What autumn leaves
The trees seem to be begging for attention. Emerald summer mantles have given way to a flurry of crimson, amber, ochre, vermillion and gold. The autumn leaves are an eye-catching spectacle – they demand a skyward gaze.
But the greatest marvel of the season is what is falling to the ground.
Quite literally tonnes of leaves fall gracefully from overhead. They carpet forest floors, neighbourhood gardens, sidewalks and streets alike. All piled deep in layers upon layers of single leaves – the literal mille-feuilles of the natural world.
Our view of fallen leaves is far from generous. Our impression is epitomised in the word we use to describe leaves carpeting a forest floor – “the litter layer”.
In keeping with the fastidious manner that characterises our species, we work to get fallen leaves out of sight. We tidy them up, rake them in piles, bundle them in bags, and cart them away in truckloads – to be composted, to satiate hungry landfills, or to kindle autumnal bonfires. We treat them like Nature’s dirty garbage – hoovering them up, and disposing of them like so much carpet detritus after a riotous party.
And this somehow sullies the amazing thing about autumn leaves.
Autumn leaves are not some undesirable by-product of a grander process. They are not leftovers. They are not garbage.
Autumn leaves are an incredible product of evolution. Their existence is a remarkable testimony to the grand arc of evolutionary time. Their existence is purposeful, useful, advantageous.
On the face of things, when trees drop leaves to the ground at the end of the growing season, it seems wasteful. Leaves are, after all, the products of intense activity over the growing season. Their emergence has required the investment of precious nutrients and water obtained by the roots. Collection of these resources has required a “foraging” root system, growing through the soil, requiring the investment of additional resources to do so. Nutrients and water have then had to be transported over great distances, from roots, through trunks and branches, to the tips of stems. This also required investment of tree resources – to construct the powerful, dynamic hydraulic system through which the water and nutrients flowed. Ultimately, the leaves themselves were also constructed, using building blocks derived from photosynthesis to, in turn, assemble an organ that would itself function as a photosynthetic factory – using the energy from sunlight to capture carbon dioxide and convert it into sugars. At the end of the growing season, this marvellous factory is seemingly discarded, cast to the ground like a piece of useless machinery.
By all appearances, Nature invented planned obsolescence long before the consumer products industry came along. Why invest so much energy into constructing something so useful, only to throw it away at the end of the growing season?
Obviously, not all trees do discard their leaves at the end of the growing season. For example, most needle-leaved, evergreen gymnosperm species, such as pines and spruces, retain their leaves year round. Following the growing season, the leaves of these trees enter a dormant state, shutting down their major activities – photosynthesis and the daily control of water transport. Clearly, it is not necessary for trees to lose their leaves at the end of the growing season, and yet the broad-leaved deciduous trees do.
One reason why the leaves of most broadleaf trees are discarded at the end of the growing season in temperate climates is that they aren’t designed to withstand the harsh winter conditions that will follow. Their cells are sensitive to freezing conditions. Ice irreparably damages the cells, so that when conditions are suitable for them to function again, they cannot. Autumn provides a juncture at which the leaves can be dropped so that the stems can set a hard bud that can withstand the winter conditions, and then give rise to new leaves the following spring.
While autumn provides an ideal time to discard leaves and set up the stem for dormancy, it might be argued that there is no reason why the leaf could not be re-assimilated into the plant body.
Why not just mobilise all the resources in the leaf to be stored by the rest of the tree until the ensuing spring, and then use them again to make new leaves?
This is a reasonable question. Deciduous trees possess means by which to mobilise resources for reuse. For example, like us, trees degrade proteins and nucleic acids to reuse amino acids or nucleotides under new conditions or in new locations. Similarly, they interconvert sugars to aid in energy generation, or to provide building blocks to construct other macromolecules. Importantly, trees can convert even large macromolecules into their constituent parts for reuse. Starch is a great case in point.
Starch is a large carbohydrate molecule, made of long chains of molecules of the sugar, glucose. During the daytime, when photosynthesis takes place, like many plants, deciduous trees generally integrate the newly constructed sugars into starch, to store it for later use. During the night-time, starch reserves are converted back to free sugars, like sucrose. These sugars are then transported throughout the plant and used to provide the carbon building blocks and energy for growth, and the development of new leaves or flowers.
Like all plants, deciduous trees also have a means by which to deconstruct the major sugar polymer in plants, cellulose. Like starch, cellulose is a polymer of glucose molecules joined end to end. Cellulose differs from starch in that the chemical bond connecting the sugars is configured in a manner that makes the polymer more rigid. This imparts the rigidity on cellulose that makes it a good construction material for making the upright, flexible, aerial architecture of plants. Plant cells are encased in cellulose, making them little flexible boxes that are stacked end-to-end and side-by-side to make the various plant tissues.
The chemistry that makes cellulose so rigid also makes it much more difficult to degrade than starch. Despite this, many organisms, including plants themselves, have evolved means by which to degrade cellulose. This is important for plants, as they need to be able to degrade cellulose in order to undergo cell division – dividing each cellular “box” in two. Cellulose must also be degraded in a limited manner in order for cells to elongate.
Given that deciduous trees can remobilise large molecules like proteins and starch, and even difficult polymers like cellulose, why do they not simply build deconstruct leaves of these materials for use in the summer, and then degrade these leaf materials in the autumn for future recycling? To a certain extent, this is precisely what does happen in the autumn. Almost anything that can be mobilised out of the leaf is transported out. Soluble proteins and nucleic acids are degraded into their constituent parts. Starch is converted into sugars. Sugars, amino acids, nucleotides and other soluble molecules are converted and transported out of the leaf. And yet something still remains behind.
What remains behind is a fair amount of cellulose plus some of the most difficult to degrade biological molecules known. Amongst the tough to degrade molecules, perhaps the toughest are lignins and tannins.
Lignins are very large, complex three-dimensional polymers. They function like an intermolecular “glue” – encrusting other macromolecules, particularly cellulose, thereby increasing rigidity, as well as making them less water permeable, and more resistant to degradation. Lignins are crucial in building the most rigid structures in trees, including all of the woody structures, as well as leaf veins.
Tannins are also polymers, built with different building blocks than lignins. In fact, many tannins are made by joining together the pigments that make the autumnal colours – pigments that at other times of the year function to protect the plants from the damaging effects of sunlight. These same compounds also deter herbivores and disease-causing microbes. In keeping with this, the polymeric tannins also function as a defence compounds, making the plant material relatively indigestible. In fact, the name tannin is derived from the tanning process – where leaves from trees were used to alter the properties of animal hides to make them more resilient. This occurs because the tannins from leaves bind to and alter the three-dimensional structure of proteins. Similarly, tannins present in tea and red wine alter protein structure – which is evident in the astringent “taste” one gets when consuming these beverages.
Generally speaking, despite the fact that tannins and lignins were a very early feature of terrestrial plants, plants generally do not degrade these molecules. Consequently, these are the molecules that are left behind in leaves, together with some cellulose and proteins. Together these represent a significant pool of carbon and nitrogen for the plant to cast on the ground. If there was value in capturing this investment, given the amount of time that lignins and tannins have existed in plants, a mechanism would have evolved to recycle them. But it hasn’t. Has evolution failed plants, causing them to lose this annual investment in tannins, lignins, proteins and cellulose? No, it hasn’t.
It’s important to remember what resides beneath the ground that the leaves fall on. Across many species, fully half of a tree’s biomass actually resides belowground. This environment is a rich ecosystem in and of itself. In addition to foraging for belowground water and nutrients, roots interact encounter a wide assortment of organisms. This ranges from microbes that engage in symbiotic exchanges of nutrients, to largely benign annelids and arthropods and other creatures that aerate the soil, to ruthless opportunists that aim to capitalise on the tree’s rich resources.
The belowground ecosystem is entirely shaped by the leaves that fall on its surface. The porosity of the carpet determines water flow and dispersal, insulation and heat retention. The molecules that leach from the leaves determine the nutrient quality that resides below. And all of these factors feed into the quality of the ecosystem that resides in the soil. Moreover, the quality and quantity of the tannins have a direct bearing on the variation found in the soil community. Genetic variation in tannin production has a direct bearing on the chemistry of the leaves that fall to the ground. In turn, that leaf chemistry shapes the microbial population that is able to reside in the soil.
The composition of lignins and tannins also determines the rate of decomposition of the fallen leaves. Both tannins and lignins limit the extent of breakdown by organisms, but enhance the extent of breakdown by light-mediated processes. As lignins and tannins absorb light at different wavelengths, the light quality that reaches the blanket of leaves at different times of year will result in different decomposition rates. These, in turn, will influence the flux of nutrients into the soil, and the organisms that are best suited to live in such nutrient conditions.
In essence, through fallen leaves the tree is shaping the nature of its root environment – from the water and nutrients it recovers, to the neighbours with which it resides.
In this light, it is easy to see the advantage conferred on trees by dropping their leaves in the autumn. Over evolutionary time, mutations that altered leaf chemistry in such a way as to optimise decomposition rates, nutrient release, soil porosity, and ecosystem composition would improve chances for growth, nutrient capture, and survival. These mutations would confer a selective advantage – competitive growth, resources capture, and survivorship are all important determinants of tree reproduction. Such advantages would only be realised over long time scales – trees take some time to reach reproductive age, and even then, opportunities for progeny to take root arise less frequently. Over the long arc of evolutionary time, the fallen leaves of deciduous trees have emerged as an incredibly useful strategy for success of these marvellous plants.
Referring to fallen leaves as "litter" is, therefore, a misnomer. Far from being a dispensable item, fallen leaves are an extension of the plant – a means by which to shape their environment. Lignin & tannins are not merely a lost investment of carbon & energy, but instead condition the very soil that will support subsequent seasons of growth. They are, if anything, a wise investment.
There’s a saying that “One person’s litter is another’s treasure.” In the case of leaf litter, when it comes to trees, the adage holds true. Autumn leaves are not merely a treasure to behold when they reside in the canopy, but a true treasure that shapes the wonderful diversity of life with which we share the planet. Enjoy your autumn leaves, wherever you find them.
Aber JD & Melillo JM (1982) Nitrogen immobilization in decaying hardwood leaf litter as a function of initial nitrogen and lignin content. Canadian Journal of Botany 60: 2263-2269
Austin AT & Ballaré CL (2010) Dual role of lignin in plant litter decomposition in terrestrial ecosystems. Proceedings of the National Academy of Sciences 107: 4618-4622
Chapman SK, Newman GS, Hart SC, Schweitzer JA & Koch GW (2013) Leaf litter mixtures alter microbial community development: Mechanisms for non-additive effects in litter decomposition. PLOS ONE 8: e62671
Cheng DL & Niklas KJ (2007) Above-and below-ground biomass relationships across 1534 forested communities. Annals of Botany 99: 95-102
Driebe EM & Whitham TG (2000) Cottonwood hybridization affects tannin and nitrogen content of leaf litter and alters decomposition. Oecologia, 123: 99-107
Fierer N, Strickland MS, Liptzin D, Bradford MA & Cleveland CC (2009) Global patterns in belowground communities. Ecology Letters 12: 1238-1249
Harrison AF (1971) The inhibitory effect of oak leaf litter tannins on the growth of fungi, in relation to litter decomposition. Soil Biology and Biochemistry 3: 167-172
Melillo JM, Aber JD & Muratore JF (1982) Nitrogen and lignin control of hardwood leaf litter decomposition dynamics. Ecology 63: 621-626
Rahman MM, Tsukamoto J, Tokumoto Y & Shuvo MAR (2013) The role of quantitative traits of leaf litter on decomposition and nutrient cycling of the forest ecosystems. Journal of Forest Science 29: 38-48
Winder RS, Lamarche J, Constabel CP & Hamelin RC (2013) The effects of high-tannin leaf litter from transgenic poplars on microbial communities in microcosm soils. Frontiers in Microbiology 4: 290
Dedication: This post is dedicated to my father, W.J. (Bill) Campbell, who introduced me to the song “Autumn Leaves” many, many years ago. It remains a great favourite. Julian (Cannonball) Adderley and Miles Davis hooked me, but, many artists later, Keith Jarrett’s interpretation is the one that continues to amaze. Of all the sung versions, Eva Cassidy’s is perhaps the most poignant; although, the original French version, “Les Feuilles Mortes”, by Yves Montand is up there as well.
Images: All photographs by Malcolm M. Campbell.