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Though autumn may appear to be primarily a transitional season, he shows how many remarkable and essential natural processes happen routinely only during this period. He describes such topics as timekeeping in plants and animals, food hoarding, seed dispersal, and animal mating behavior among the large mammals of the North. The book is organized by theme rather than by species, so that similar adaptation mechanisms of different species can be compared and contrasted.
Marchand has a demonstrated skill in making scientific facts easily understood, while also conveying the beauty and wonder of what he describes. Also an accomplished photographer, his many beautiful full-page photographs show unusual aspects of the season.
Keepers of Time
One mid-August morning in the high desert of Arizona, with the promise of monsoon rains rumbling in distant clouds and the metallic buzz of cicadas portending another hot summer day, I stepped out of my rock house on top of the mesa and immediately registered a sensation of fall. It took me quite by surprise, like a rustling in the bushes, and I looked around expecting to see what it was that struck me, as if "fall" were an object of some kind. There was nothing, of course; but the sensation was real and lingered like early morning frost in a deep shade.
It was a fleeting incident, one I might have shrugged off without further contemplation, except that something kept drawing me back to it, kept puzzling me as to why I should have experienced it at all. By the calendar, autumn was undoubtedly progressing in the north, but why, I wondered—without any conscious awareness and indeed after many years removed from my native forest environment—was my sense of timing still so closely synchronized with events I could not see? Was there some biological value to humans in monitoring seasonal progression, a vestige of our mammalian heritage, a carryover from an ancient ancestry in which an ability to anticipate change well in advance of its arrival may have been of adaptive value, especially in the face of winter's uncertainty? And what could I have been responding to on that clear desert morning? Were there cells in my brain coded to measure day length and send out hormonal messages that say "pay attention, things are changing"? To suchquestions, Edward O. Wilson, the eminent naturalist and sociobiologist from Harvard, would likely have answered yes, suggesting that with only a few thousand years of separation we might well carry genes from ancestral hunter-gatherers whose fitness might have increased with the sensitivity of their own biological calendars.
If this were so, I wondered, could such a link to our distant past account for other emotions that seem deeply rooted within my core—a need, for example, to be roaming the woods at this time of year; an urge to pick up the gun and go in search of game, though I am not a hunter? Others have felt it too: "the time of the chase, the season of the buck and doe and of the ripening of all forest fruits; the time when all men are incipient hunters." Even Thoreau, one of the most sensitive observers of natural history, who hunted as a boy but had long since shunned meat and given up the gun even for his bird studies, admitted to such feelings:
I found myself ranging the woods, like a half-starved hound, with a strange abandonment, seeking some kind of venison which I might devour, and no morsel could have been too savage for me. The wildest scenes had become unaccountably familiar. I found in myself, and still find, an instinct toward a higher ... spiritual life, as do most men, and another toward a primitive rank and savage one, and I reverence them both. I love the wild not less than the good ... even in civilized communities, the embryo man passes through the hunter stage of development.
Perhaps these responses—the sense of timing, the urges within—simply are conditioned by experience and long habit. Yet if such learned behavior were to be of any benefit to ourselves or our ancestors in preparing for difficult times ahead, it is implicit that we have the ability to anticipate change, which requires measuring something and then comparing this to yesterday's measurement, or last week's, or last month's, which also requires memory in one form or another.
That we might possess the ability to carry out these computations subconsciously does not tax the imagination excessively, but the problem gets much more difficult when we consider an animal of temperate or northern latitudes, born in the spring, whose first winter's survival now requires numerous timely adaptations in physiology and behavior. Less than 6 months old, it must register impending change and make profound adjustments for a season it does not know. Plants, too, must have a way of anticipating change, for in acclimating to winter conditions, timing is critical. Survival of freezing, for example, requires many alterations at the cellular level that must be completed well in advance of the immediate need.
Virtually every organism, then, native to temperate or higher latitudes—and that lives for more than one growing season—must possess a physiological yardstick or calendar, a means of tracking time either directly through internal rhythms or indirectly by monitoring changes in some tangible quality of the environment. What might provide such a precise clock or calendar? Understanding the possibilities first requires understanding the nature of seasonal change.
* * *
Autumn, though more subtle in some areas than others, is universal outside of the tropics. When a newcomer to an area such as the shortgrass prairie that generally lacks colorful deciduous trees remarks that he or she misses the seasons, what the person usually means is that he or she misses the turning leaves of aspens, maples, birches, or some other conspicuous indicator of the season. But the changing of leaves does not bring on the season. The absence of deciduous trees in the shortgrass prairie does not mean that fall, any more than spring, fails to come to the grasslands. It is the steady, progressive change in angle of incoming sunlight as the earth, tilted on its axis, voyages around the sun that determines the annual march of seasons; and it is only the intensity of change, increasing with distance from equatorial regions, and the vegetative expression of seasons that varies geographically.
The earth in its voyage around the sun traces an imaginary path that defines the plane of our orbit in space. The sun does not sit at the center of the plane, however, for our orbit is elliptical, with the earth passing closest to the sun in the latter part of December and reaching its most distant point in June. It is not, therefore, our elliptical orbit, our varying distance from the sun, that results in seasons of warmth or coolness, for if this were so our northern summers would occur from December through February as in the southern hemisphere. Rather, it is the tilt of the earth's own axis from vertical (with respect to the plane of our orbit around the sun) that results in large monthly variations of incoming solar radiation at any given location outside the tropics. As we approach the aphelion of our orbit on the summer solstice, the northern hemisphere is tilted prominently toward the sun, giving all the northern latitudes longer, warmer days and keeping the region above the Arctic Circle entirely within the sunlit sector as the earth turns. Six months later, we (in the northern hemisphere) are closer to the sun but tilted away from it. Between these two turning points in our annual journey are the seasons of spring and fall.
Transitions between summer and winter are seldom as smooth, however, as described by planetary physics. Of spring and fall Burroughs remarked, "the [season] comes like a tide running against a strong wind; it is ever beaten back, but ever gaining ground, with now and then a mad push upon the land." Indeed, many of us live in places where spring is occasionally turned on its heels with a surprise May snowstorm, or the bite of autumn frost is temporarily soothed with a placid "Indian summer" of unseasonably warm weather (see p. 122). Though the annual course of incoming solar radiation is entirely predictable, it gives rise to much less dependable changes, on a daily or weekly basis, in the surface temperatures that drive our weather systems. This unpredictability of temperature results from considerable variation over time and distance in the amount of radiant energy absorbed by the earth, as affected by cloudiness, the presence of snow and ice cover, and differences in vegetation or soil character. Thus, temperature alone is too uncertain to serve as the primary signal for the myriad changes that must take place in the fall. The environmental cues by which plants and animals coordinate their seasonal rhythms must be far more dependable.
Three direct aspects of incoming solar radiation, apart from its varying effect on surface temperatures, offer potentially useful measures in predicting the seasons. Each of these is related to the ever-changing angle at which sunlight strikes the earth, as the earth's poles alternately dip toward and then away from the sun. One obvious result of a lower angle of incidence is an increase in shadow length. In order for changes in shadow length to be used effectively as a calendar, however, they would have to be referenced to some fixed measure and standardized for time of day—a practice that was used successfully by prehistoric cultures to anticipate important events, but that would require extraordinary neural function on the part of animals to accomplish subconsciously. More useful, perhaps, is the change in light quality—the spectral character of light reaching us—that accompanies the seasonally changing angle of incidence. Light passing through the atmosphere interacts with various gasses and particulates, each of which tends to absorb or reflect differently, depending on the wavelength or energy of the light. As the distance a beam of sunlight must travel through the earth's atmosphere increases, more attenuation occurs in the ultraviolet and infrared portions of the spectrum, energy just beyond the two ends of the visible spectrum. While this is light the mammalian eye can't see, both ultraviolet and infrared wavelengths are, nonetheless, capable of regulating the biological clocks in rodents (see later discussion). In addition, some attenuation occurs in the blue-green region of the spectrum, which corresponds to the wavelength of peak sensitivity in the biological rhythms of many mammals. Insofar as light quality also changes on a daily basis, however, as between early morning, noon, and evening (hence the prevalence of reds, yellows, and oranges when the sun is near the horizon), some means of time correction would be required if this were to serve as an effective calendar.
The one seasonal trend in solar radiation that seems to affect humans most in fall is the accelerated rate of decrease in total daylight hours or night length. This is particularly dramatic at higher latitudes, of course, where the transition from long summer days ("days" that last for weeks above the Arctic Circle) to equally long winter nights is especially rapid. Day length at 65° N latitude (Fairbanks, Alaska, for example) shortens by 31/2 hours between August 1 and September 1. But even at mid latitudes the changing day length at this time of year is easily perceptible, and it is not difficult to imagine this as furnishing a useful calendar, provided some mechanism exists by which an organism can keep track of total hours of sunlight or darkness. The problem seems simple enough for us, as we subconsciously register the slowly changing light conditions of morning and evening relative to clock time and our daily habits of working and eating, but we must keep in mind that for plants and animals the only clocks are internal, and environmental information of this nature must not only be quantified, but also translated into a physiological response.
* * *
If light is to serve as a clock or calendar, something has to receive information about it and translate that information into action. The eye, of course, gathers light, but there is compelling information that it is not the only organ to do so. A congenitally blind person may still show chemical responses to light despite absence of any pupillary reflex, outer retinal functioning, or conscious awareness of a light stimulus. Retinally degenerate mice show normal biological rhythms under simulated day and night light regimes in spite of near total loss of visual perception. And in both birds and insects, the primary receptor of environmental signals regulating reproduction, migration, and fat deposition lies somewhere other than in the retina. Experimentally covering portions of a bird's skull, but not its eyes, can make a vast difference in its developmental response to simulated long or short days. The light receptor in birds and insects, for nonvisual information, appears to lie in the brain, not the eyes.
While the exact location and nature of the extraretinal light sensor in birds and insects remains elusive, in mammals the pineal appears to be the "third eye." The pineal is a single, somewhat club-shaped organ usually found in the midline of the brain at the point where the cerebral hemispheres and the cerebellum come together. It is a distinct organ, separate from the brain, and is highly vascularized—a condition linked to its ultimate function.
The pineal exhibits several properties that suggest a central role as a biological calendar. For one, it is photoreceptive by itself, meaning that it is capable of detecting and responding to light, both electrically and metabolically, even when surgically isolated from the body. The pineal of many animals contains cells with a photosensitive pigment, probably rhodopsin (the same pigment that receives light in the rod cells of the retina). But the pineal also functions in conjunction with neural signals from the brain or nervous system, and is thus capable of integrating light information from the retina as well. Most importantly, the pineal acts like an endocrine gland, a ductless gland that, in response to nerve signals, secretes chemical products into the blood—in this case melatonin, a hormone that regulates a number of developmental and reproductive processes.
In essence, then, while the pineal shows independent light sensitivity, its function as a principal light-monitoring organ in mammals seems tied directly to both the retina and a biological clock that is located in the hypothalamus of the brain. Light impulses from the retina are sent via a special nerve tract directly to the hypothalamus, which coordinates general biological rhythms with day-night cycles. The light information is then relayed by neural signals to the pineal, where it is translated into chemical information via the secretion of melatonin and sent to other parts of the body.
It is the pineal's sensitivity to light information received from the brain and its ability to translate this into chemical information (its endocrine function) that places it in a central position with regard to time measurement. The pineal is essentially the endpoint of an optic system for processing day-length information, not unlike that of the visual cortex in the brain for processing images. The usefulness of the pineal as a calendar, however, hinges on the importance of melatonin as a chemical messenger and its sensitivity to day-night duration. Melatonin is a key player in mediating a number of seasonal changes in animals, many of which are critical to its overwintering success. Introduced artificially into the blood of white-footed mice, melatonin can induce fall molt, even during long days. It can stimulate the accumulation of brown adipose tissue, increase the animals' basal metabolic rate, and double their number of spontaneous daily torpor bouts. Injected into weasels and djungarian hampsters it can prevent molting from white to brown during the lengthening days of spring. Melatonin is also a key hormone in regulating reproductive development in animals. In every case, whether an animal be diurnal or nocturnal in habit, melatonin is synthesized at night, and therefore its quantity in the bloodstream provides direct information to other tissues regarding day length.
A single enzyme is responsible for the light sensitivity of melatonin production. NAT, as it is called (for N-acetyltransferase), one of 59 enzymes present in the pineal, catalyzes the conversion of the chemical serotonin to an intermediate product, which is then acted upon by another light-insensitive enzyme to complete the production of melatonin. NAT itself is so sensitive to light that its concentration in the pineal can increase more than 20-fold during a 12-hour night. Longer dark periods result in extended periods of high NAT activity, and hence melatonin secretion, thereby converting light information into a biochemical message in the form of nocturnal melatonin peaks of variable length. Long-duration melatonin peaks signal short days. (Continuous darkness, however, does not continuously stimulate NAT activity and melatonin production. The biological clock functioning in the hypothalamus restricts this activity to periods falling within the "expected" nighttime.) In essence, then, it is night length, rather than day length, that is being measured. If animals are exposed experimentally to a strong light pulse in the middle of the night, NAT levels plummet rapidly, halving in 3 to 5 minutes, with a subsequent drop in melatonin production signaling the end of the dark period. The light pulse is read as a new dawn, and the animal responds physiologically as if the long days of summer had returned.
* * *
If the sensitivity of the mammalian clockworks and calendar seems extraordinary, the timekeeping mechanism of plants is no less so. Lacking a central nervous system to process light impulses from optic sensors, and lacking the integrative function and information distribution capability of an endocrine gland, plants nonetheless anticipate the coming season unfailingly, and do so by acting on the same light information that animals receive. That two such disparate organisms might evolve similar strategies should not be so surprising; the two kingdoms have, after all, responded to the same need, with day length generally the only completely reliable environmental cue available to either. And when it comes to monitoring light, few organisms are better suited to the challenge than green plants, for if photoreceptors depend universally on light-sensitive pigments, then plants have no equal.
Photosynthetic plants have developed an impressive array of pigments designed to capture energy from almost the entire spectrum of visible sunlight. Chlorophyll molecules are the most abundant pigment in leaves, but their light-gathering effectiveness is limited to a relatively narrow color band at the blue-green and red ends of the spectrum. Because environments may differ in the spectral quality of light available to plants (energy reaching the forest floor, for example, has had much of the blue-green and red light filtered out by chlorophyll in the leaves overhead), additional pigments are often employed to capture light of different wavelengths and transfer its energy to the chlorophyll molecules, which are the only ones to participate directly in the chemical reactions of photosynthesis. The accessory pigments are themselves brilliantly colored—primarily shades of yellow and orange—but are not often seen until fall because they are masked by the sheer abundance of chlorophyll during the growing season.
With many different pigments present in the leaf, it would not be illogical to assume that one or another was somehow involved in the measurement of day length. Yet the identity of the clockwork pigment escaped discovery for a long time. When finally the timekeeper was revealed, it turned out to be a very different pigment indeed—one that seemed almost a contradiction in form and function to those previously known. This pigment wasn't found where the others usually were, yet it was almost everywhere in the plant. Its color wasn't far from that of chlorophyll, yet its appearance depended entirely on how, or when, you looked at it. New and fresh, it was bright blue, but the moment the molecule was illuminated by sunlight, it switched to an olive-green color. Exposed specifically to light at the far-red end of the spectrum, however, it reverted immediately back to blue. The pigment had nothing to do with energy capture for photosynthesis, yet by its absorption of light it seemed to have a decided influence on the timing of such key events as the onset of flowering, senescence, and dormancy.
The pigment is called phytochrome, and among all those light-absorbing (and -reflecting) molecules that excite our senses in the fall, this one, the least noticeable, may in many ways be the most important at this time of year. We now know it to serve as both a calendar and an on/off switch for a number of processes essential to the orderly cessation of growth and onset of winter acclimation in the plant. The exact mechanism by which phytochrome works remains a mystery, but the details of its timekeeping ability have largely been solved, and its split personality seems to be the key.
Phytochrome exists in two distinct and reversible states, each sensitive to light of slightly different color and each having greatly different effects on physiological processes in the plant. As noted already, when phytochrome is synthesized from protein by the plant it is blue in color and has an absorption peak (the color band to which a pigment is most responsive) centered in the orange-red region of the spectrum, specifically at a wavelength of 660 nm. In this state, which we call phytochrome-red (or Pr) for its reaction color, the molecule is essentially biologically inert. In fact, when present in abundance it seems to actually block specific developmental processes in the plant.
When phytochrome is excited by absorption of energy in the red band of sunlight, however, it changes state to a form in which it is biologically active, behaving almost like a hormone to open a gate for a number of developmental processes in the plant. In this state the pigment is olive-green in color and no longer reactive to orange-red light. Instead, it absorbs much more efficiently in a region centered near the far end of red, close to the limit of our color vision (we label this form Pfr for far-red)—but when it does so, strangely, it reverts back to its original state. Phytochrome undergoes this same reversal in darkness, too—or decays altogether—and this, it turns out, may be its most useful property as a timekeeper.
How, then, might phytochrome work as a calendar to regulate seasonal events? Sunlight is constantly converting phytochrome from its initial inhibitory form to the active state. But the reverse is also true because the full spectrum of sunlight contains energy affecting both forms of phytochrome (the absorption ranges of Pr and Pfr overlap slightly). Here's an important twist, though: The efficiency with which Pr absorbs red light is greater than the absorption efficiency of Pfr for far-red light, so sunlight acts more like a red source, with more of the Pr ending up as Pfr during the day than is true of the reverse. The result is establishment of an equilibrium concentration of the two forms of phytochrome in daylight, with the biologically active form dominant.
Length of day, by itself, doesn't change this balance, for sunlight will always drive the opposing reactions at about the same rate; but length of night is another matter. With Pfr undergoing either complete destruction and loss during the night, or chemical reversion to its original, inhibitory form, the length of night will have much to do with how long the active form of phytochrome, Pfr, or the blocking form, Pr, is present in the plant at any given season. And in much the same way that exposure to light in the middle of the night will cause a rapid plummet in NAT and melatonin synthesis in mammals, so too will even a 2- or 3-minute dark interruption signal a new dawn in plants. Thus, the parallels are nearly complete. Like the pineal in the animal, phytochrome is capable of responding to light information and communicating that information via chemical messengers, behaving much like a hormone to switch critical plant processes on or off. It is almost certain that phytochrome does not act alone (calcium is now suspected as an important accomplice), but it is the master. Concealed by the fall brilliance of other pigments, the unseen phytochrome, to the plant at least, may be the real glory of the season.
* * *
By mid-August the gradual increase in the length of night begins to make a difference to plants and animals. There are few outward signs of fall yet, for the days are still warm enough to nurture the progeny of summer, but the solstice is already two months past and the shadows of a year's new growth are testing their reach. In the Arctic, the caribou are entering their period of autumnal reproductive development even before the return of true night, with their biological calendar registering the progressively diminishing light intensity around midnight as shortening days. Some of the grasses are browning at their tips as they begin to move reserves of energy and nutrients to the safety of below-ground tissues. Flowers are maturing into seed heads of a different beauty. The signs are subtle at first, but the internal timekeepers are at work sending the first messages of impending cold. There is a quiet revolution starting behind the mask of August calm.
WHAT SAITH SEPTEMBER?
A Fair month, truly—golden fair, spiced with breath of the orchards, the vineyards' winy smell ...
All the earth lies dry and warm, and palpitant in sunshine. The touch of it is vital. Lie at length here in the pasture, prone on its springy turf, and let the strength of it, the sweetness, the balm of healing, lap your tired soul to the Elysium, sleep—such sleep as comes never within four walls, or to the downiest couch ever fashioned by man's hand. Sleep, and dream not. This the hour of fruition, needs not to borrow charm of such insubstantial stuff. A full world and goodly lies all about. Upland orchards blush red and yellow; lowland, stubble, meadow, corn-field, chant in high, colorful notes a swelling prelude to Nature's harvest-home.
What scent comes out of the corn-land—rare, fine, subtile as breath of elfin flowers? All the russet rustling stretch is steeped in its balm. You drink it in long gasps, and turn away, sighing—it is full, so full, of spring, and dew, and dawn, and hope ...
A jocund time this should be. The earth, the fulness thereof, lies smiling peace to a perfect heaven. Yet somehow there creeps in an under-note—a wailing minor of loss and waste. Faint, ah, so faint! you hear it in the singing waters, the full, rich, rustling leaves, the low winds sighing out of the sky to lose them as wafts of balm. Through them September saith to this fair world, "Laugh, dance, lie in the sun; eat, drink, and be merry. To-morrow you must die."
Walk afield [then] every day ... Whether sun shines, or rain drips, or white frost bites and stings, you should find a liberal education in the hectic beauty of death; not cruel death, but a tender doom, sweet with the glory of full harvest, and spanned with the rainbow of spring resurection.
— MARTHA MCCULLOCH WILLIAMS, 1892
|I||A Season of Change|
|Keepers of Time||3|
|A Touch of Frost||29|
|Reflections on the Pond||36|
|Down the Long Wind||45|
|The Improbable Flight of Insects||67|
|Walking the Whole Way South||72|
|A Dangerous Chill||82|
|III||A Time to Sow, A Time to Reap|
|Finding New Ground||95|
|Harvesting the Future||103|
|Season of the Vole||115|
|The Fall of the Leaf||127|