A survey of the mountains of Papua New Guinea (PNG) has revealed forests full of trees more than twice as high as are seen at similar elevations in other mountain ranges, challenging the way we think about high altitude ecosystems.

“Current thinking is that tall mountains make small trees,” said Dr Michelle Venter of the University of Northern British Columbia in a statement. Although there were known to be exceptions to this, they were thought to be sparse. “However, we recorded more than 15 tree families with individuals growing 30-40 meters tall at extreme altitudes, which brings this assumption into question,” Venter added.

Before her move to Canada's north, Venter was based in a very different climate at James Cook University in far north Queensland. From there she measured the trees in 193 forest plots from PNG's coastal lowlands to heights of 3,100 meters (10,000 feet). Instead of the anticipated decline as she moved to higher ground, Venter found tall trees all the way up, and in fact recorded that the biomass (weight of living things) peaked sharply above 2,200 meters (7,200 feet).

In Global Change Biology, Venter described what she found as some of the most carbon-rich forests at high altitudes anywhere. Venter pointed out the average temperatures in these forests, and the high rainfall, make them climatically similar to the homes of the world's tallest trees – the North American west coast and Tasmania.

The tallest trees Venter measured, the 41-meters-high (135 feet) southern beech (Nothofagus starkenborghii), pale into insignificance height-wise compared to coastal redwoods (Sequoia sempervirens) reaching 115.8 meters (380 feet) or a 99.82-meters (327.5 feet) mountain ash (Eucalptytus regnans). However, what these forests lose in sheer height, they make up in diversity – the tallest forests of the world usually involve just one very big species, instead of more than a dozen.

Co-author Dr John Dwyer of the University of Queensland told IFLScience it is the combination of latitude and altitude that produces this richness. “The same elevations further away from the equator would be way too cold to support such large trees, e.g. Mt Kosciusko in Australia is 2,300 meters (7,500 feet) above sea level and even tough trees like snow gums can’t grow right near the summit.”

Dwyer added that the most remarkable aspect to the find was the number of tall trees found. Although the occasional specimen had been noticed before, no one had done the work to discover how common they are. He said there could be many reasons why these forests have not produced specimens as awe-inspiring as those in low-altitude temperate forests, including frequent landslides, and a genetic incapacity of the species. “In New Zealand, the climate is ideal for growing tall trees, but the native tree species have not evolved to take advantage of this,” he pointed out.

The finding raises hopes that some forests in other parts of the world may be able to move to higher altitudes, rather than dying out, as the whole world becomes more tropical. However, Dwyer told IFLScience we don't know enough to be sure if this will happen. “Growing tall comes at a price. Tall trees need to invest in strong trunks to support their enormous weight. It's also difficult to move water from the roots all the way to the canopy,” he said. This means large trees “may be especially vulnerable to small changes in temperature that make it more difficult to maintain healthy functioning. We are not sure how the existing trees [at lower elevations] will respond.” Moreover, as long-lived species, even trees suited to higher altitudes may struggle to colonize new territory quickly enough.” 

Senior author Professor Michael Bird of James Cook University said, “Believe it or not, why and how trees grow large is still a topic for investigation and reasons for the persistence of large old trees are still not clearly known," said senior author Professor Michael Bird of James Cook University. "Large trees are susceptible to many interacting threats, from disease to climate change, and because of their size and age, they don’t adapt well to rapidly changing, human-modified environments.”