By Nic Rawlence 17/03/2021

I’m deep in the middle of the Kā Tiritiri o te Moana Southern Alps with Michael Knapp collecting beech leaves and ripping apart rotting logs on the hunt for giant collembola. Some 17 years later, these precious beech samples would allow Michael and I to answer one of the longest-running debates in New Zealand botany.

When Polynesians arrived on that fateful day in Aotearoa New Zealand some 750 years ago, around 80% of the country was covered in large tracts of forest that would have made an Ent proud. However, wind back the clock to the height of the last Ice Age 19,000-29,000 years ago and we enter an alien world. Sea levels were 120 metres lower, connecting the three main islands of New Zealand – you could walk from Tāmaki Makaurau Auckland to Ōtepoti Dunedin without having to take the Interisland Ferry across Raukawa Moana Cook Strait. The Lake Taupō super volcano had just erupted burying most of Te Ika-a-Māui North Island under ash, while at the same time the Southern Alps were covered in a vast ice sheet. Forests became climate refugees and retreated to warmer northern areas of both main islands, and White Walkers roamed the land…nope, sorry, that’s Game of Thrones.

As the climate warmed after the last Ice Age, forests expanded in a wave out of these northern refugia to recolonise their former home. This cycle repeated itself over numerous cold glacial and warm interglacial periods throughout the past 2.5 million years.

But here’s where the mystery starts. While forests, in general, are widespread across Aotearoa in the warm interglacial we currently find ourselves in, tawai southern beech forest (whose five Nothofagus species make up nearly half of New Zealand’s forest) has a remarkably disjunct distribution. The most famous is the ‘beech gap’ across Te Wai Pounamu South Island right through glacier country. Other gaps can be found on Rakiura Stewart Island, and in the Manawatu and Taranaki in the North Island.

Adding to the mystery, there is also tantalising evidence that suggests not all forests retreated to warmer northern climes when the Ice Age hit. Low levels of fossil pollen in sediments from southern Aotearoa hint that beech forest survived the long winter in small pockets in situ, like hardy locals staying put, only to recolonise areas when the climate warmed – indeed this is seen as a rapid increase in beech pollen in sediments left by retreating glaciers. Similar trails of breadcrumbs are seen in southern rata, where fossil pollen suggests in situ refugia contrary to genetic data that argues for post-glacial recolonization from northern areas.

With these intriguing mysteries in mind, Michael and I assembled the Scooby gang, with researchers from Otago University, The University of Auckland, and Manaaki Whenua Landcare Research. We hoped that we could use variable pieces of DNA from silver beech (Nothofagus menziesii), that record their biological heritage like the pages of a book, to test whether Aotearoa’s beech forests were climate refugees or hardy locals. What we discovered would challenge how we thought our forests responded to climate change.

A technicolor dream coat of genetic diversity in silver beech. Figure from Rawlence et al. (2020).

The stronghold of genetic diversity of silver beech, with six lineages, is the northern South Island, indicating the survival of huge tracts of Ent friendly forest during the height of the Ice Age. In contrast, in southern Te Wai Pounamu, south of the beech gap, there is a single bottlenecked lineage. Likewise, single equally bottlenecked lineages can be found in the southern, and northern Te Ika-a-Māui, separated by the Manawatu beech gap. What surprised us is that all three of these isolated lineages were not present in the silver beech stronghold.

The genetic landscape of silver beech we had reconstructed strongly suggested that rather than becoming climate refugees, sticking with the locals and staying put in small isolated stands was the norm, riding out unfavourable climatic conditions until your luck turned. If the genetic lineages in the southern parts of both main islands had also been found in their respective northern refugia, we would have assumed the climate refugee hypothesis was correct. Well, that’s science for you. The extreme climate and glaciations in Te Wai Pounamu probably caused the bottleneck in the southern lineage. In contrast, silver beech in the North Island preferred the cooler conditions of the Ice Age, flourishing in situ, with bottlenecks caused by unfavourable warm interglacial conditions.

While our Scooby gang didn’t manage to solve the mystery of what caused the ‘beech gaps’, it’s likely they exacerbated the genetic divergence between lineages in silver beech. The harsh climatic conditions and glaciations of the Ice Age probably caused the South Island beech gap, yet the origins of those in the North Island are a little more complicated. If you stepped back in time to three million years ago, Cook Strait didn’t exist. Instead, the southern North Island was joined to the South Island. The Manawatu Strait separated this ancient landmass from the rest of the North Island. This strait closed around one million years ago, with rapid mountain uplift providing favourable conditions for southern beech. Lowland areas probably acted as a barrier to beech dispersal – a no man’s land where beech shall not pass. It was only 500,000 years ago, in the blink of an eye on the geological timescale, that the formidable Cook Strait formed. Whatever the cause of the beech gaps, maybe some mysteries are not meant to be solved.

On a recent family holiday driving up the Te Tai o Poutini West Coast, I tell the kids about the natural history of New Zealand’s beech forests and how their present-day distribution is largely the result of Ice Age environmental changes. As we enter the beech gap, we leave the last of these hardy, true southern locals behind in their lonely outpost. The botany nerd in me is excitedly awaiting to see their stronghold further north, on the other side.