The Secret Lives (and Deaths) of Leaves

It’s autumn now, and the trees outside my window have already burst into brilliant shades of yellow and red. Soon their gorgeous leaves will fall to earth, ready to be raked, bagged, and shipped off to Pacific Region Compost.

It seems a little wasteful; these trees have put a surprising amount of energy into growing something that only lasts for six or seven months. In a 2002 study, researchers looking at 14 California blue oaks found that, by July, each of those oaks had invested about 6.6 kg of new plant material in their leaves. (This is dry mass, not including the ~85% of the leaves that were water.) One outlier had 29 kg of dry leaf mass, which in life could have weighed about a quarter-ton.

6.6 kg (or even 29 kg) might seem like chump change for a tree weighing several tons, but remember that trees need hundreds of years to get that big. Annual tree growth can be measured in the low tens of kilograms per year . . . so that extra 6.6 kg of leaf-matter represented a significant fraction of the tree’s net production, poured into something that it would soon toss aside like so much trash.

Waste Not, Want Not

Except . . . the tree doesn’t actually throw its leaves away. Not completely, at least.

By the time a leaf has started to yellow, the tree—like an angry tenant facing imminent eviction—has ransacked it for anything that isn’t firmly bolted down. A flood of enzymes, reactants and catalysts steal sugars, dismantle proteins, metabolize cell membranes, and transport precious minerals back into the plant stem. 80% of the leaf’s phosphorus, nitrogen and potassium, and over half of several other minerals, are reclaimed from each leaf in this staggeringly thorough-and-orderly process. Even life-giving chlorophyll is ultimately torn apart, if only to detoxify it as the plant dismantles its light-harvesting complexes to reclaim the abundant proteins within.

Phosphorus-rich DNA is left alone until the very end, allowing the leaf’s pillaged cells to continue functioning well enough to see off all the rest of their chemical treasures. But DNA, too, is eventually taken back into the tree, stripped down to its building-blocks and ready to be put to use elsewhere.

By now the leaf is fully yellow (or red, or brown) and what once was verdant photosynthesizing machinery is now a dry, empty husk. The tree has reclaimed everything it can, leaving largely cellulose, tannins, and a handful of other tough compounds . . . the cellular equivalent of the drywall and floorboards. All these compounds (except the tannins) could probably still be broken down and reclaimed, but instead the plant chooses to let it go, severing its connection to what’s left of the leaf and letting it fall, unwanted, to the earth below.

Burn It Down And Salt The Earth

Except, even that isn’t actually wasted.

Trees don’t ask for very much in life: a little water, a little sun, a chance to plant a few seeds of their own . . . and the slow, painful death of anything that stands in their way.

As science writer Malcolm Campbell explains so well, all those tannins served the tree in life, making its leaves unpalatable (or indigestible) to a host of leaf-hungry micro- and macro-organisms. In death, as the thick blanket of fallen leaves smothers above-ground plants (that might compete with the tree for water and soil nutrients) the tannins in those decaying leaves seeps deep into the ground, altering the nutrient balance and pH of the soil and effectively poisoning anything the tree doesn’t want growing in and around its roots. The specific chemical makeup of what the tree leaves in its leaves will also affect their decomposition rate, allowing the tree to alter soil aeration and permeability to its liking.

Thus, by “wasting” its leaves year after year, a tree gains mastery over its below-ground environment, ultimately creating soil that is favorable to it and its symbiotic organisms, and is hostile to everything else.

Leaf decomposition is also the final step toward the tree reclaiming its initial investment in its leaves. The nutrients that weren’t reclaimed before the leaf fell can be taken up by the tree’s roots instead. So, in the end, the tree that seemed so wasteful was actually quite efficient, reclaiming nearly everything it expended, and poisoning its enemies at the same time.

As The Worm (Or Rake) Turns

Which brings me back to those trees outside my window, whose leaves would normally be headed for our green yard waste bins.

Humans (surprise) can disrupt the normal nutrient reclamation cycle of deciduous trees. By raking and bagging all their leaves, we deprive the trees of nitrogen and minerals they might otherwise have reclaimed through their roots. We also (depending on the species of tree) leave their roots vulnerable to soil-borne organisms that might otherwise have been held back by the tannins and breakdown products of their fallen, rotting leaves.

In North America specifically, we’ve also interrupted the leaf nutrient cycle in a completely different way, even in places where nobody rakes: with earthworms.

Earthworms were dealt a pretty severe blow in North America over the last few ice ages, and up until a few hundred years ago, northern American forests were actually worm-free. But humans have since brought around 60 species of earthworms into North America, mostly from Europe and Asia . . . and these worms are now disrupting leaf-nutrient cycles in forests across the continent. By spreading and stealing nitrogen and other nutrients, and by altering the structure of the forest soil, invasive earthworms are ravaging North American forests: starving plants, compacting the soil and wrecking animal habitats, all because they’re messing with the fallen leaves of trees.

So this autumn, do your favorite tree a favor: don’t rake. Who cares if your neighbors complain about the rancid foot-deep muck of rotting leaves slowly poisoning your lawn? So what if you need to murder a few thousand earthworms to make sure the leaves decay according to the tree’s own natural rhythm? By killing all worms and refusing to rake, you’re helping a beautiful tree to thrive, and/or to murder everything around it.

And in the end, isn’t that what really matters?

Photo credit: Borut Trdina

This post originally appeared on the Observation Deck.