From Everest to the Sea, Part 2: The “Beautiful Forest”


On a map, northern India looks like flat land from the Himalayas all the way to the sea.

Jeroen via Wikimedia, CC BY-SA 2.5

But waterways exiting the hills and crossing the Terai encounter a landscape feature — the 1100-foot-high Delhi Ridge — that turns them either westward toward the Arabian Ocean or eastward, as the Ganges river system, toward the Bay of Bengal.

Once upon a time, within human memory, this land hosted grassy savannahs where lions and cheetahs hunted the same sorts of big game they now pursue in Africa, while tigers and leopards prowled the woods.

Today, more 400 million people live here. Northern India is now a densely populated and intensively farmed mosaic of cities, villages, and agricultural fields

The Ganges nevertheless flows on toward the sea, where it meets and mingles its waters with other great rivers to form one of the world’s largest deltas.

At the Mouths of the Ganges, millions of trees still grow. These are mangroves of the Sundarban — a Bengali word for “beautiful forest.”



See how the puffy white clouds stop at open water? The Sundarbans are “breathing,” emitting water vapor during photosynthesis that cools the air above and condenses into clouds.


All trees “breathe” like that — some can even influence their weather — but only mangrove forests grow in the muddy coastal interface between ocean and land.

In such places, trees face life-threatening challenges from both fresh water and salt water.

Here in the Ganges Delta, for example, a subcontinent’s worth of water comes flooding in during the rainy season, submerging the land for weeks at a time.

At such times, animals can seek refuge or move away. That’s not an option for plants. The Sundarban mangroves, like all trees, need oxygen and will drown in flood water or mud if they can’t get it.

This mangrove tree rejects your salt, Sea! (Image: Ulf Mehlig via Wikimedia, CC BY-SA 2.5)

That’s not all.

Every day a salt-water tide covers about a third of the Delta. And salt is deadly to plants.

Things only get worse at low tide.

That’s when salty brine and white crystalline crusts build up in the soil as seawater evaporates under the hot sun.

Why did mangroves evolve ways to survive anoxia, salinity, and other problems in this harsh environment instead of simply moving inland, as many other land plants have done?

Because there are opportunities to exploit here.

  • Sediments derived from the Himalayas and Indo-Gangetic Plain are incredibly rich in nutrients. The sea brings even more fertilizers into the mix.
  • Trees that can adapt to such an extreme setting won’t have to deal with the fierce competition for light and other necessities that goes on in most forests.
  • They can engineer the environment. No, not just clouds and rain. Mangrove root systems slow down water movement enough to filter out most fine sediment. Over time, this builds up into more land to grow on.

Here’s the basic picture:



Yes, there are cats in mangrove forests. We’ll get to them shortly.


Mangroves, and where they come from

Judging by fossil pollen and plant parts, mangroves seem to have worked this out towards the end of the Cretaceous period and at the dawn of the Age of Mammals.

That’s right. Some mangrove species survived the end-Cretaceous extinction.

Back then, there was no Bay of Bengal. Instead, the tropical waters of Tethys flowed around the globe. No wonder Earth was a greenhouse!

As time passed, plate tectonic events like India’s collision with Asia and the northward movement of Australia and South America — both of which used to be attached to Antarctica — closed off Tethys and brought other dramatic changes to our planet.

Through it all, mangroves have thrived along tropical and subtropical coasts.

There’s no single genus called “mangrove” — that’s a common name given to a relatively small number of different plant species found where the sea and land meet.

The Sundarban mangrove forest is the world’s largest stand. Almost 8,000 square miles of mangroves grow on 54 islands in the Mouths of the Ganges.

However, other forests do well in Asia and elsewhere — basically anywhere they can get sufficiently warm temperatures and enough rainfall.

Special geographic and climatic conditions allow mangroves to grow as far south as New Zealand, but they haven’t been able to get any farther north than Florida, southern Texas, and southernmost Louisiana:



Water carves deltas into beautiful forms, whether it’s the Ganges or the Mississippi River, but these Louisiana mangroves are really struggling this far north.


Cats and mangroves

Anyone who has ever tried to give a house cat a bath will be surprised at the number of wild cats that live in these coastal wetlands.

Predators like to take advantage of productive ecosystems. That’s why we see a variety of large and small felines in the Sundarbans and other mangroves.



Not totally safe, though. We’ve already looked at the tiger attacks on local people. Tigers are sometimes the victims there, too. In May 2019, authorities had a shootout and killed four tiger poachers in the Sundarbans!


In addition to tigers, jungle cats, fishing cats, and some leopard cats (Prionailurus bengalensis), the Beautiful Forest also reportedly hosts a few clouded leopards (Neofelis nebulosa).

Although I haven’t come across mention of leopards (Panthera pardus) in these ecosystems, marbled cats (Pardofelis marmorata) have been reported in Borneo’s mangroves.

Wilting et al. think that the elusive flat-headed cat (Prionailurus planiceps) would like this habitat, too, though it hasn’t been reported in mangroves yet.


Featured image: Laskar Sarowar via Wikimedia, CC BY-SA 4.0.


Sources:

Cannon, C. H.; Morley, R. J.; and Bush, A. B. 2009. The current refugial rainforests of Sundaland are unrepresentative of their biogeographic past and highly vulnerable to disturbance. Proceedings of the National Academy of Sciences, 106(27): 11188-11193.

Chen, Y.; Hou, Y.; Guo, Z.; Wang, W.; and others. 2015. Applications of multiple nuclear genes to the molecular phylogeny, population genetics and hybrid identification in the mangrove genus Rhizophora. PloS One, 10(12): e0145058.

Ellison, A. M.; Farnsworth, E. J.; and Merkt, R. E. 1999. Origins of mangrove ecosystems and the mangrove biodiversity anomaly. Global Ecology and Biogeography, 8(2): 95-115.

Lo, E. Y.; Duke, N. C.; and Sun, M. 2014. Phylogeographic pattern of Rhizophora (Rhizophoraceae) reveals the importance of both vicariance and long-distance oceanic dispersal to modern mangrove distribution. BMC Evolutionary Biology, 14(1): 83.

Wikipedia. 2019. Ganges. https://en.wikipedia.org/wiki/Ganges Last accessed August 11, 2019.

___. 2019. Ganges Delta. https://en.wikipedia.org/wiki/Ganges_Delta Last accessed August 11, 2019.

___. 2019. Indo-Gangetic Plain. https://en.wikipedia.org/wiki/Indo-Gangetic_Plain Last accessed August 11, 2019.

___. 2019. Mangrove. https://en.wikipedia.org/wiki/Mangrove Last accessed August 11, 2019.

___. 2019. Sundarbans. https://en.wikipedia.org/wiki/Sundarbans Last accessed August 11, 2019.

Wilting, A.; Cord, A.; Hearn, A. J.; Hesse, D.; and others. 2010. Modelling the species distribution of flat-headed cats (Prionailurus planiceps), an endangered South-East Asian small felid. PloS One, 5(3): e9612.



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