MoniGett

27/01/2022

Trees are dying of thirst in the Western drought – here’s what’s going on inside their veins
Like humans, trees need water to survive on hot, dry days, and they can survive for only short times under extreme heat and dry conditions.

During prolonged droughts and extreme heat waves like the Western U.S. is experiencing, even native trees that are accustomed to the local climate can start to die.

Central and northern Arizona have been witnessing this in recent months. A long-running drought and resulting water stress have contributed to the die-off of as many as 30% of the junipers there, according to the U.S. Forest Service. In California, over 129 million trees died as a consequence of a severe drought in the last decade, leaving highly flammable dry wood that can fuel future wildfires.

Firefighters are now closely watching these and other areas with dead or dying trees as another extremely dry year heightens the fire risk.

What happens to trees during droughts?
Trees survive by moving water from their roots to their leaves, a process known as vascular water transport.

Water moves through small cylindrical conduits, called tracheids or vessels, that are all connected. Drought disrupts the water transport by reducing the amount of water available for the tree. As moisture in the air and soil decline, air bubbles can form in the vascular system of plants, creating embolisms that block the water’s flow.

The less water that is available for trees during dry and hot periods, the higher the chances of embolisms forming in those water conduits. If a tree can’t get water to its leaves, it can’t survive.

A dyed cross section of a ponderosa pine sapling shows the water transport tissue and conduits. Raquel Partelli Feltrin
Some species are more resistant to embolisms than others. This is why more pinon pines died in the Southwest during the drought in the early 2000s than juniper – juniper are much more resistant.

Drought stress also weakens trees, leaving them susceptible to bark beetle infestations. During the 2012-2015 drought in the Sierra Nevada, nearly 90% of the ponderosa pines died, primarily due to infestations of western pine beetles.

Fire damage + drought also weakens trees
Although fire is beneficial for fire-prone forests to control their density and maintain their health, our research shows that trees under drought stress are more likely to die from fires. During droughts, trees have less water for insulation and cooling against fires. They may also reduce their production of carbohydrates – tree food – during droughts, which leaves them weaker, making it harder for them to recover from fire damage.

Trees that suffer trunk damage in a fire are also less likely to survive in the following years if drought follows. When trees have fire scars, their vascular conduits tend to be less functional for water transport around those scars. Traumatic damage to the vascular tissue can also decrease their resistance to embolisms.

So, burned trees are more likely to die from drought; and trees in drought are more likely to die from fire.

What does this mean for future forests?
Trees in Western forests have been dying at an alarming rate over the past two decades due to droughts, high temperatures, pests and fires. As continuing greenhouse gas emissions warm the planet and drive moisture loss, increasing the frequency, duration and intensity of droughts, research shows the U.S. and much of the world will likely witness more widespread tree deaths.

The impact that changing drought and fire regimes will have on forests farther in the future is still somewhat unclear, but several observations may offer some insight.

There is evidence of a transition from forests to shrublands or grasslands in parts of the Western U.S. Frequent burning in the same area can reinforce this transition. When drought or fire alone kills some of the trees, the forests often regenerate, but how long it will take for forests to recover to a pre-fire or pre-drought condition after a large-scale die-off or severe fire is unknown.

In the past decade, the Western U.S. has witnessed its most severe droughts in over 1,000 years, including in the Southwest and California. A recent study found subalpine forests in the central Rockies are more fire-prone now than they have been in at least 2,000 years.

If there is no change in greenhouse gas emissions, temperatures will continue to increase, and severe drought stress and fire danger days will rise as a result.

Read more: Rocky Mountain forests burning more now than any time in the past 2,000 years

27/01/2022

Why do plants grow straight?
Have you ever been at a sporting event or concert and had to wiggle and reposition to get in just the right spot to see the action? Maybe you needed to shift left or right to see between two people. Perhaps you even had to squat on your seat to see over the person in front of you.

Well, plants often have to do something similar so that they can “see” as much light as possible. Plants need light to perform photosynthesis – making sugars from water and carbon dioxide in the air to feed themselves.

If sunlight is directly above them, then plants will grow straight up toward it.

Sometimes, though, it’s not that simple. For example, you might have seen house plants bending towards a window rather than growing straight and tall. When light comes from an angle, plants will curve toward it to get better access to the light they need to grow. Hormones in the plant’s tissues, called auxins, make cells on the dark side of the plant grow taller, bending the plant toward the light.

Plants contain hormones called auxins that make them grow in the direction of light sources.
In a forest, plants may branch out so that their leaves are in open patches of sun, rather than in the shade. This often happens if taller bushes and trees tower over them, or if they are growing in a crowd of other plants. It’s much like humans seeking out sunny spots or reaching their hands toward a campfire to warm up when they feel cold outdoors.

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Humans twist or bend by changing our body positions temporarily, but when plants twist, bend or elongate, they are actually growing toward the light. The types of plants that reposition themselves to see the light are species that grow in a slow but determined way.

Other types of plants may not grow straight because they have different strategies. For example, strawberries grow close to the ground and spread sideways by sending out runners – stems that spread out just above the ground to create new plants.

Stone house with ivy reaching up one wall to second story.
Ivy is climbing up the side of this house in Aberdeen, Scotland, to expose its leaves to as much sun as possible. Stuart Caie/Flickr, CC BY
Other plants, like ivy, grow as vines that climb up trees, walls and fences and use them for support. Climbing vines may grow straight, to the side or at angles, depending on what kinds of support structures they find to grow on. The purpose is to expose their leaves to as much sunlight as possible.

In my recent book, “Lessons from Plants,” I explore how plants are usually positioning themselves to see the light. It’s fascinating that we humans, too, are often positioning ourselves to see something interesting.

So the next time you see a plant growing straight, take notice of whether light is directly above it. Or if you see a plant that’s not straight, notice whether it’s bending toward light coming from the direction it’s facing. Or maybe it’s a vine climbing on a structure and using that support to take a different route toward the sun.

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And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

27/01/2022

Dinosaur food and Hiroshima bomb survivors: maidenhair trees are ‘living fossils’ and your new favourite plant
Most of us are captivated by the thought of a “living fossil”, which is any organism that appeared millions of years ago in the fossil record and survives today, relatively unchanged.

The maidenhair tree, Ginkgo biloba, ticks all the boxes of this definition. The genus Ginkgo is well known in China and Japan where it has special significance in Buddhism and Confucianism, and first became known to European botanists in the late 1600s.

Today, ancient ginkgo fossils can be found all over the world, some of which are almost 300 million years old – a time when dinosaurs roamed the planet. Let’s delve further into what makes this species so remarkable: from its ability to survive nuclear bombs, to its vomit-smelling seeds, to it’s beautiful autumn display.

Hardy survivors
The ancestral ginkgo evolved so long ago it spread across the super continent Pangaea and was present in both the northern component (Laurasia) and the southern part (Gondwana, which included Australia) when the continents fragmented.

As a result there are fossils, Ginkgo australis, from the cretaceous period about 65-140 million years ago in the Koonwarra Fish Fossil beds near Leongatha, Victoria. There are also much more recent (about 20 million years old) fossils from Tasmania.

Ginkgo biloba has an intriguing appearance. It can grow up to 35 metres tall with a spreading canopy, and its leaves are a wonderful fan shape, often with a little cleft or notch.

The wonderful fan shape of their leaves. Photoholgic/Unsplash, CC BY
As you might imagine for a genus dating back almost 300 million years, the maidenhair tree is both hardy and resilient, tolerating a wide range of soil and climatic conditions.

The tree is known to be very long-lived and some specimens at temple sites are thought to be over 1,000 years old which, in part, explains the mystique associated with the species.

They have a lignotuber – a modified stem at the base of the trunk containing lots of buds – which allows for sprouting at ground level and multiple stems. The lignotuber allows for rapid recovery from serious environmental stresses such as fire and defoliation.

Read more: Tree ferns are older than dinosaurs. And that's not even the most interesting thing about them

In fact, six trees not only survived the bombing of Hiroshima, but recovered quickly, are healthy and growing still. Their survival showed the resilience of the ginkgo and the trees became an important symbol that recovery from disaster was, indeed, possible.

Australians can empathise with this as the vibrant re-sprouting of trees after bushfires often plays a similar symbolic role.

Stinky seeds and dinosaur food
Things continue to get interesting when you consider there are separate male and female trees; a relatively rare feature in modern trees. The male reproductive structures have mobile s***m that swim to the ovule for fertilisation, which is considered a primitive or ancient characteristic.

If fertilisation occurs, the female tree produces a seed that resembles a fruit. The seed’s soft fleshy layer is malodorous, with people often describing it as being revolting or smelling of human vomit.

The seeds, resembling fruits, are known for its repulsive smell. Shutterstock
A pair of female maidenhair trees was planted outside the entrance to the Old Geology building at the University of Melbourne in the 1920s. Since then, staff and students have had to use the side entrance when trees held seeds. This will probably continue for decades to come.

Likewise, I know of a couple of female trees that were planted outside the entrance to a major bank branch in Hawthorn, Victoria. It was considered karma by disgruntled customers, until their sudden removal by a desperate manager.

The male tree doesn’t smell but produces pollen, which has been known to cause allergies, so be wary of which s*x you plant and where you plant them.

The seed’s strong scent has been linked to its dispersal, as many animals are drawn to strong, even rancid smelling fruits. There’s little evidence as to which animals or birds eat ginkgo seeds today, but there has been speculation the seeds may have been eaten by dinosaurs.

Read more: Friday essay: trees have many stories to tell. Is this our last chance to read them?

Ginkgos coexisted with dinosaurs for millions of years. It’s easy to imagine a huge herbivorous dinosaur munching on tall maidenhair trees. Sadly, there’s no evidence of gingko seeds in fossilised dinosaur droppings. But for those who are captivated by the connection of a living fossil and dinosaurs, perhaps that fossil is still to be found.

Ginkgo biloba in Huishan Temple of Huishan Ancient Town, China. Jerry Wang/Unsplash, CC BY
Ginkgo for gardeners
Ginkgo biloba has been cultivated for more than 3,000 years, and so whether it grows naturally in the wild is uncertain. Even in China, it grows most often in homes and temples, and there’s very little genetic diversity within the plants suggesting they’ve been grown from cuttings.

The tree has been so widely planted it now occurs in major cities and botanic gardens around the world. In Australia, many of us live within a few kilometres of a recent planting.

Many of the ginkgo trees planted in urban landscapes are males grown from cuttings. But there are different cultivars available from nurseries, with some being all female varieties that are highly prized for their brilliant yellow autumnal colour.

Bright yellow Ginkgo tree
Female trees have a stunning autumn display. Shutterstock
Apart from allergenic pollen and vile smelling seeds, Ginkgo biloba can have another very annoying or perhaps frustrating habit for gardeners. Young plants can grow very tall before their side branches begin to grow and develop. This form of growth, called bolting, is considered an adaptation to stressed environments, but it’s little consolation when you’ve been growing a gingko for 20 years, it’s over 6m tall and still looks like a bean pole.

You have to be patient with slow-growing, long-lived trees, but they’re worth the wait! They rarely, if ever, have pest or disease problems, they are hardy and, despite being cultivated in Europe and North America for centuries, have never become weedy.

Read more: Running out of things to do in isolation? Get back in the garden with these ideas from 4 experts

They may well be described as living fossils, but they are in fact a resilient genus of modern plants that can cope with whatever the environment has thrown at them for over 300 million years.

They are the epitome of great survivors and I would not be betting against their chances of surviving for millennia to come.

27/01/2022

27/01/2022