed-off by a poison drop and how a wide range of threatened native birds, reptiles and invertebrates were introduced and have thrived in their new home. 

They may also hear about our efforts to keep out invasive mammalian predators and the constant war on weeds.

And while the island is surely impressive, and a testament to inspired leadership and the hard work of thousands of willing volunteers, they may not realise that the bush at about 30 to 40 years old is still far from mature. To compare it with mature bush, take a stroll through Waterfall Gully in Shakespear Regional Park or, even better, visit Bushy Park/Tarapuruhi where you will see massive trees festooned with epiphytes and surrounded by luxuriant vegetation all the way to ground level. At around one third of the area of Tiritiri, Bushy Park supports around the same number of tīeke/saddleback as Tiritiri does.

The planting programme on the island has greatly accelerated the change from farmland to bush but there is still a long way to go. This is most obvious underfoot, where the leaf litter which decomposes and forms new topsoil has so far only replaced a fraction of the soil lost during the farming period.

Other aspects of bush development are well underway as short-lived trees like cabbage tree and coprosmas have died and fallen, eventually being broken down by fungi and invertebrates. Last year’s Cyclone Gabrielle also brought down trees, broke large branches and ripped the canopy clear of leaves. It was sad to see so much damage, but the outcome is more rotting timber, more leaf litter and more light reaching the forest floor where tree seedlings, previously starved of light, are growing rapidly and will help to increase the complexity and quality of the bush.

The Supporters of Tiritiri Matangi and the Department of Conservation have worked hard for what is already an amazing outcome. There is more that they can and will do in the coming years but for some things – like a deep friable topsoil where hundreds of thousands of burrow-nesting seabirds can excavate their nests – we will just have to be patient.

John Stewart, previously our SOSSI columnist, has been volunteering on Tiritiri Matangi for around 14 years and has switched to writing this new column. Initially he joined a small team on the island monitoring nesting kōkako, which he is still doing although the team is larger now. He works on projects related to Tiritiri’s land and seabirds and recently started citizen science projects with volunteers on plant phenology and bird diets. John was chair of the Supporters of Tiritiri Matangi for three years and is now a member of the biodiversity sub-committee.