Extraordinary Pohutukawa Trees

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Humans aren’t the only living creatures who modify entire ecosystems. Ever since we have been coming to New Zealand, we have been charmed and amazed by the Pohutukawa trees Metrosideros excelsa that grace the shorelines like enormous, billowing, green and red cumulus clouds that hold onto the cliffs with many octopus tentacles.

During our return visit to New Zealand in 2004, I visited Rangitoto Island then Tiri Tiri Matangi Island in the Hauraki Gulf at Auckland. What I saw there amazed and delighted me. I learned that the Pohutukawas, over hundreds of years can single-handedly convert volcanic islands to forested islands filled with birds, geckos and more. The same process has been performed in the Hawaiian Islands by five related Metrosideros species which are endemic to Hawaii.

In 2004 I wrote the following article to document my new understanding of the Pohutukawas and their extraordinary role in establishing the ecosystems of the Northern New Zealand islands and coasts. When we returned to New Zealand in 2009 we learned that there are now numerous organizations dedicated to replanting native vegetation on the Bay of Islands, then re-introducing native birds which once lived here. We humans can now restore the islands to the lovely forests that the Pohutukawa trees made possible.

From Volcanoes to Pohutukawa Forests in New Zealand
By Dorothy Darden
Photos by Dorothy Darden

I am standing on the rim of a volcano which last erupted 600 years ago, looking down into the volcano cone which is filled by a beautiful forest of pohutukawa trees. Rangitoto Island is located only five nautical miles to the northeast of downtown Auckland, but unlike most of the other islands near Auckland, it was never deforested and farmed by Maori and European people because there is practically no soil. Most of the island is jumbled, black a’a lava rocks. So how on earth can there be a forest here?

Known to the locals as “Rangi”, the island is perfectly round, about 3-1/2 nautical miles in diameter. The shape of the cone is very similar to the classic shape of Mt. Fuji in Japan. The height of the cone is 259 meters above sea level. In the early 1900’s, people from Auckland build small homes fronting several of the bays, but made a very small impact on the island. These little houses are now being restored as historic sites. There is one small house near the ferry dock where a park ranger lives.

During my tour of the island I was witnessing natural forest regeneration unlike anything I have ever seen or read about. The island appeared dark and menacing as we sailed past it entering Auckland Harbour aboard ADAGIO several days ago. The dark color is due to areas of bare lava rock that still remain. This means that we can still see the way in which the pohutukawa trees are bringing back the forest.

The pohutukawa tree Metrosideros excelsa is one of our favorite New Zealand native trees. By the time we arrived in New Zealand, most had been cut down, except for the giant specimens which cling to the rocky coastline, some estimated to be more than 800 years old. They are the only New Zealand tree that can lean way out over the water, clinging to bare rock by their octopus tentacle-like branches and roots, providing a brilliant display of red blossoms at Christmas time during the New Zealand summer. Many New Zealanders call them the New Zealand Christmas tree and plant these trees in their gardens and in parks. A large pohutukawa blooms during the San Francisco summer on Alcatraz Island, and many have been planted along some of the Bay Area streets. But we have assumed that these trees had always only grown clinging to the rocky shoreline.

The problem we have in understanding natural systems is that most of them have been greatly modified or destroyed. Most plants and animals that we see have been marginalized to places where people cannot farm or live, or to parks, botanical gardens and zoos. I found it exciting and wondrous to discover the vital role that the pohutukawa trees have evolved to play in the regeneration of volcanic islands.

From the ferry landing I boarded a wagon hauled by a tractor, driven by a park ranger who gave us a running commentary of what we were seeing, as he carried us along a black road up to the stairway leading to the island summit. I have never been to Hawaii, so it took me a while to realize that the huge jumbles of black lava rocks were not devoid of vegetation because they had been bulldozed or poisoned. They had not yet been vegetated. The vegetation that we did see grew in “islands” in a “sea” of lava scoria, with a pohutukawa tree in the center of each island.

After the lava had cooled, the only seed that could germinate among the a’a rubble was a pohutukawa tree seed. When I was gardening in New Zealand, and learning about the native trees, I learned that pohutukawa tree seeds will not grow if planted in your well prepared loamy garden soil, but must germinate in the crack of a rock! I thought that this was a puzzling adaptation, but seeing the rocky shorelines of the Bay of Islands, I assumed that this was its normal habitat. But better than that, the pohutukawa evolved to be the first tree in the forest succession that restored vegetation to the many volcanic scoria cones and volcanic islands of New Zealand.

Most of the islands that we sail among between Auckland and the Bay of Islands, that are now mostly pasture land with a fringe of pohutukawa trees clinging to the shoreline cliffs, were once fully forested with pohutukawa trees. How magnificent they must have appeared when they were solid red with their Christmas blooms.

The pohutukawa flower produces hundreds of tiny seeds, each connected at one end to a thin silver ribbon. The wind blows these ribbons, carrying the seeds far and wide. At the Auckland Museum library, I read the instructions used by volunteers to germinate pohutukawa seeds for planting in forest restoration areas: “Sprinkle the seeds on the top of a mixture of scoria sand and sieved peat, water with sea water and cover with a plastic bag.”

After a seed has germinated in the crack of a rock, it develops a “net” of roots for trapping debris and water and eventually for binding the rocks to hold its massive size and weight along the shoreline cliffs – and in the jumble of scoria rocks on a volcanic island.

For five hours I walked from the top to the bottom of the island, along the trails under the forest canopy and over the rocks, among moist, iridescent green ferny glens and across dry mosses. I noted the stages of forest regeneration that is still occurring. I took photos of the tiny pohutukawa tree which was growing in isolation, in the center of a field of black lava rock. I call this “stage one”.

Next I photographed a medium sized pohutukawa tree under the shade of which was growing a small large-leafed puka tree and a perching lily plant which usually grows as an epiphyte on the branch of a tree. “Stage two”. This “island” of vegetation was still isolated, but it was growing, upwards and outwards.

Not far away was a larger “island”. The pohutukawa tree was now fifteen feet tall. It had grown many branches which it spread out to provide the beginnings of a humic soil with its leaf litter and shade for many smaller plants, and even some other species of trees. This I called “stage three”.

In the distance I could see that many islands of vegetation had spread towards each other and had merged into a forest. I called this “stage four”.

The following is the text from a Dept. of Conservation interpretation sign: “Here on Rangitoto’s a’a lava fields, succession leads to the creation of islands of vegetation and soil – clusters of trees and plants which grow together, usually under the spread of a pohutukawa tree. You are standing right in front of a large vegetation island. An “island” usually extends as far as the outer and upper branches of the large pohutukawa. This island will grow and expand to join with neighboring islands. Eventually, in many hundreds of years, Rangitoto will be completely clothed with forest.”

As I walked through the most mature areas of the forest I could still see the original “island” structure of a large assortment of plants grouped around a central pohutukawa tree. On the rainy side of the island, and in the ravines, the understory was a carpet of kidney ferns which have the adaptation of shriveling up when the weather is dry, but quickly greening and expanding after a rainfall. Many types of mosses, lichens, liverworts and other ferns grew in the cool shade.

In the dryer areas, no ferns were growing, but several types of dry mosses carpeted the rocky ground. The park ranger told us that of the 400 or so native plant species which grow on the mainland, approximately 200 plant species have made it to Rangitoto Island. And they are still arriving, and the pohutukawa is providing the habitat for them to become part of the forest.

Several plants are showing “behavior” on Rangitoto that is different from the way they live elsewhere in New Zealand. One example is the tufted lily Collospermum, which usually grows as an epiphyte on the branch of a tree, happily grows in great numbers on the lavarocks in the shade of the pohutukawas.

Another is the mangrove. These trees have colonized the rocky beaches in the coves of the island. Mangroves are known for living in a muddy habitat, but they seem happy at Rangitoto with their roots wrapped around volcanic rocks. Gradually, silt is accumulating around their roots, and mud will form over time. Small pebbles have become sand where bivalves are burrowing. The mangrove habitat is evolving, with the mangrove trees playing the same role in creating an ecosystem along the shoreline as the pohutukawas are creating the forest ecosystem on the land.

The following day I visited a very different island which is named Tiritiri Matangi, and called by the locals, “The Singing Island,” and referred to as “Tiri”. This is not a volcanic island, and only 1-1/2 nautical miles long and 3/4 nautical mile wide. The highest point is 79 meters above sea level. There are buildings below the light house and a few small houses along the shore. Twenty years ago the island was covered by pasture land that had been planted after the original settlers cut down the forest. The only pohutukawa trees that remained were very large specimens, some of which are at least 800 years old. The government would not allow the farmers to cut down these magnificent trees, which spread their boughs high up into the sky and down towards the beaches, all along the island’s rocky shoreline.

The Friends of Tiritiri Matangi raised the funds to germinate from seed and plant 280,000 trees on this island between 1984 and 1994. As the forest matured, many of New Zealand’s native birds returned to the island, and several endangered species which have been captive bred were released on the island. Two years ago sixty of the very rare Tuataras, which lived at the same time as the dinosaurs, were released on the island. At one time Tuataras lived throughout the North Island. This forest planted by volunteers is mature enough to provide for many birds, which were singing constantly as we walked along the pathways under the trees.

Above the cliffs the trail took us past three nesting boxes where little blue penguins were moulting. They crawl into their burrows and stay there for two weeks or more waiting for their new feathers to develop. We could lift the lid on each box and see the penguin through a glass roof over its burrow. Not far along the trail we found what the park rangers call “the petrel station”. She showed us several holes where petrels have burrowed into the soil for nesting, and pointed out that in the hills below us was a network of petrel burrows. Petrels cannot burrow into the lava rocks of Rangitoto. She said that there is competition between the penguins and the petrels for some of the burrows. She has been amazed that the little blue penguins can make their way up the high cliffs to the burrows, and they have even been seen at the top of the island near the lighthouse.

Along the beach, under a gigantic pohutukawa tree, kingfishers are nesting in holes they have made in the bank. The guide told us that the kingfishers make the holes by flying directly into the bank many times, using their strong beak to break through the soil until the hole is large enough for them to perch on the edge and continue chiseling out their burrow.

By comparison, fewer species of native birds are found on Rangitoto, because the lava rock is inhospitable to the many species of burrowing and ground living birds that are found on Tiri.

On Tiri, with the exception of the ravines where the farmers had left some of the native forests, the plant community is still young and simple. On Rangitoto I could see dozens of species of plants per square foot of forest floor, but on Tiritiri Matangi it will be a hundred years before the complexities and biodiversity of a mature forest will have developed.

Some forest succession processes result in a simpler forest with low biodiversity, such as the Hemlock and rhododendron forests of the east coast of the US. Tropical rain forests have developed high biodiversity over millions of years.

For many thousands of years, pohutukawa flowers have been pollinated mainly by geckos. Introduced predators have decimated the gecko populations, and bees have become the primary pollinators of the pohutukawas. This provides us with lovely pohutukawa honey, however scientists believe that the trees will evolve shorter flowers, as the bees selectively pollinate those flowers which are easiest for them to access. I photographed bees in a pohutukawa flower. I did not see any geckos.

The pohutukawa tree is responsible for recreating forest ecosystems after volcanic cataclysms. This tree has grown in our esteem as we have learned more about how it has evolved to play this vital ecological role.

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