I like really tall trees.

I suppose the possession of this adoration of our planet’s living, heaven-raking spires comes as a kind of birthright. I grew up in the Pacific Northwest, an area not only richly coated with swaths of the densest temperate rainforests in the world, but also the tallest forests in the world. I came of age spending a great deal of time hiking and navigating forests largely consisting of several tree species that are among the world’s tallest. Coast redwood (Sequoia sempervirens), douglas-fir (Pseudotsuga menziesii), and Sitka spruce (Picea sitchensis) are all found in the lush coastal forests of Oregon and far Northern California where I spent many long, summer days of my youth; each of them generally regarded as being within the top five tallest tree species on the planet, based on the consistency and frequency of superlatively monstrous individuals within each. Even the “smaller” trees in the region seem to reach uniformly towering heights. Western redcedar (Thuja plicata) can top out at 200 feet (61 m) or more above the soft, spongy soil of the dark, coastal woods of Washington and Oregon. Western hemlock (Tsuga heterophylla), a very common sight in the Pacific Coast Ranges, can easily grow to some 250 feet (76 m) at its droopy crown. The bottom levels of the canopy in a Pacific Northwestern old-growth rainforest can potentially be no less than 150 feet (46 m) high, which is a value not often matched in any other forested region on Earth.

It’s no wonder that growing up immersed in this place has left me with a love for these great trees; old-growth forests full of venerable, enormous trees are incomparably majestic places. The sense of perspective and scale that these trees provide is invariably humbling. It’s difficult not to walk alongside them in a kind of hushed reverence, as if you were traversing the floor of an ancient and solemn temple or cathedral, one crafted from humongous, gnarled pillars of wood and moss, rounded with smoothed with deep time and dark silence. The temperate rainforest springs to life in intense bursts of emerald from wherever these trees have embedded their water-ravenous feet, with lithe lances of ferns and the ghostly baubles of root-associating mushrooms erupting wherever soil space is available. These dampest and darkest of woods, blanketed from the sun a football field’s length upwards, have been described as primordial, as a place of senescence and decay, but I think this is a misplaced conceptualization. The sites where the greatest of these trees grow is positively choked with life; life that clings to and parasitizes other life, life that reaches achingly skywards in even the weakest, most diluted sunbeam to touch down on the forest floor. In my mind, these are places of as much birth and flourishing as they are museums.

This aesthetically spell-binding quality, mixed with these forests’ complex ecology and somewhat unique, insular propensity to harbor endemic species…creatures found nowhere else in the world…is what persistently attracts me back to them time and time again (and also inspires me to write about them…multiple times…because I’m a little insufferable).

It is these types of places, misty, verdant groves of titanic conifers, that come to the mind of most when they envision the world’s tallest trees…granted they call the Northern Hemisphere home. It’s somewhat widely known that California’s coast redwoods are the world’s tallest species, and across the North American continent the sheer size of Pacific Northwest forest trees is no secret…especially when compared against the far more “compact” deciduous trees that are common on the Eastern Seaboard. But a very close contender for the title of the most gravity-taunting plant in the world comes from a location not often associated with impenetrable forests. One of the tallest organisms on Earth is an altogether different kind of plant than the behemoth redwoods, and it hails from the opposite side of the globe from the dewy haunts of Cascadia…a place far more associated with rust-colored, alien deserts, blinding heat, and a faunal assemblage that constitutes the world’s largest bucket of shorts-soiling “hell fucking no.”

I’m of course talking about Australia.

Yes, Australia is a place of extremes…where the venom flows like water, the coral reefs are supersized, and summer turns the landmass into a not-so-metaphoric broiling pan of unending solar-powered punishment  (one that keeps getting hotter). From a biological perspective, Australia is a continent perpetually locked in rebellious teenager mode, deviating from the rest of the world’s biota and letting its freak flag fly proudly for millions of years in a parade of pouches, flightless birds, weird plants, fangs, spikes, and scales. It is therefore quite fitting that one of the tallest trees in the world, the only one in the top five that is not a conifer, in pure contrarian style, is Australia’s Eucalyptus regnans…the “mountain ash” or “swamp gum.”

Eucalyptus regnans holds the title of the world’s tallest flowering plant, or “angiosperm” (which is in contrast with the coniferous redwoods, firs, and spruces…which are cone-bearing “gymnosperms”). It is also the tallest tree in the Southern Hemisphere, and the only trees to compete halfway decently for this distinction are other members of the genus Eucalyptus. The only explanation I can think of is nepotism…..

…or, alternatively, that Eucalyptus is a highly diverse group of over 700 species of plants that dominate the Australian flora, with an extensive array of growth forms and habits, and that a giant species or three thrown in is not by itself unusual. Eucalyptus is known for its role as koala chow and its Altoid-scented foliage, as well as a particularly attractive, psychedelic species native to Indonesia and the Phillipines, “rainbow eucalyptus” (Eucalyptus deglupta)…which looks like it was bred by Willy Wonka’s horticulturist.

More broadly, Eucalyptus is a part of a large, wide-ranging family of flowering plants found in the warmer latitudes; Myrtaceae. The family contains a staggering number of species of plants (more than 5600 estimated), including well-known examples like allspice, guava, clove, myrtle, and the enigmatic, endemic (and threatened) ohi’a of the Hawaiian Islands. They are all united in having hard, woody stems and generally having obvious, brushy flowers that strongly resemble exploding fireworks.

This is a variable and successful group of plants, and the stately mountain ash is a jewel in this family’s crown.

It grows in the relatively cool, mountainous, far southern Australian states of Victoria and the island of Tasmania in upper elevations. Mountain ash inhabits the rainy “wet eucalypt” forests that characterize much of the very southern tip of the Great Dividing Range, where it grows at a breakneck rate of roughly one human height every couple of years. The tree grows faster than the mammoth conifers of the Northern Hemisphere, but ends up, in maturity, being a less voluminous, lanky sight to behold; a thin, columnar colossus culminating in a noble tuft of aromatic, evergreen leaves. The mountain ash is to the coast redwood/giant sequoia as Manute Bol is to Andre the Giant. As a pepperoni stick is to a pork chop. As a taquito is to a burrito.
Anyways, you get the point, and I’m going to stop before I make myself hungry.

Some exceptionally altitudinous mountain ash specimens can exceed 300 feet (91 m) in height after as much as four centuries of growth. As of 2014, the largest living mountain ash is the aptly-named (on account of its 100 m (327 foot) height), baronial Centurion, a tree from Tasmania. This sits solidly in the mid-range among the tallest Californian redwoods, but there is an account of a mountain ash from Victoria in the late-1800s near the community of Thorpdale that was measured at about 375 feet (114 m) in height…which would not only put it within mere feet of the tallest reliably recorded tree ever (a coast redwood named Hyperion, at just over 379 feet), but would mean that at the time, it would have been the tallest tree known. The Thorpdale tree was felled more than a century ago, but the site of its former stump remains marked to this day.

Most eucalypt species have a close relationship with the fires that regularly blaze their way across the dry woodlands and plains that border the vast interior desert of the Australian continent. They have a suite of adaptations that allowed a lineage of plants that, tens of millions of years ago, originated in the region’s rainforests, to survive the aridification of Australia and to prosper over a newer, drier continent: stringy, flammable bark, oil-rich leaves that break down very slowly in the leaf litter, and, commonly, a means of rapidly re-sprouting from fire-protected seeds and/or resilient buds hidden underneath the bark, stimulated to grow after severe damage to outside layers in a bushfire. The majority of eucalypts in these dry, open woodlands are reborn again and again from frequent fires. On hot, dry season days when the sky is roiling with angry, charcoal-colored storm clouds, it is the crack of thunder and lightning that marks their labor pains.

However, the relatively moisture-loving, rainforest-associated mountain ash is a bit of a wuss, comparatively, when it comes to fire. While the species certainly has the dilapidated, vestigial remains of the structures associated with post-fire sprouting from the charred bark, the main strategy appears to reproduce from the seeds left behind following an especially damaging fire (and spectacularly so, with as much as 2.5 million seedlings per hectare (an area about the size of a baseball field) sprouting from seed following a burn…obviously, this is pared back substantially as the trees mature and compete with one another for light and resources in a case of the most cutthroat sibling rivalry conceivable). If the fire is intense enough, the mountain ash often dies outright, never to rise again from the ashes like a Phoenix, save for its genetic torch being carried forward by great multitudes of its progeny.

Mountain ash is superbly adapted to deal with the effects of relatively infrequent fires. It can take a good decade or more after a fiery armageddon for the new crop of young trees to be mature enough to produce seed, so if another super-fire cuts through the area before that point, the entire region becomes effectively deforested of mountain ash. This is obviously a big damn problem, but under normal circumstances, the chances of catastrophic engulfment with earth-cleansing, surface-of-the-sun scale hellfire occurring more than once per decade in the same spot is fairly low.
But…the key word is “normal” in “normal circumstances.”

I linked to a source several paragraphs above that illustrates that Australia is getting hotter as global, human-induced climate change progresses. The other side of that doom-and-gloom coin of the heat being imminently and permanently cranked up to 11 is the fairly solid recent prediction (from CSIRO, Australia’s national science agency) that in the coming decades, rainfall will decline in the range of mountain ash in southern Australia, and severity of drought conditions will increase. Both of these factors contribute heavily to promoting not only more frequent fires, but the types of destructive events that bake the soil into blackened sterility. There are predictions that suggest that the wet, southern, mountain forests of Victoria and Tasmania might not have quite the same jump in overall risk of severe bushfires in response to coming climate change that other regions of the continent will likely endure, but mountain ash habitat will also surely suffer substantial drops in rainfall and an uptick in the amount of that moisture that is evaporated away, sucked right into the unyielding blow drier that is Australia’s future atmosphere.

The seemingly inevitable transition of every single corner of Australia into a fucking tinderbox isn’t a particularly potent threat to mountain ashes all by itself. There’s a second element that comes into play with the relentless dehydration of the continent; one with a very direct human component. Can you guess what it is?

Mountain ash is a valuable source of lumber in this part of the world, and, curiously enough, it turns out that threshing the everloving hell out of old-growth stands, and laying immense tracts of forest bare for the timber industry, has lasting, awful ecological effects. The double-whammy threat to these trees comes in the form of the junction between fire susceptibility and timber harvest practices. Mountain ash forests that are 1) younger and 2) fairly homogenously so are pitifully prone to the super-hot, super-aggressive fires Australia has become known for. Until mountain ash grows up to be big, strong, and resilient against the licking of flames, the young trees are as vulnerable to incineration as a box of kerosene-soaked matches at a KISS concert. If these baby-faced whippersnappers are all about the same size/age as well, then there isn’t much diversity, tree to tree, in the forest’s resilience against fire; the fire can spread completely unimpeded due to the absence of burn-slowing bigger individuals.

The most dramatic force shaping mountain ash forests towards a uniform crop of spark-wary sitting ducks is that of unfettered clearcut logging of these trees. What occurs is a tragically tight positive feedback loop that begins to “trap” entire landscapes in a self-perpetuating process of rapid change and potentially irreversible shifting to an entirely different ecosystem structure at the elimination of what once was. Logging promotes the growth of fire-prone mountain ash forests, and the subsequent increased fire activity and severity constricts the ability of that area to allow the re-establishment of old-growth forest…which encourages yet more bushfires in young, dense, regenerating groves of trees.

At this point in time, mountain ash isn’t immediately faced with the dodo’s fate; there’s no indication that the tallest flowering plant in existence will die out in the next couple of decades. As far as we can tell, Eucalyptus regnans is rather “safe” at this moment. However, mountain ash still isn’t specifically protected in Australia, and timber harvest continues largely unabated. The potential threats to mountain ash haven’t yet been evaluated by the IUCN (International Union for Conservation of Nature) as grave enough to warrant the recommendation of protection from exploitation, but there are a couple reasons why we should be concerned with still declining numbers and the health of mature mountain ash forests.

The first of these is that, like other huge, forest-dominating trees like redwoods and sequoias, the mountain ash has a wide-reaching role to play in its ecosystem, and a great many species depend on its presence for their own survival. When you are big enough to influence the entire dynamic progression of the little world that surrounds you, any change or absence has a reverberating, “echo” effect. The best example of this in mountain ash forests is the stubborn dependence of the Leadbeater’s possum (Gymnobelideus leadbeateri) on the accessibility of a very specific mix of moderately mature eucalyptus trees and wattle (Acacia). This nondescript, nocturnal, squirrel-like marsupial, closely related to the watery-eyed, frenetic, more publicly familiar sugar gliders, is now found only in a tiny stretch of forested uplands in central Victoria. It is a victim of the punctuated, but extensive, loss of mountain ashes old enough to have tree-holes as a daytime refuge (because apparently nocturnal animals aren’t big into the whole good ol’ vitamin sunshine thing); cataclysmic fires in the region, paired with regular clearcutting, have decimated available habitat for the possum in recent years, causing a drop to an estimated 1000 remaining animals in the wild. That may seem like a lot of Leadbeater’s possums left in the world, but in reality, a series of marginally inferno-y summers could evict these little guys off the planet for good. Considering that every single individual alive is now constricted to an area smaller than that of metropolitan L.A., the idea of them getting snuffed out by a bad case of climactic heartburn, or indirectly through a temporary spike in the price of mountain ash timber, doesn’t seem all that ridiculous. If you aren’t yet feeling a bit morose about the possum’s likely eventual, brutal blaze-and-blade initiated gentrification into oblivion, please consider that Gymnobelideus leadbeateri continuously makes a face like you just stole food out of its mouth and told it its birthday was canceled:

Here, the equation is simple. No mountain ash = no Leadbeater’s possum. No Leadbeater’s possum = no inspiration for a new Pokemon in the next series of games. No new Pokemon = I actually have to grow up and begin behaving like a fully functioning adult human.

This is an unspeakable tragedy.

The other reason the loss of mountain ash forests is worrisome, in particular the old-growth forests full of the largest trees, is that gargantuan trees are uniquely suited for siphoning off the carbon dioxide pollution responsible for global climate change, and converting it back into biomass. Forests, more broadly, are the truthfully important engines on land for grabbing carbon out of the atmosphere and incorporating it into new growth…but for the world’s largest trees (like mountain ash and redwoods), with an increase in size and age, that capacity for transforming greenhouse gases into board feet of lumber climbs in scale almost exponentially. Rather than slowing down and taking it easy on the growth as they enter retirement age, the world’s giant tree species appear to do the opposite, ratcheting up their efficiency as a “carbon sink” and accelerating their carbon mass gain. Big trees, not unlike humans, tend to pack on the pounds faster in their golden years.

Since one big, ancient mountain ash or redwood far more adequately gobbles up greenhouse pollutants year to year than a stand of dozens of smaller, more junior trees, we should start considering the last remaining fragments of old-growth forest, specifically, as a minor means of mitigating some of the output of global carbon dioxide emissions; these exemplary trees are worth saving as both crucial, ecosystem-influencing habitat for scores of other species, as well as a drain on the atmosphere-bound outflow of greenhouse gases. It’s important to note that mountain ash specifically has been shown to have groves that are estimated to be the most carbon-dense in the world, hinting at their potential as especially effective carbon traps. If we lose the biggest mountain ashes, we simultaneously shoot ourselves in the foot a little bit on combating climate change.

Since we now recognize the utility of incredibly large trees as veritable carbon vacuums, we would be wise to understand the scope of what kind of a loss it would be to have these plants fade into extinction. Mountain ash, as a species, seems to be holding on (for now), but the coast redwood was, depressingly, newly included in 2013’s IUCN Red List as an endangered species, still in decline from human harvesting and encroachment.

We are just now starting to understand the most tenuous, subtle relationships between the most massive organisms (among which the mountain ash is a prime example) to ever evolve on Earth, and their surrounding neighbors…both in a direct, ecological framework, and indirectly through recycling climate-altering carbon dioxide. Hastening our pace on keeping the Tallest Trees in the South, or the North, or wherever, from collectively biting the dust at our own misguided and careless hand does far more than save some of the handsome hiking and photography locales we supposedly love so much…it also avoids a world of ecological pain.

Because that’s the thing about really big trees, evidently; the taller they are, the harder they fall.

Image credits: introduction image of E. regnans, rainbow eucalyptus, mountain ash with cars, logging photo, Leadbeater’s possum

© Jacob Buehler and “Shit You Didn’t Know About Biology”, 2012-2014. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Jacob Buehler and “Shit You Didn’t Know About Biology” with appropriate and specific direction to the original content.