Monday, June 14, 2021

Money Trees

nickel treeIf money grew on trees it seems that could result in vast monocultures, with ruinous environmental impacts. I suppose it depends on currency. If the money tree produced only Iranian rials or Venezualan bolivars, we’d likely consider it a noxious weed.

On the South Pacific island of New Caledonia, there’s a rainforest understory tree that doesn’t bear money; it is money. More or less. The milky sap of Pycnandra acuminata is 25% nickel, the exact same percentage of the shiny metal that the US has been putting in its nickels for the past 155 years (for perspective, nickel ore of 2% is high). To me, the fact a tropical tree can bleed money is nowhere near as strange as the fact that the thing is alive at all, given that even small amounts of nickel – we’re talking below one percent – will kill most plants.

The ability of Pycnandra acuminata to scavenge nickel from the soil and concentrate it is called hyperaccumulation. Most plants take from the soil the nutrients they need for optimal growth, and leave the rest. But for reasons unknown, a few species have fetishes for certain minerals. Alpine pennycress (Thlaspi caerulescens) gathers zinc and chromium for a hobby, while a Japanese tree called Chengiopanax sciadophylloides (I can’t find an English name) takes first prize for its manganese collection.  Since there’s no strict definition of what qualifies a plant to be a member of this exclusive guild, estimates of the number of hyperaccumulator species worldwide range from 400 to 700.

Aside from the point that such ability is “wicked cool,” in technical parlance, folks are excited about hyperaccumulator plants because they can help us clean up some of our worst messes: toxic metals. In the presence of oxygen, petroleum eventually breaks down to carbon dioxide and water, thanks to native microbes. Even persistent chemicals like DDT will biodegrade. Metals don’t, which is generally good. It would be a drag to unearth a stash of buried gold, only to find it had rotted.

Back in 1867, farmers in the central US got their hands on “Paris Green,” a copper/arsenic rat poison, to see if it was effective against Colorado potato beetles. It was a brilliant success, and its use on a range of crops became widespread. In 1898, the insecticide lead arsenate was introduced, which thrilled apple growers to death (sometimes literally) because it killed codling moths and other apple pests. Then in the early 1900s, dimethylmercury and other mercury-based pesticides came into use. For almost a hundred years, we sprayed fields with lead, arsenic, mercury, and copper; stuff that lasts forever. What could possibly go wrong?

Well, the answer to that is obvious. Plenty can go wrong, and children have been poisoned by these heavy metals (another term lacking a definition – all metals feel heavy to me). Untold numbers of kids have been unwittingly exposed. Old apple orchards are among the more heavily contaminated sites we know of. Some have been identified and remediated at great cost, usually by removing the topsoil to a hazardous-waste landfill. Other such places grew homes many years ago. Once a lawn is established, exposure is negligible, but I’m sure some people would like to garden.

Back to hyperaccumulators. Some are in active use helping to mitigate polluted sites, but unfortunately, the plants that collect agro-metals are either not very efficient, or not hardy to our zone. The Chinese brake fern (Pyeris vitata) vacuums arsenic from the soil at a furious clip, but is tropical. Salix miyabeana, a species of willow native to northern Japan, is moderately good at mercury removal, and is actually available here. Ironically, two of the most pernicious invasive aquatic plants, hydrilla (Hydrilla verticillata) and water hyacinth (Eichhornia crassipes), are super-efficient as toxin accumulators. Dang!

Research on hyperaccumulator plants continues to focus on remediation, but the possibility of phytomining, the extraction of minerals from the ground using plants and without disturbing the soil, is being investigated too.

Paul Hetzler is an arborist and author. He denies being a hyperaccumulator, saying he just needs a barn. Or two.

Photo: The New Caledonia tree Pycnandra acuminata bleeds a latex exudate that contains 25% nickel.
By Antony van der Ent, CC BY 4.0, via Wikimedia Commons


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Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.You can read more of his work at or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World

4 Responses

  1. Tim says:

    New Caledonia’s biggest export is nickel, understandably.

  2. Steve Stofelano Jr says:

    Good thing you’re not a “pack (pycnandra) rat” Paul…!

  3. icheckemout says:

    Great article! Thanks.

  4. Patrick McNamara says:

    Very interesting, thanks!

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