New flora discovered for Te Tauihu

There are over 90 species of Coprosma throughout the Pacific and Australasia, half of which can be found in New Zealand. But there was one in particular that ecologist Rowan Hindmarsh-Walls wasn't expecting to find on a recent stroll through Brown River Scenic Reserve — Coprosma pedicellata.

He was on an organised trip with the Nelson Botanical Society, and as he observed the flora of the reserve's oxbow wetland, his eyes locked onto a single sapling tucked in amongst the other Coprosmas. It reminded him of the Coprosma pedicellata that he become familiar with while working around wetlands in Southland and eastern Fiordland with the Department of Conservation.

“I thought to myself, 'That looks very much like Coprosma pedicellata, but it shouldn’t be here.' My heart started to race in what can only be described as plant-induced wild excitement," Rowan said.

Before this discovery, there were no known populations of this wetland species in Te Tauihu. In fact, the closest known populations in the South Island were thought to be in North Canterbury, south of Amuri Range. This Coprosma is at-risk declining, which is one step away from a threatened status. It's often restricted to the margins of small oxbow lakes and ponds, or former stream channels. According to Rowan, the species is a relic of a bygone era.

"The current scattered distribution is presumed, in part, to be an artefact of major fragmentation and decimation of the species habitat," he said. "Most of these habitats — old flood channels, oxbow wetlands in forest, and shrubland in lowland valley systems — have been systematically cleared and drained for farming in the past 150 years."

Brown River Scenic Reserve is what Rowan describes as a "small but fairly well-preserved piece of lowland podocarp and beech forest, surrounded by exotic forestry and farmland on all sides." The reserve is one of a handful of smaller reserves in Rai Valley that's managed by Department of Conservation. It’s also where Forest & Bird have previously found pekapeka (long-tailed bats) in its years of monitoring this threatened native species in the Te Hoiere catchment. Through Te Hoiere Project over the past year, Forest & Bird expanded its predator monitoring to this reserve with 20 tracking tunnels to better understand pekapeka predators in the area, namely rats.

"Being next to a highway, the reserve has been extensively botanised, so the last thing on my mind was looking for anything profoundly new," Rowan said.

Rowan is currently based in Renwick, as a supervisor for DOC's biodiversity team in South Marlborough.Below is an edited excerpt from his April 2024 article about the discovery in Trilepidea, the monthly newsletter for the New Zealand Plant Conservation Network.

Notes on the population at Brown River Scenic Reserve

Population size and structure

Nine sub-populations of the species were found at the site, each associated with a different piece of flood channel or wetland. There are likely to be more sub-populations present, as the entire area of suitable habitat was not systematically searched. In total, 106 adult plants, 230 saplings, and many thousands of seedlings were counted across the entire site.

All plants were healthy, with no obvious disease. Most adult plants had at least some fruit on them, with a few plants fruiting plentifully. There were a number of very large old individuals, the largest being approximately 7 metres tall with a trunk diameter of approximately 20 cm near the base.

Threats

There were a number of weed species present within the forest at the site that would likely have a significant negative impact on the Coprosma pedicellata population if allowed to expand. These included: cherry laurel (Prunus laurocerasus), Old Man’s Beard (Clematis vitalba), Wandering Willy (Tradescantia fluminensis), and Blackberry (Rubus sp.).

Wild pig rooting was also noted at one of the sites, and pigs could have an impact on recruitment of the species if not managed appropriately.

Myrtle rust was present in abundance in the area, on both Ramarama and Rōhutu. As these species make up a notable portion of the sub-canopy at some sites, if myrtle rust kills many of the adult Lophomyrtus trees in the area, the forest structure could be altered to a point that minorly impacts the Coprosma.