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Giant sloths: killed by rainy weather? Kamraman/Wikimedia Commons, CC BY-SA

Wet weather at the end of the last ice age appears to have helped drive the ecosystems of large grazing animals, such as mammoths and giant sloths, extinct across vast swathes of Eurasia and the Americas, according to our new research.

The study, published in Nature Ecology and Evolution today, shows that landscapes in many regions became suddenly wetter between 11,000 and 15,000 years ago, turning grasslands into peat bogs and forest, and ushering in the demise of many megafaunal species.

By examining the bone chemistry of megafauna fossils from Eurasia, North America and South America over the time leading up to the extinction, we found that all three continents experienced the same dramatic increase in moisture. This would have rapidly altered the grassland ecosystems that once covered a third of the globe.

The period after the world thawed from the most recent ice age is already very well studied, thanks largely to the tonnes of animal bones preserved in permafrost. The period is a goldmine for researchers – literally, given that many fossils were first found during gold prospecting operations.

Our work at the Australian Centre for Ancient DNA usually concerns genetic material from long-dead organisms. As a result, we have accrued a vast collection of bones from around the world during this period.

But we made our latest discovery by shifting our attention away from DNA and towards the nitrogen atoms preserved the fossils’ bone collagen.

Lead Author Tim Rabanus-Wallace hunts for megafaunal fossils in the Canadian permafrost in 2015. Julien Soubrier Chemical signatures

Nitrogen has two stable isotopes (atoms with the same number of protons but differing number of neutrons), called nitrogen-14 and nitrogen-15. Changes in environmental conditions can alter the ratio of these two isotopes in the soil. That, in turn, is reflected in the tissues of growing plants, and ultimately in the bones of the animals that eat those plants. In arid conditions, processes like evaporation preferentially remove the lighter nitrogen-14 from the soil. This contributes to a useful correlation seen in many grassland mammals: less nitrogen-14 in the bones means more moisture in the environment.

We studied 511 accurately dated bones, from species including bison, horses and llamas, and found that a pronounced spike in moisture occurred between 11,000 and 15,000 years ago, affecting grasslands in Europe, Siberia, North America, and South America.

Alan Cooper inspects ice age bones from the Yukon Palaeontology Program’s collection, Canada, 2015. Julien Soubrier

At the time of this moisture spike, dramatic changes were occurring on the landscapes. Giant, continent-sized ice sheets were collapsing and retreating, leaving lakes and rivers in their wake. Sea levels were rising, and altered wind and water currents were bringing rains to once-dry continental interiors.

The study shows that a peak in moisture occurred between the time of the ice sheets melting, and the invasion of new vegetation types such as peatlands (data shown from Canada and northern United States). http://nature.com/articles/doi:10.1038/s41559-017-0125

As a result, forests and peatlands were forming where grass, which specialises in dry environments, once dominated. Grasses are also specially adapted to tolerate grazing – in fact, they depend upon grazers to distribute nutrients and clear dead litter from the ground each season. Forest plants, on the other hand, produce toxic compounds specifically to deter herbivores. For decades, researchers have discussed the idea that the invading forests drove the grassland communities into collapse.

Our new study provides the crime scene’s smoking gun. Not only was moisture affecting the grassland mammals during the forest invasion and the subsequent extinctions, but this was happening right around the globe.

Extinction rethink

This discovery prompts a rethink on some of the key mysteries in the extinction event, such as the curious case of Africa. Many of Africa’s megafauna — elephants, wildebeest, hippopotamus, and so on — escaped the extinction events, and unlike their counterparts on other continents have survived to this day.

It has been argued that this is because African megafauna evolved alongside humans, and were naturally wary of human hunters. However, this argument cannot explain the pronounced phase of extinctions in Europe. Neanderthals have existed there for at least 200,000 years, while anatomically modern humans arrive around 43,000 years ago.

We suggest instead that the moisture-driven extinction hypothesis provides a much better explanation. Africa’s position astride the Equator means that its central forested monsoon belt has always been surrounded by continuous stretches of grassland, which graded into the deserts of the north and south. It was the persistence of these grasslands that allowed the local megafauna to survive relatively intact.

Our study may also offers insights into the question of how the current climate change might affect today’s ecosystems.

Understanding how climate changes affected ecosystems in the past is imperative to making informed predictions about how climate changes may influence ecosystems in the future. The consequences of human-induced global warming are often depicted using images of droughts and famines. But our discovery is a reminder that all rapid environmental changes — wet as well as dry — can cause dramatic changes in biological communities and ecosystems.

In this case, warming expressed itself not through parched drought but through centuries of persistent English drizzle, with rain, slush and grey skies. It seems like a rather unpleasant way to go.

Alan Cooper receives funding from the Australian Research Council

Matthew Wooller receives funding from US National Science Foundation

Tim Rabanus-Wallace does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond the academic appointment above.

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Trump’s top advisers were set to meet on Tuesday to provide the president with a recommendation ahead of a G7 meeting in May. However, a White House official said the meeting had been postponed due to conflicting schedules. It is unclear when it will now take place.

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Claxton, Norfolk At Thorpe Hall near Haddiscoe, 340 pairs of rooks once nested, but this spring there is not one

Assessing the rook population in the Yare valley has long been a favourite ritual of my springs. Since the nests are coarse bundles of sticks in the bare treetops it is easy to combine the serious census work with the season’s wider pleasures: the sounds of first chiffchaffs or blackcaps, the lemon wings of male brimstone butterflies, and the year’s first glamorous colour from primroses, marsh marigolds and walls of blackthorn blossom.

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Wollemia pine pollen cone. Wollemia pines (found in the wild only in Australia) are one of the most ancient tree species in the world, dating back 200 million years. Velela/Wikipedia

They say that trees live for thousands of years. Like many things that “they” say, there is a germ of truth in the saying (even though it is mostly false).

The vast majority of trees that burst forth from seeds dropped on the Australian continent die before reaching maturity, and in fact most die within a few years of germination.

But depending on how you define a tree, a very select few trees can live for an astoundingly long time.

What are the oldest trees?

If we define a “tree” as a single stemmed woody plant at least 2 metres tall, which is what most people would identify as a tree, then the oldest in Australia could be a Huon Pine (Lagarostrobos franklinii) in Tasmania, the oldest stem of which is up to 2,000 years old.

However, the Huon Pine is also a clonal life form – the above-ground stems share a common root stock. If that common root stock is considered to be the base of multi-trunked tree, then that tree could be as old as 11,000 years.

But if you accept a clonal life form as a tree, even that ancient Huon age pales into insignificance against the 43,000-year-old king’s holly (Lomatia tasmanica), also found in Tasmania.

King’s Holly, or Lomatia tasmanica, can form clones nearly 50,000 years old. Natalie Tapson/Flickr, CC BY-NC-SA

Once you accept that a common, genetically identical stock can define a tree, then the absolute “winner” for oldest tree (or the oldest clonal material belonging to a tree) must go to the Wollemi Pine (Wollemia nobilis). It may be more than 60 million years old.

The Wollemi pine clones itself, forming exact genetic copies. It was thought to be extinct until a tiny remnant population was discovered in Wollemi National Park in 1994. The trunk of the oldest above-ground component, known as the Bill Tree, is about 400-450 years old. But the pine sprouts multiple trunks, so the Bill Tree’s roots may be more than 1,000 years old.

There is also substantial evidence that the tree has been cloning itself and its unique genes ever since it disappeared from the fossil record more than 60 million years ago.

How do you date a tree?

If no humans were around to record the planting or germination of a tree, how can its age be determined? The trees themselves can help tell us their age, but not just by looking at their size. Big trees are not necessarily old trees - they might just be very healthy or fast-growing individuals.

A much more reliable way to determine age of a tree is through their wood and the science of dendrochronology (tree-ring dating).

Dendrochronology involves counting tree rings to date a tree. The wider the ring, the more water the tree absorbed in a given year. sheila miguez/flickr, CC BY-SA

Many trees lay down different types of cell wall material in response to seasonal patterns of light, temperature or moisture. Where the cell walls laid down at the beginning of the growth season look different to those laid down at the end of the season, rings of annual growth can be seen in cross-sections of the tree.

This map of growth patterns can also be cross-dated or correlated with major events like multi-year droughts or volcanic eruptions that spewed material into the atmosphere to be incorporated into the wood of the tree. But the cell walls are more than just calendars.

Why so old?

Individual tree stems can live for so long because of the structure of the wood and the tree’s defence mechanisms. The woody cell walls are very strong and resist breakage.

In fact, scientists have recently discovered that these walls contain a structure – nanocrystaline cellulose – that is currently the strongest known substance for its weight.

Wood can, however, be broken down by insects and fungi. Even though there is little nutrition or energy in wood, there is some – and there are plenty of organisms that will try and use it.

But trees are not defenceless, and can fight back with physical barriers or even chemical warfare. When one tree is attacked by these destructive forces, individuals may even signal to other trees to be aware and prepare their own defences to fight off death and decay.

The death of trees

So why don’t all trees live for centuries or millennia, and why do so many die before even reaching maturity?

Adult Wollemi pines in the wild. J.Plaza/Van Berkel Distributors

Seedlings and young trees may die because they have germinated in an area where there’s not enough water, nutrients or light to keep them alive as adults. Young trees also haven’t had much time to develop barriers or defences against other organisms and may be browsed or eaten to death.

Some trees simply fall prey to accidents: wind storms, fires or droughts. This is just as well, because there is a vast number of plants and animals – including humans – which rely on the wood and other components of these dead trees for their food and shelter.

But increasingly we may see trees dying because the environment is changing around them and they may not be able to cope. This is not just due to climate change; urban development and agricultural expansion, pollution and even too much fertiliser acting as a poison – even our most remote environments are subject to these changes.

But that doesn’t necessarily mean we will have no more very old trees. The Wollemi Pine’s genes have already survived over millions of years, multiple ice ages and warming periods and even the fall of the dinosaurs and rise of humans. And now, people have deliberately spread Wollemi Pine trees all around the world so they are living in a wide range of countries and climates, meaning that the risk of them all dying out is substantially reduced.

Maybe we can do the same for other trees, ensuring that trees will outlive us all.

Cris Brack is a member of the Institute of Foresters of Australia and Director National Arboretum, Canberra Foundation.

Matthew Brookhouse does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond the academic appointment above.