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Tropical forests flee uphill to escape heat

The tropical forests of the Andes are responding to climate change. But a tree can climb only so far before it has nowhere to go.

LONDON, 22 November, 2018 − Tropical forests are racing uphill to escape global warming. Some of them may lose the race.

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A meticulous and sustained study of nearly 200 plots of forest in Colombia, Ecuador, Peru and northern Argentina has found that where they can, tropical species are moving uphill as the thermometer rises. But there is a problem: can a species that flourished in one ecosystem in the Andes and Amazon migrate and colonise another at higher altitude?

A new study in the journal Nature finds that some of them cannot. “Andean forests must be added to the growing list of ecosystems and species that lack the ability to quickly and cohesively respond to climate change and thus face high risk of extinction, biodiversity loss and functional collapse,” they conclude.

Plants and animals in mountain communities everywhere in the temperate world seem to be on the move: many of the studies however focus on observations of selected species in one country or mountain zone, or even on one mountain.

Wider perspective

Belén Fabrique and Kenneth Feeley of the University of Miami and colleagues went looking for the big picture. They selected 186 closely-monitored tracts of forest in what scientists call the Tropical Andes Biodiversity Hotspot, at altitudes of from 300 to 3,000 metres. These forest plots together are home to 120 different plant families divided into 528 genera and 2,024 tree named species, including palms, tree ferns and lianas.

They then looked for a way to measure change in a mix of such diversity − in effect, a local ecosystem − and selected a measure called the community temperature index, already used to monitor shifts in bird and butterfly populations elsewhere. Since most of the plots had been surveyed each year over a 20-year period, they had a way of detecting and tracking change.

Temperate species of trees are adapted to big seasonal shifts in temperature. Trees in the lowland tropics are not. Tropical trees that migrate uphill run the risk of encountering an environmental roadblock, a shift in the ecosystem.

“In the Andes, the ecosystems can change very fast and very dramatically, for example from sunny and dry premontane forests to sopping wet cloud forests. These changes, called ecotones, appear to be blocking species migrations,” said Belén Fabrique, who designed the study.

“The faster climate change happens, the faster we will lose our tropical forests, which means that climate change will happen even faster”

“These ecotone barriers make it harder for plants to relocate their populations – and if they can’t relocate, they will go extinct.”

In response to climate change – driven by ever-higher emissions of greenhouse gases from the combustion of fossil fuels – the researchers confirmed that the thermophilic or heat-loving species were shifting to higher ground, while the abundance of the species adapted to cool conditions was declining. But the rate of change in the mix of these forest plots was not uniform: some Andean species were being driven out as rainfall and cloud cover conditions became intolerable.

“Thermophilisation is a mouthful of a word but it means that forests are becoming more heat-loving over time because as the world warms up, the species that prefer cold are being kicked out or are dying off and the heat-loving species are moving up and taking their place,” said Professor Feeley.

“Everything is moving up the mountain, so the species near the tops of the mountains are running out of places to go and may soon face the risk of mountain-top extinction.”

Taxonomical logjam

The next step is to try to work out how climate operates on specific Andean plants: a challenge because many of them have yet to be identified and named. Only then can researchers work out the ecological consequences of their loss.

The irony is that forest ecosystems play a key role in moderating climate change.

“Tropical forests are one of the most important players in the world’s global carbon cycle. They slow down climate change by taking a lot of carbon out of the atmosphere and putting it into their growth,” Professor Feeley said.

“So the faster climate change happens, the faster we will lose our tropical forests, which means that climate change will happen even faster.” – Climate News Network