A lament for the trees that made us

A Tasmanian-led team is watching trees in their death throes, and grieving.

Trees are better than poetry any day, wrote US poet Joyce Kilmer a few years before dying in battle in World War I. “Poems are made by fools like me,” he ended his hymn of praise, “but only God can make a tree”.

Amen to that. While I don’t have Kilmer’s religious faith, living on a mountain’s heavily-forested slopes I too feel a deep devotion to these marvels of creation. Deep inside us I’m sure all of us do. As descendants of tree-dwelling primates it’s probably in our DNA.

Besides lifting our spirits, trees matter to us in countless ways – shelter, carbon store, soil health, home for wildlife are just a start. Every land-dwelling species on Earth needs them, including us. Without them we wouldn’t be here. Without them, our future would be bleak indeed.

Tim Brodribb’s dedication to trees has taken him to new places in science and the human experience, to the point where he and his fellow-scientists can actually see a tree’s death throes as its water supply system collapses under the stresses of drought and extreme heat.

Brodribb, a plant physiologist at the University of Tasmania, led an international team of biologists whose paper on forests and climate was recently published in the US journal Science. The paper concludes that human-induced climate change raises the prospect of a world without forests.

Titled “Hanging by a thread? Forests and drought”, the paper notes that heat and drought have already begun the process of forest decline and set in motion further unavoidable changes in forest ecology and structure. If we can’t get greenhouse gas emissions significantly below their present (pre-pandemic) rate, forests will be an early casualty.

Brodribb and UTAS technicians have developed advanced optical technology that enables him to pinpoint the moments when extreme soil dryness leads to a tension in the tiny threads of water being drawn up a tree’s trunk, pulling in minute bubbles of air that cut off the supply of water to the tree’s leaves.

Brodribb and his co-authors also note the failure of another life-sustaining mechanism, the tiny openings in a plants’ leaves, called stomata. They used gas analysers to “see” how sustained hot weather forces the stomata to leak precious moisture into the atmosphere, causing the leaves to die.

We have heard much of the impact of fire on trees, through both deliberate acts of deforestation for farming and cropping and devastating wildfire, most tellingly across Australia last summer. That is in itself a tragedy, for the forests and their wildlife and also for the human communities that depend on them.

But fire is a small part of the story. Prolonged drought and periods of extreme heat pose a much more widespread, and ultimately deadlier, threat to the future viability of our forests. The Brodribb paper indicates that threat is already playing out, here and around the world.

The implications of unmitigated climate change are dire for trees on our own island. Last landfall on the edge of the circumpolar Southern Ocean, Tasmania is home to many plant species found nowhere else, including those wilderness icons, pencil pine and King Billy pine.

You’ve seen images of gnarled old specimens (they can live for over 1000 years), standing proud on a misty plateau, perhaps alongside an alpine tarn. Evolving over 150 million years ago during the breakup of the supercontinent of Pangaea, they lived through the breakup of another one, Gondwana.

Thirty years ago my wife and I planted pencil and King Billy seedlings in our mountain garden. For 15 years they grew well, but repeated bursts of summer heat and dryness are now killing them. For these cool climate species, 500 metres up is no longer high enough.

At our pre-pandemic emission rate Earth’s surface will warm by 3C within half a century. By then much of the planet (including most of Australia) will be unliveable for humans, but at least we can move. With nowhere to go, those two iconic conifer species will be early victims of that heating.

But even trees well-adapted to today’s hot, dry conditions will struggle to survive the level of heat that we are headed for. If humanity is not prepared to countenance drastic emissions cuts, it must prepare for a world without forests.

Tim Brodribb and his colleagues used to feel alarmed at our failure to curb emissions. Now, seeing things play out, they are grieving over our prospective loss, and so should we all. The prospect of living without forests is beyond tragic. It is unthinkable.

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