Gender bias seems behind vulnerable Black Poplar’s imbalance - but at least there’s cloning

In 2001, while travelling through Clarecastle, south of Ennis in Clare, Daniel Buckley caught sight of a tree in “Flynn’s Field”, opposite the national school. It was monumental, with imposing lopsided branches. Its gnarly bark was deep brown and covered in thick, knotted burrs and swollen fissures. It looked like a tree that belonged in a Grimms’ fairytale, not a field in a west of Ireland village.

What he had spotted was a 200-year-old Black Poplar (Populus nigra). Buckley, who was in the middle of his PhD dissertation on whiskered bats at the time, had never seen anything like it. What he saw in Flynn’s Field triggered an immediate fascination with this rare species, and he set off on a path to find out everything he could.

The natural home of the Black Poplar is a watery one; they’re found in floodplains and along the banks of rivers and lakes. But they’re a tree with specific needs, and water is just one of them. They like space and plenty of full sunlight, and will not tolerate shade or mixing too closely with other trees. Black Poplars cannot blend in; they are born to stand out.

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Buckley has spent years searching for these trees and he speaks of individual specimens with affectionate familiarity. There’s a particularly remarkable 150-year-old one in Spencer Harbour, along the northwestern side of Lough Allen, a lake on the River Shannon in Leitrim, which has a seven-metre girth and is on the national tree register for champion specimens. Black Poplars are either male or female; this one is a male, and he shows no signs of decline, Buckley says, despite his age.

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The Spencer Harbour tree got there after a remarkable journey. His parents were 15 years old before they became sexually active – that’s circa 165 years ago – and they needed to be near each other to reproduce. On these trees, just a few weeks before their shiny green leaves unfurl in April – when flooding is common – catkins emerge on the upper parts. Female catkins are lime-green and more prominent than males, which are attractive burnt auburn.

Roots will sprout directly from the tree, or fragments will be transported downstream, where they will settle along the bank. This way of reproducing is like a waiting strategy

When fertilised in June, the female catkins swell and produce bursts of more than two hundred off-white cotton-like seeds, which are disseminated by wind and water. The trees then look as if they’re shedding snow, and this has, historically, been to the dislike of some landowners and farmers, who have cut them down. This may be one reason why female trees are so rare (we don’t have figures for Ireland, but in the UK there are just 7,000 Black Poplars left; of these, just 700 are female).

The diminutive seeds have tufts of hair which help them travel, but to keep them lightweight they have little to no stored energy reserves. Once they’re shed, they need to quickly find a place to germinate, or else they will die. Moist soil recently cleared from retreating flood waters is ideal, and if they land here within 24 hours, they will take up enough water and bath in enough summer sunlight to grow.

If the conditions aren’t right – if the floods don’t recede in spring, or the tree becomes damaged – they have another way of reproducing as a safety mechanism for survival: they clone themselves. Roots will sprout directly from the tree, or fragments will be transported downstream, where they will settle along the bank. This way of reproducing is like a waiting strategy; it’s more favourable to have a clone out there than to sit and do nothing.

Since he first saw the Black Poplar in Clarecastle, Buckley has sought to find out more about the history of the species here. We know that its cousin, the Aspen (Populus tremula), is native and was found before Ireland separated from the rest of Europe during the last Ice Age. But the status of the Black Poplar is unresolved.

A few years ago, Buckley applied for a Praeger grant from the Royal Irish Academy to conduct genetic analysis on a portion of the 700 trees he had identified across Ireland. He wanted to know their genetic purity and what it could say about their uniqueness compared to those in Britain.

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He sent samples to a lab in Scotland, and the results he got back were revealing. The genetic makeup of some trees here is similar to those found in Britain, which suggests they may have been imported into Ireland during the 18th and 19th centuries, when Anglo-Irish estates were planted.

But the situation at Lough Allen is different. The trees here are genetically distinct from all other trees in Ireland and Britain, and unlike all other Black Poplars in the country, they are successful at sexual reproduction. This marks Lough Allen’s population as being of international importance, especially given that Black Poplar is one of Europe’s most threatened tree species.

Does it matter if Black Poplar is native or not? Perhaps not. It’s extremely rare and isn’t invasive or harmful to other species. In fact, given its ability to hold back and store water, it may have a future in managing flood risk and restoring river banks. For this reason, Buckley worked with dairy and tillage farmers in northeast Cork and planted 173 Black Poplars along the edges of the Bride River.

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Now, to increase the chances of the Black Poplar’s long-term survival in Ireland, Buckley and Bernard Carey, a forester in Clare, want to propagate them, and they’re looking for a fertile, wet site of about half an acre to create a nursery for these trees. They’re hopeful that a local authority or State agency will offer a suitable location to give these fairytale trees a future.