What autumn leaves – their evolutionary history

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“The falling leaves – drift by my window. The autumn leaves – of red and gold.” from Autumn Leaves, English lyrics by Johnny Mercer (1909-1976)

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 literalmille-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.

Written By: Malcolm Campbell
continue to source article at scilogs.com

2 COMMENTS

  1. 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.

    In fact most trees lose leavers after a time. Its just that in temperate climates the loss is synchronised with autumn, and in other places, synchronised with the dry season – rather than scattered through the year.

    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.

    The leaves reabsorb what nutrients they can from the leaves, but the soil bacteria and fungi do the rest of the work to liberate nutrients for the roots to take up later.

    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.

    There are very complex systems of symbiotic relationships with soil Mycorrhiza.

    http://en.wikipedia.org/wiki/Mycorrhiza – A mycorrhiza is a symbiotic (generally mutualistic, but occasionally weakly pathogenic) association between a fungus and the roots of a vascular plant.[2]

    In a mycorrhizal association, the fungus colonizes the host plant’s roots, either intracellularly as in arbuscular mycorrhizal fungi (AMF or AM), or extracellularly as in ectomycorrhizal fungi. They are an important component of soil life and soil chemistry.

    Mycorrhizae form a mutualistic relationship with the roots of most plant species. While only a small proportion of all species has been examined, 95% of those plant families are predominantly mycorrhizal.[3] They are named after their presence in the plant’s rhizosphere (root system).

    These Mycorrhiza can form huge networks, transferring nutrients around miles of forest floor ecosystems.

  2. Great article. And, I have to agree that Eva Cassidy’s version of the song endures as a poignant example of her remarkable talent.

    As for the autumn leaves, I am very allergic (with asthma), but that hasn’t stopped me from marveling about how the trees time the abscission event. The coordination of the auxin to ethylene ratio and the measure of light…. I just cannot stop loving the living world.

    The only problem I have with the article is the use of the world “designed”. Otherwise a well written, lovely tribute to the beauty of nature.

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