The changes in color of our native foliage here on Eastern Long Island is not well-synchronized among species, with our tupelos often bare of leaves while the oaks are still green, but the fall colors generally peak during the third week in October. It looks like peak colors will be later this year, perhaps a result of the warm September and October weather we’ve been treated to.Autumn colors are the result of the breakdown of the green pigments, chlorophylls, and the unmasking of other leaf pigments, called anthocyanins and carotenoids. This process is triggered by two factors: the change in daylight, a cue that does not change from year to year, and temperature, a much more variable factor.
The intensity of the fall colors is affected by a third factor, precipitation. Anthocyanin is not only unmasked as chlorophyll breaks down, it is manufactured in leaves in autumn and its production is limited by abnormally dry weather. Perhaps this is the reason for the intense red colors in tupelos and red maples, both species found in the wet soils of swamps.
The manufacture of anthocyanin takes energy. Why produce this red pigment when leaves are being dismantled and dropped? Scientists have speculated that anthocyanin must have some adaptive value to plants at this time of year, but the two theories that have been proposed—one being that anthocyanin protects leaves from sun damage during the brief period when chlorophyll is being dismantled and nutrients are being transported from leaves to stems and roots for winter storage, and the other claiming the bright red coloration discourages insect pests, such as aphids, from laying eggs on nearby twigs and branches in autumn—have not held up to close scrutiny.
This annual botanical phenomenon remains a mystery.
Some of the most brilliant red colors adorn winged sumac (a shrub) and Virginia creeper (a vine). The ubiquitous poison ivy, found growing as both a shrub and vine, also produces a striking reddish color in its leaves.
While not brilliant, our flowering dogwoods add a nice splash of deep purplish-red to the autumn landscape. Often overlooked at other times of the year, the two common shrubs found in our oak forests, lowbush blueberry and huckleberry, take on a mix of orange-red-purple that can be so striking as to appear fluorescent.
Why do plants drop their leaves? That is best answered by looking at the function of leaves.
Leaves are essentially solar panels designed to collect sunlight energy. That energy is used to manufacture food in a process called photosynthesis. After a year of exposure to the elements and insects, a leaf’s ability to collect sunlight and contribute to the growth of the plant is greatly diminished. At some point, the solar panel becomes more of a liability than an asset, and before that point is reached, valuable compounds that can be reused elsewhere in the tree are extracted, and the leaf is cast off.
Everyone knows that our deciduous trees, by definition, drop their leaves every year. Most in this area drop their leaves before winter, when snow and ice could collect on the leaves and cause damage to the tree, and frozen ground could prevent the replacement of water lost through leaves and result in severe desiccation. But some people are not aware that evergreens also drop their leaves. The difference, of course, is that the evergreens don’t shed all their leaves at once. They drop their oldest leaves, those that have put in at least two full growing seasons, and hang on to their most recently developed ones that still function adequately as solar collectors.
A close look at the branches of a white pine or pitch pine at this time of year illustrates this clearly. The golden brown leaves, or needles, being shed are located back from the tip of the twigs on last year’s growth. Needles that sprouted from buds in the spring of 2016 are kept for the 2017 season, while the class of 2015 is dropped to the forest floor for recycling.
This also can be seen among arborvitaes, although the tiny scale-like leaves are shed still attached to the twig in a large, flat clump. It’s harder to see in Eastern red cedars, as they drop their older needles more gradually over a period of many months. And American holly will not shed its oldest leaves until spring.
Among the evergreens, the design of the solar panels must take into account the challenges of winter. The process of photosynthesis involves the exchange of gases through tiny pores, called stomata, in the surface of the leaf. Carbon dioxide is taken in and utilized in the formation of carbohydrates. A very important byproduct of photosynthesis, oxygen, is excreted, along with some water vapor. Loss of the latter, called transpiration, is dangerous to the plant under conditions when it cannot be easily replaced, such as during a prolonged drought.
During a cold winter, when precipitation falls as snow and the ground is frozen, water is not available to evergreens. There are several adaptations to avoid desiccation, and chief among them is to form a thick, waxy coating on the leaves, as is found on mountain laurel and American holly. Another strategy is to grow low to the ground (e.g. wintergreen and trailing arbutus), where the impact of wind is lessened and the winter snow pack provides a protective cover.
A third strategy is to shape the leaves in such a way as to minimize surface area: good for water conservation but a trade-off in terms of functioning as a solar collector. Extreme examples of the latter strategy are seen among our needled evergreens (e.g. pitch pine and white pine), and it is the trademark among the spruces and firs that dwell in the far north where the ability to shed snow is also important.