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ambika kamath | Updated on January 22, 2018

The gum acacia is the backbone of the thorn-scrub forest in Kutch, Gujarat. Thesweetgum rules the temperate hardwood in Florida

An artist’s reconstruction of ‘Hallucinochrysa’. Credits: Ja Peñas, Proceedings of the National Academy of Sciences

BLink_sweet gum.JPG

Ambika Kamath   -  BUSINESS LINE

How gum from tree trunks has given us the most telling fossils

Imagine yourself walking into a forest. What does it feel like? Do your eyes need to adjust to the darkness under the forest canopy, or do you need to squint to keep the sun’s glare out? Can you feel the wind blow your hair into your eyes, or does sweat collect your hair into slick clumps? Do dry leaves crunch under your feet as you step forward, or do your shoes sink into moist ground? Forests that occupy different climates can feel dramatically different from each other, and many of these differences stem from a single source: the trees.

I’ve spent quite a bit of time in two types of forests: tropical thorn-scrub and temperate hardwood. On the face of it, the two forests could not feel more different. The trees that form the backbone of the thorn-scrub forest in Kutch are the gum acacias. In the summer months before the monsoon showers, the gum acacias lose all their leaves. The thorn-scrub landscape is thus coloured by the yellows and greys of the gum acacia bark and bare thorny branches, matched by the yellow-brown sand and dark grey volcanic rock. Growing on this substrate of sand and rock, the gum acacias maintain a cautious distance from one another, forming only a slight buffer against the strong breeze sweeping across the landscape. The sun beats down oppressively, the leafless trees offer minimal shade, and your sweat evaporates in seconds.

The trees that ruled over my patch of temperate hardwood in Florida are called sweetgums, and in many ways, sweetgums are the opposite of gum acacias. They hang on to their leaves through the summer, casting a dark, cool shadow on the ground below. Their thornlessness makes the forest seem inviting. But within minutes, you’ll find yourself stewing in still, moist summer air.

Stepping away from the immediate experience of these two forests and taking a longer, broader view of gum acacias and sweetgums, some similarities between the two trees come into focus. For example, both groups of trees have interesting distributions across the globe. Acacias are found today in Africa, Asia, Australia, and the Americas, and over the last 50 million years, these trees have moved many times between the continents. Of course, thanks to the ever-changing landscape of the Earth’s surface, the continents themselves were not in the same place 50 million years ago. So we don’t quite know where these wanderers originated.

Like the acacias, sweetgums are also spread across many different parts of the world. Their current distribution, however, is oddly discontinuous — they’re found in East and Southeast and West Asia (but not Central Asia), and in eastern North America. But fossils and pollen grains suggest that sweetgums were once widespread across Asia and Europe. How then did they end up in these disconnected fragments? The most likely culprits are cycles of glaciation many thousands of years ago, which eliminated sweetgums from parts of the world that became too cold. Florida, however, acted as a safe haven for the sweetgum, because the region managed to stay warm even in the face of glaciers covering large parts of North America. In today’s hotter temperatures, sweetgums have spread northward and westward again.

But of course the most fascinating similarity between gum acacias and sweetgums is — you guessed it —their gum. Gum starts out as a liquid exuded from tree trunks, and quickly coagulates into pure stickiness, nearly impossible to get off your fingers if you touch it. Gum protects trees from herbivores, and has been harvested by humans for centuries to turn into medicines, glue, and furniture varnish. But tree gum has one purpose more important than any other — it fossilises into amber. If an unsuspecting animal happens to get stuck in some gum as it’s hardening, then millions of years later, it might turn into a fossil. Fossils found in amber let us catch a glimpse of what lived alongside gummy trees many aeons ago. And the best fossils show more than just what the creatures were — they show us how they lived.

My very favourite fossil is of an insect that was locked into amber more than a hundred million years ago. This curious beast, with menacing jaws and long, finely-branched, spine-like projections all over its body, certainly looks, at first glance, like an alien of the imagination. Even the scientists who discovered it thought so, naming it Hallucinochrysa (think ‘hallucination’). But the insect wasn’t the only thing to fossilise in the amber; the gum also trapped a clue as to why the insect looked this way. A ‘trash packet’, comprising bits of fern, was found enmeshed among the projections, and suddenly everything made sense. Being covered in fern must have let this creature hide its true, tasty self from anything that might want to eat it, and the projections on its body helped pin the fern camouflage jacket in place. Exactly this strategy — covering yourself in plant matter to avoid being seen — is present in modern-day relatives of Hallucinochrysa, implying that this behaviour has proven beneficial to insects for a staggeringly long time. This trans-epochal similarity completely overshadows the transcontinental difference between my patches of thorn-scrub and hardwood forest, or the global similarity of gum acacias’ and sweetgums’ odd geographic distributions. Contemplating how gum-producing trees live at the heart of this vast difference and vaster similarities stretches my brain to the fullest.

(Ambika Kamath studies organismic and evolutionary biology at Harvard University)

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Published on September 25, 2015

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