Now, at the height of summer, when the fresh green of the spring leaves has darkened, I will start this week’s column with a question: “Why is it that northern Japan’s Mongolian oak and Europe’s common beech retain their rustling brown leaves all winter, while sharing their temperate forest habitat mainly with deciduous trees that drop theirs each autumn and regrow them in spring?”
A supplementary question is: “Why do most trees from mid-Honshu southward and westward not shed their leaves at all?”
Growing up in temperate northwestern Europe, spring was, for me, synonymous with snowdrops, cowslips, catkins, and flower and leaf buds bursting forth color and greenery. Yet any resident of central or southern Japan, though no doubt annually entranced by the waves of plum and cherry blossom marching up the country, will be more familiar with trees that remain green all year.
From a human perspective, trees seem faced with a dilemma: “Do I, don’t I, do I, don’t I drop my leaves?” Fall, that American season, takes its name from the seasonal descent of leaves, yet the needle-leaf trees — spruces, firs and pines — hold on to theirs, with only the unusual larches being both needle-leaved and deciduous, giving us glorious autumnal golds and vibrant spring pale greens.
Broad-leaved deciduous trees such as oaks, maples, birches and alders produce new sets of leaves each year in a seemingly miraculous flurry of vernal growth powered merely by sunshine, water, and minerals and nutrients supplied by the soil. Yet equally, evergreen broad-leaved trees — such as many of the species in southern Japan and most of those in subtropical and tropical regions — retain their leaves all year, dropping them a few at a time rather than in one seasonal rush.
What is more, these evergreen broad-leaved species typically have thicker, waxy, shiny leaves that are dark green — very unlike the leaves in delicate shades of green of deciduous broad-leaved trees.
In areas that experience high temperatures and high rainfall, trees are exposed to a double dose of stress. The first involves water, with high temperatures causing evaporative water loss from exposed surfaces. This water loss, which cools the trees, also provides the means by which minerals and nutrients are transported along with water up from the soil via the roots. But constant high temperatures can cause the loss of too much water, causing plants to wilt. Thicker waxy coats on leaves are a common way of minimizing the risk of this excessive water loss.
The second type of stress occurs when heavy rains leach chemicals and minerals from leaves. A waxy leaf coating helps again in that it retards water loss due to heavy rainfall. Chemical compounds in the waxy layer also serve to repel herbivores and inhibit fungal growth.
Take a closer look the next time you stand beneath a tall canopy tree and give a thought to how many leaves there are. In fact there are commonly as many as 6-8 sq. meters of leaf area covering every 1 sq. meter of the ground. That is a phenomenal output of growth; all derived from natural resources and able to tap solar energy at rates far exceeding our best solar panels.
But why are so many of the leaves intact? Why, relatively speaking, are so few eaten away by insects or other animals? The combination of repulsive physical characteristics such as spines, fibers and waxy layers, and chemical defenses seems to do the trick.
If deciduous species suffer a fungal or insect attack they can shed the parasites along with their leaves in autumn. Evergreens must do their best to avoid the possibility, as they hang on to their leaves so much longer.
Deciduous broadleaf trees produce all their leaves at once during a mild season when ground frosts have generally finished, the days are lengthening and more sunlight is available. In autumn the same trees withdraw valuable compounds from their living solar panels, the leaves, and take the opportunity to load toxic wastes into leaves that are reaching the end of their useful life and due to be discarded.
During this process color changes are profound, yielding the gorgeous yellows, golds and reds of autumn. Then, through a process called abscission, the leaf and its petiole are progressively separated from the twig or branch until finally wind or gravity carry them away.
Yet by some fluke, red is not the color of autumn in evergreen broadleaf forests — it is the color of new, young leaves. These leaves are richer in nutrients, higher in protein, and lower in fiber than older leaves, making them more nutritious to herbivores. Fiber makes leaves more difficult to digest while protein makes them more attractive and palatable. These fresh leaves, before they are laden with their protective compounds, are what herbivorous insects and mammals favor because they are tender.
Then, as the growing leaves accumulate secondary or defense compounds, including alkaloids, phenolics, tannins, and terpenoids, they become less and less palatable. Some of these compounds serve to repel insects, as is the case of the camphor tree, which commonly grows in southern Japan. Other compounds inhibit fungal growth. These insecticidal or fungicidal properties are sometimes exploited by people directly or indirectly in their use of particular timbers or products made from wood.
I recall once working in a deciduous forest on the eastern shore of Loch Lomond in Scotland. In the very early summer morning, the dawn chorus of birds began against a backdrop of stillness. However, there was another sound — like a fine pattering — that took me a while to locate and even longer to identify. Eventually I noticed nearby leaves covered with tiny particles.
On looking up I discovered that caterpillars, hordes of them, were munching their way through the canopy of young oak leaves high above, and it was their debris that was raining down around me. I have seen mass infestations of caterpillars a few times since, in each case affecting a single species of tree, sometimes one specimen alone — and once, in Alaska, I encountered it across a whole hillside of conifers.
The toxic defense compounds produced by trees turn an apparent sea of food into a minefield of chemical toxins. As a result, many insect species have extremely conservative diets, some feeding exclusively on just one species to which they have evolved a tolerance through producing detoxifying enzymes. Hence the periodic outbreaks of defoliating caterpillars, but limited to just one species of tree, while neighboring species are untouched.